PisaVisionLab

Anobile, G., Cicchini, G. M. & Burr, D. C. (2012). Linear mapping of numbers onto space requires attention,Cognition, 3 (122), 454-459. PDF

Mapping of number onto space is fundamental to mathematics and measurement. Previous research suggests that while typical adults with mathematical schooling map numbers veridically onto a linear scale, pre-school children and adults without formal mathematics training, as well as individuals with dyscalculia, show strong compressive, logarithmic-like non-linearities when mapping both symbolic and non-symbolic numbers onto the numberline. Here we show that the use of the linear scale is dependent on attentional resources. We asked typical adults to position clouds of dots on a numberline of various lengths. In agreement with previous research, they did so veridically under normal conditions, but when asked to perform a concurrent attentionally-demanding conjunction task, the mapping followed a compressive, non-linear function. We model the non-linearity both by the commonly assumed logarithmic transform, and also with a Bayesian model of central tendency. These results suggest that veridical representation numerosity requires attentional mechanisms.

Pooresmaeili, A., Cicchini, G. M., Morrone, M. C. & Burr, D. (2012). "Non-retinotopic processing" in Ternus motion displays modeled by spatiotemporal filters,J Vis, 1 (12), PDF

Recently, M. Boi, H. Ogmen, J. Krummenacher, T. U. Otto, & M. H. Herzog (2009) reported a fascinating visual effect, where the direction of apparent motion was disambiguated by cues along the path of apparent motion, the Ternus-Pikler group motion, even though no actual movement occurs in this stimulus. They referred to their study as a "litmus test" to distinguish "non-retinotopic" (motion-based) from "retinotopic" (retina-based) image processing. We adapted the test to one with simple grating stimuli that could be more readily modeled and replicated their psychophysical results quantitatively with this stimulus. We then modeled our experiments in 3D (x, y, t) Fourier space and demonstrated that the observed perceptual effects are readily accounted for by integration of information within a detector that is oriented in space and time, in a similar way to previous explanations of other motion illusions. This demonstration brings the study of Boi et al. into the more general context of perception of moving objects.

Cicchini, G. M., Arrighi, R., Cecchetti, L., Giusti M. & Burr, D. C. (2012). Optimal Encoding of Interval Timing in Expert Percussionists, J Neurosci, 3 (32), 1056-1060. PDF

We measured temporal reproduction in human subjects with various levels of musical expertise: expert drummers, string musicians, and non-musicians. While duration reproduction of the non-percussionists showed a characteristic central tendency or regression to the mean, drummers responded veridically. Furthermore, when the stimuli were auditory tones rather than flashes, all subjects responded veridically. The behavior of all three groups in both modalities is well explained by a Bayesian model that seeks to minimize reproduction errors by incorporating a central tendency prior, a probability density function centered at the mean duration of the sample. We measured separately temporal precision thresholds with a bisection task; thresholds were twice as low in drummers as in the other two groups. These estimates of temporal precision, together with an adaptable Bayesian prior, predict well the reproduction results and the central tendency strategy under all conditions and for all subject groups. These results highlight the efficiency and flexibility of sensorimotor mechanisms estimating temporal duration.

Zimmermann, E., Morrone, M. C. & Burr, D. (2012). Visual motion distorts visual and motor space, J Vis, 2 (12), PDF

Mapping of number onto space is fundamental to mathematics and measurement. Previous research suggests that while typical adults with mathematical schooling map numbers veridically onto a linear scale, pre-school children and adults without formal mathematics training, as well as individuals with dyscalculia, show strong compressive, logarithmic-like non-linearities when mapping both symbolic and non-symbolic numbers onto the numberline. Here we show that the use of the linear scale is dependent on attentional resources. We asked typical adults to position clouds of dots on a numberline of various lengths. In agreement with previous research, they did so veridically under normal conditions, but when asked to perform a concurrent attentionally-demanding conjunction task, the mapping followed a compressive, non-linear function. We model the non-linearity both by the commonly assumed logarithmic transform, and also with a Bayesian model of central tendency. These results suggest that veridical representation numerosity requires attentional mechanisms.

Cicchini, G. M. (2012). Perception of duration in the parvocellular system, Front Integr Neurosci 6:14 PDF

Both theoretical and experimental evidence suggests that duration perception is mediated preferentially by the color-blind but high temporally sensitive luminance pathway. In this experiment we tested whether color modulated stimuli and high spatial frequency luminance modulated stimuli, which are known to be relayed mostly by the slow parvocellular system, are able to elicit reliable sense of duration. We show that ramped color modulated stimuli seem to last less than luminance modulated stimuli matched for visibility. The effect is large, about 200 ms and is constant at all durations tested (range 500–1100 ms). However, high spatial frequency luminance stimuli obtain duration matches similar to those of low spatial frequency luminance modulated stimuli. The results at various levels of contrast and temporal smoothing indicate that equiluminant stimuli have higher contrast thresholds to activate the mechanisms which time visual stimuli. Overall the results imply that both the magnocellular and the parvocellular systems access reliably the timing mechanisms with a difference only in the way these are engaged

Schnier, F., Zimmermann, E. & Lappe, M. (2010). Adaptation and mislocalization fields for saccadic outward adaptation in humans,Journal of Eye Movement Research, 4 (3), 1-18. PDF

Adaptive shortening of a saccade influences the metrics of other saccades withina spatial window around the adapted target. Within this adaptation field visualstimuli presented before an adapted saccade are mislocalized in proportion to thechange of the saccade metric. We investigated the saccadic adaptation field andassociated localization changes for saccade lengthening, or outward adaptation. Wemeasured the adaptation field for two different saccade adaptations (14 deg to 20deg and 20 deg to 26 deg) by testing transfer to 34 different target positions. Wemeasured localization judgements by asking subjects to localize a probe flashedbefore saccade onset. The amount of adaptation transfer differed for different targetlocations. It increased with increases of the horizontal component of the saccade andremained largely constant with deviation of the vertical component of the saccade.Mislocalization of probes inside the adaptation field was correlated with the amountof adaptation of saccades to the probe location. These findings are consistent withthe assumption that oculomotor space and perceptual space are linked to each other.

Toscani, M., Marzi, T., Righi, S., Viggiano, M. P. & Baldassi, S. (2010). Alpha waves: a neural signature of visual suppression,Exp Brain Res, 3-4 (207), 213-219. PDF

Alpha waves are traditionally considered a passive consequence of the lack of stimulation of sensory areas. However, recent results have challenged this view by showing a modulation of alpha activity in cortical areas representing unattended information during active tasks. These data have led us to think that alpha waves would support a 'gating function' on sensorial stimulation that actively inhibits unattended information in attentional tasks. Visual suppression occurring during a saccade and blink entails an inhibition of incoming visual information, and it seems to occur at an early processing stage. In this study, we hypothesized that the neural mechanism through which the visual system exerts this inhibition is the active imposition of alpha oscillations in the occipital cortex, which in turn predicts an increment of alpha amplitude during a visual suppression phenomena. We measured visual suppression occurring during short closures of the eyelids, a situation well suited for EEG recordings and stimulated the retinae with an intra-oral light administered through the palate. In the behavioral experiment, detection thresholds were measured with eyes steady open and steady closed, showing a reduction of sensitivity in the latter case. In the EEG recordings performed under identical conditions we found stronger alpha activity with closed eyes. Since the stimulation does not depend on whether the eyes were open or closed, we reasoned that this should be a central effect, probably due to a functional role of alpha oscillation in agreement with the 'gating function' theory.

Morrone, M. C. (2010). Brain development: critical periods for cross-sensory plasticity,Curr Biol, 21 (20), R934-936. PDF

Recent work has shown that visual deprivation of humans during a critical period leads to motion area MT+ responding to auditory motion. This cross-sensory plasticity, an important form of brain reorganization, may be mediated by top-down brain circuits from pre-frontal cortex.

Schutz, A. C. & Morrone, M. C. (2010). Compression of time during smooth pursuit eye movements,Vision Res, 24 (50), 2702-2713. PDF

Humans have a clear sense for the passage of time, but while implicit motor timing is quite accurate, explicit timing is prone to distortions particularly during action (Wenke & Haggard, 2009) and saccadic eye movements (Morrone, Ross, & Burr, 2005). Here, we investigated whether perceived duration is also affected by the execution of smooth pursuit eye movements, showing a compression of apparent duration similar to that observed during saccades. To this end, we presented two brief bars that marked intervals between 100 and 300 ms and asked subjects to judge their duration during fixation and pursuit. We found a compression of perceived duration for bars modulated in luminance contrast of about 32% and for bars modulated in chromatic contrast of 14% during pursuit compared to fixation. Interestingly, Weber ratios were similar for fixation and pursuit, if they are expressed as ratio between JND and perceived duration. This compression was constant for pursuit speeds from 7 to 14 deg/s and did not occur for intervals marked by auditory events. These results argue for a modality-specific component in the processing of temporal information.

Giacomelli, G., Volpe, R., Virgili, G., Farini, A., Arrighi, R., Tarli-Barbieri, C., et al. (2010). Contrast reduction and reading: assessment and reliability with the Reading Explorer test,Eur J Ophthalmol, 2 (20), 389-396. PDF

PURPOSE: To investigate the reliability of the Reading Explorer (REX) charts and to assess the impact of text contrast reduction (1.5 cycle/degree) on reading speed in subjects with normal and low vision. METHODS: Standard visual acuity (ETDRS charts), reading speed (MNread charts), and contrast sensitivity (Pelli-Robson charts) measurements were obtained in 3 groups of subjects stratified by visual acuity level in the better eye from 0.0 to 1.0 logMAR, with intermediate cutoffs at 0.3 and 0.6 logMAR. Measurements of reading speed for decreasing levels of text contrast were obtained with the REX charts using a 1.5 cycle/degree text. RESULTS: Since in many patients with lower vision a plateau of maximum reading speed across different levels of text contrast was not found, reliability indexes were computed for average reading speed and reading contrast threshold. In the group with lower visual acuity, 95% limits of agreement were +/-0.134 log word/minute and +/-0.175 log contrast sensitivity, suggesting good reliability. The proportion of subjects with a 20% loss of reading speed from 90% to 45% text contrast was estimated to be 1/3 at 0.6 logMAR visual acuity level and 2/3 at 1.0 logMAR. CONCLUSIONS: The adverse effect of decreased text contrast, which may be found in ordinary reading material, on the reading performance of subjects with advanced and initial low vision is probably underestimated. The REX test proved to be a reliable investigation tool for this phenomenon.

Ricci, D., Cesarini, L., Gallini, F., Serrao, F., Leone, D., Baranello, G., et al. (2010). Cortical visual function in preterm infants in the first year,J Pediatr, 4 (156), 550-555. PDF

OBJECTIVE: To assess visual function in low-risk preterm infants at 3, 5, and 12 months corrected age to determine whether the maturation of visual function in the first year is similar to that reported in term-born infants. STUDY DESIGN: Seventy-five low-risk infants (25.0-30.9 weeks gestation) underwent ophthalmological examinations and a battery of tests (fix and follow, visual fields, acuity, attention at distance, and fixation shift) designed to assess various aspects of visual function at 3, 5, and 12 months corrected age. RESULTS: The results were comparable with normative data from term-born infants in all tests but fixation shift, suggesting that maturation of most aspects of visual function is not significantly affected by preterm birth. In contrast, >25% of preterm infants failed the fixation shift test at 3 months, with a higher percentage of failing at 5 and 12 months. CONCLUSIONS: There is a specific profile of early visual behavior in low-risk preterm infants, with a high percentage of infants failing a test that specifically assesses visual attention and provides a measure of cortical processing.

Zimmermann, E. & Lappe, M. (2010). Motor signals in visual localization,J Vis, 6 (10), 2. PDF

We demonstrate a strong sensory-motor coupling in visual localization in which experimental modification of the control of saccadic eye movements leads to an associated change in the perceived location of objects. Amplitudes of saccades to peripheral targets were altered by saccadic adaptation, induced by an artificial step of the saccade target during the eye movement, which leads the oculomotor system to recalibrate saccade parameters. Increasing saccade amplitudes induced concurrent shifts in perceived location of visual objects. The magnitude of perceptual shift depended on the size and persistence of errors between intended and actual saccade amplitudes. This tight agreement between the change of eye movement control and the change of localization shows that perceptual space is shaped by motor knowledge rather than simply constructed from visual input.

Guzzetta, A., D'Acunto, G., Rose, S., Tinelli, F., Boyd, R. & Cioni, G. (2010). Plasticity of the visual system after early brain damage,Dev Med Child Neurol, 10 (52), 891-900. PDF

The aim of this review is to discuss the existing evidence supporting different processes of visual brain plasticity after early damage, as opposed to damage that occurs during adulthood. There is initial evidence that some of the neuroplastic mechanisms adopted by the brain after early damage to the visual system are unavailable at a later stage. These are, for example, the ability to differentiate functional tissue within a larger dysplastic cortex during its formation, or to develop new thalamo-cortical connections able to bypass the lesion and reach their cortical destination in the occipital cortex. The young brain also uses the same mechanisms available at later stages of development but in a more efficient way. For example, in people with visual field defects of central origin, the anatomical expansion of the extrastriatal visual network is greater after an early lesion than after a later one, which results in more efficient mechanisms of visual exploration of the blind field. A similar mechanism is likely to support some of the differences found in people with blindsight, the phenomenon of unconscious visual perception in the blind field. In particular, compared with people with late lesions, those with early brain damage appear to have stronger subjective awareness of stimuli hitting the blind visual field, reported as a conscious feeling that something is present in the visual field. Expanding our knowledge of these mechanisms could help the development of early therapeutic interventions aimed at supporting and enhancing visual reorganization at a time of greatest potential brain plasticity.

Gori, M., Sandini, G., Martinoli, C. & Burr, D. (2010). Poor haptic orientation discrimination in nonsighted children may reflect disruption of cross-sensory calibration,Curr Biol, 3 (20), 223-225. PDF

A long-standing question, going back at least 300 years to Berkeley's famous essay, is how sensory systems become calibrated with physical reality. We recently showed [1] that children younger than 8-10 years do not integrate visual and haptic information optimally, but that one or the other sense prevails: touch for size and vision for orientation discrimination. The sensory dominance may reflect crossmodal calibration of vision and touch, where the more accurate sense calibrates the other. This hypothesis leads to a clear prediction: that lack of clear vision at an early age should affect calibration of haptic orientation discrimination. We therefore measured size and orientation haptic discrimination thresholds in 17 congenitally visually impaired children (aged 5-19). Haptic orientation thresholds were greatly impaired compared with age-matched controls, whereas haptic size thresholds were at least as good, and often better. One child with a late-acquired visual impairment stood out with excellent orientation discrimination. The results provide strong support for our crossmodal calibration hypothesis.

Paci, M., Matulli, G., Baccini, M., Rinaldi, L. A. & Baldassi, S. (2010). Reported quality of randomized controlled trials in neglect rehabilitation,Neurol Sci, 2 (31), 159-163. PDF

The aim of this study is to assess the reported quality of randomized controlled trials (RCTs) on the effectiveness of neglect rehabilitation using a standardized scale. A search of seven electronic databases was carried out. Selected articles were scored using the PEDro scale and classified as high or low quality study both with the original cut off of 6 and a modified cut off of 5. A linear regression analysis between year of publication and quality rate was used to test whether the quality of the studies improved with time. A total of 18 RCTs were selected. Six articles (33.3%) and 10 articles (55.56%) were classified as having high quality when the original cut off or the modified cut off of the PEDro scale were used, respectively. Analysis shows no time-related changes in PEDro scores. The results show that reported quality is moderate for RCTs in neglect rehabilitation.

Burr, D. C., Ross, J., Binda, P. & Morrone, M. C. (2010). Saccades compress space, time and number,Trends Cogn Sci, 12 (14), 528-533. PDF

It has been suggested that space, time and number are represented on a common subjective scale. Saccadic eye movements provide a fascinating test. Saccades compress the perceived magnitude of spatial separations and temporal intervals to approximately half of their true value. The question arises as to whether saccades also compress number. They do, and compression follows a very similar time course for all three attributes: it is maximal at saccadic onset and decreases to veridicality within a window of approximately 50ms. These results reinforce the suggestion of a common perceptual metric, which is probably mediated by the intraparietal cortex; they further suggest that before each saccade the common metric for all three is reset, possibly to pave the way for a fresh analysis of the post-saccadic situation.

Morrone, M. C., Cicchini, M. & Burr, D. C. (2010). Spatial maps for time and motion,Exp Brain Res, 2 (206), 121-128. PDF

In this article, we review recent research studying the mechanisms for transforming coordinate systems to encode space, time and motion. A range of studies using functional imaging and psychophysical techniques reveals mechanisms in the human brain for encoding information in external rather than retinal coordinates. This reinforces the idea of a tight relationship between space and time, in the parietal cortex of primates.

Filippi, L., Cavallaro, G., Fiorini, P., Daniotti, M., Benedetti, V., Cristofori, G., et al. (2010). Study protocol: safety and efficacy of propranolol in newborns with Retinopathy of Prematurity (PROP-ROP): ISRCTN18523491,BMC Pediatr, (10), 83. PDF

BACKGROUND: Despite new therapeutic approaches have improved the prognosis of newborns with retinopathy of prematurity (ROP), an unfavourable structural and functional outcome still remains high. There is high pressure to develop new drugs to prevent and treat ROP. There is increasing enthusiasm for anti-VEGF drugs, but angiogenic inhibitors selective for abnormal blood vessels would be considered as an optimal treatment.In an animal experimental model of proliferative retinopathy, we have recently demonstrated that the pharmacological blockade of beta-adrenoreceptors improves retinal neovascularization and blood retinal barrier breakdown consequent to hypoxia. The purpose of this study is to evaluate the propranolol administration in preterm newborns suffering from a precocious phase of ROP in terms of safety and efficacy in counteracting the progression of retinopathy. METHODS/DESIGN: Preterm newborns (gestational age at birth lower than 32 weeks) with stage 2 ROP (zone II-III without plus) will be randomized, according to their gestational age, to receive propranolol added to standard treatment (treatment adopted by the ETROP Cooperative Group) or standard treatment alone. Propranolol will be administered until retinal vascularization will be completely developed, but not more than 90 days. Forty-four participants will be recruited into the study. To evaluate the safety of propranolol administration, cardiac and respiratory parameters will be continuously monitored. Blood samplings will be performed to check renal, liver and metabolic balance. To evaluate the efficacy of propranolol, the progression of the disease, the number of laser treatments or vitrectomies, the incidence of retinal detachment or blindness, will be evaluated by serial ophthalmologic examinations. Visual function will be evaluated by means of behavioural standardized tests. DISCUSSION: This pilot study is the first research that explores the possible therapeutic role of beta blockers in ROP. The objective of this research is highly ambitious: to find a treatment simple, inexpensive, well tolerated and with few adverse effects, able to counteract one of the major complications of the prematurity. Any favourable results of this research could open new perspectives and original scenarios about the treatment or the prevention of this and other proliferative retinopathies. TRIAL REGISTRATION: Current Controlled Trials ISRCTN18523491; ClinicalTrials.gov Identifier NCT01079715; EudraCT Number 2010-018737-21.

Burr, D. C., Turi, M. & Anobile, G. (2010). Subitizing but not estimation of numerosity requires attentional resources,J Vis, 6 (10), 20. PDF

The numerosity of small numbers of objects, up to about four, can be rapidly appraised without error, a phenomenon known as subitizing. Larger numbers can either be counted, accurately but slowly, or estimated, rapidly but with errors. There has been some debate as to whether subitizing uses the same or different mechanisms than those of higher numerical ranges and whether it requires attentional resources. We measure subjects' accuracy and precision in making rapid judgments of numerosity for target numbers spanning the subitizing and estimation ranges while manipulating the attentional load, both with a spatial dual task and the "attentional blink" dual-task paradigm. The results of both attentional manipulations were similar. In the high-load attentional condition, Weber fractions were similar in the subitizing (2-4) and estimation (5-7) ranges (10-15%). In the low-load and single-task condition, Weber fractions substantially improved in the subitizing range, becoming nearly error-free, while the estimation range was relatively unaffected. The results show that the mechanisms operating over the subitizing and estimation ranges are not identical. We suggest that pre-attentive estimation mechanisms works at all ranges, but in the subitizing range, attentive mechanisms also come into play.

Binda, P., Morrone, M. C. & Burr, D. C. (2010). Temporal auditory capture does not affect the time course of saccadic mislocalization of visual stimuli,J Vis, 2 (10), 7 1-13. PDF

Irrelevant sounds can "capture" visual stimuli to change their apparent timing, a phenomenon sometimes termed "temporal ventriloquism". Here we ask whether this auditory capture can alter the time course of spatial mislocalization of visual stimuli during saccades. We first show that during saccades, sounds affect the apparent timing of visual flashes, even more strongly than during fixation. However, this capture does not affect the dynamics of perisaccadic visual distortions. Sounds presented 50 ms before or after a visual bar (that change perceived timing of the bars by more than 40 ms) had no measurable effect on the time courses of spatial mislocalization of the bars, in four subjects. Control studies showed that with barely visible, low-contrast stimuli, leading, but not trailing, sounds can have a small effect on mislocalization, most likely attributable to attentional effects rather than auditory capture. These findings support previous studies showing that integration of multisensory information occurs at a relatively late stage of sensory processing, after visual representations have undergone the distortions induced by saccades.

Zimmermann, E., Schnier, F. & Lappe, M. (2010). The contribution of scene context on change detection performance,Vision Res, 20 (50), 2062-2068. PDF

The gist of a visual scene is perceived in a fraction of a second but in change detection tasks subjects typically need several seconds to find the changing object in a visual scene. Here, we report influences of scene context on change detection performance. Scene context manipulations consisted of scene inversion, scene jumbling, where the images were cut into 24 pieces and randomly recombined, and scene configuration scrambling, where the arrangement of the objects in the scene was randomized. Reaction times, where significantly lower in images with normal scene context. We conclude that scene context structures scene perception.

Lunghi, C., Binda, P. & Morrone, M. C. (2010). Touch disambiguates rivalrous perception at early stages of visual analysis,Curr Biol, 4 (20), R143-144. PDF

Binocular rivalry is a powerful tool to study human consciousness: two equally salient stimuli are imaged on the retinae, but at any given instant only one is consciously perceived, the other suppressed.The suppression takes place early, probably in V1. However, a trace of the suppressed signal has been detected along the dorsal visual pathway (BOLD responses) and demonstrated with psychophysical experiments. The suppressed image of a rotating sphere during rivalry is restored to consciousness when the observer actively controls the rotation and a similar effect on the suppressed signal has been shown for motion perception and reflexive eye movements (see Supplemental References). Here, we asked whether cross-modal sensory signals could selectively interact with rivalrous visual signals that are analyzed at a very early stage, probably V1. An auditory stimulus, when attended, can influence binocular rivalry, extending dominance times for a congruent visual stimulus. Tactile information can  also disambiguate unstable visual motion and can fuse with vision to improve discrimination (e.g. slant). Our results indicate that a haptic oriented stimulus can disambiguate visual perception during binocular rivalry of gratings of orthogonal orientation, not only by prolonging dominance but also by curtailing suppression of the visual stimulus of matched orientation. The effect is selective for the spatial frequency of the stimuli, suggesting that haptic signals interact with early visual representations to enhance access to conscious perception.

Ross, J. & Burr, D. C. (2010). Vision senses number directly,J Vis, 2 (10), 10 11-18. PDF

We have recently suggested that numerosity is a primary sensory attribute, and shown that it is strongly susceptible to adaptation. Here we use the Method of Single Stimuli to show that observers can extract a running average of numerosity of a succession of stimuli to use as a standard of comparison for subsequent stimuli. On separate sessions observers judged whether the perceived numerosity or density of a particular trial was greater or less than the average of previous stimuli. Thresholds were as precise for this task as for explicit comparisons of test with standard stimuli. Importantly, we found no evidence that numerosity judgments are mediated by density. Under all conditions, judgements of numerosity were as precise as those of density. Thresholds in intermingled conditions, where numerosity varied unpredictably with density, were as precise as the blocked thresholds. Judgments in constant-density conditions were more precise thresholds than those in variable-density conditions, and numerosity judgements in conditions of constant-numerosity showed no tendency to follow density. We further report the novel finding that perceived numerosity increases with decreasing luminance, whereas texture density does not, further evidence for independent processing of the two attributes. All these measurements suggest that numerosity judgments can be, and are, made independently of judgments of the density of texture.

Burr, D. C. & Morrone, M. C. (2010). Vision: keeping the world still when the eyes move,Curr Biol, 10 (20), R442-444. PDF

A long-standing problem for visual science is how the world remains so apparently stable in the face of continual rapid eye movements. New experimental evidence, and computational models are helping to solve this mystery.

Campanella, F., Sandini, G. & Morrone, M. C. (2010). Visual information gleaned by observing grasping movement in allocentric and egocentric perspectives,Proc Biol Sci, 1715 (278), 2142-2149. PDF

One of the major functions of vision is to allow for an efficient and active interaction with the environment. In this study, we investigate the capacity of human observers to extract visual information from observation of their own actions, and those of others, from different viewpoints. Subjects discriminated the size of objects by observing a point-light movie of a hand reaching for an invisible object. We recorded real reach-and-grasp actions in three-dimensional space towards objects of different shape and size, to produce two-dimensional 'point-light display' movies, which were used to measure size discrimination for reach-and-grasp motion sequences, release-and-withdraw sequences and still frames, all in egocentric and allocentric perspectives. Visual size discrimination from action was significantly better in egocentric than in allocentric view, but only for reach-and-grasp motion sequences: release-and-withdraw sequences or still frames derived no advantage from egocentric viewing. The results suggest that the system may have access to an internal model of action that contributes to calibrate visual sense of size for an accurate grasp.

Burr, D., Banks, M. S. & Morrone, M. C. (2009). Auditory dominance over vision in the perception of interval duration,Exp Brain Res, 1 (198), 49-57. PDF

The "ventriloquist effect" refers to the fact that vision usually dominates hearing in spatial localization, and this has been shown to be consistent with optimal integration of visual and auditory signals (Alais and Burr in Curr Biol 14(3):257-262, 2004). For temporal localization, however, auditory stimuli often "capture" visual stimuli, in what has become known as "temporal ventriloquism". We examined this quantitatively using a bisection task, confirming that sound does tend to dominate the perceived timing of audio-visual stimuli. The dominance was predicted qualitatively by considering the better temporal localization of audition, but the quantitative fit was less than perfect, with more weight being given to audition than predicted from thresholds. As predicted by optimal cue combination, the temporal localization of audio-visual stimuli was better than for either sense alone.

Arrighi, R., Arecchi, F. T., Farini, A. & Gheri, C. (2009). Cueing the interpretation of a Necker Cube: a way to inspect fundamental cognitive processes,Cogn Process, (10 Suppl 1), S95-99.PDF

The term perceptual bistability refers to all those conditions in which an observer looks at an ambiguous stimulus that can have two or more distinct but equally reliable interpretations. In this work, we investigate perception of Necker Cube in which bistability consists of the possibility to interpret the cube depth in two different ways. We manipulated the cube ambiguity by darkening one of the cube faces (cue) to provide a clear cube interpretation due to the occlusion depth index. When the position of the cue is stationary the cube perceived perspective is steady and driven by the cue position. However, when we alternated in time the cue position (i.e. we changed the position of the darkened cube face) two different perceptual phenomena occurred: for low frequencies the cube perspective alternated in line with the position of the cue; however for high frequencies the cue was no longer able to bias the perception but it appears as a floating feature traveling across the solid with the cube whole perspective that returns to be bistable as in the conventional, bias-free, case.

Arrighi, R., Marini, F. & Burr, D. (2009). Meaningful auditory information enhances perception of visual biological motion,J Vis, 4 (9), 25 21-27. PDF

Robust perception requires efficient integration of information from our various senses. Much recent electrophysiology points to neural areas responsive to multisensory stimulation, particularly audiovisual stimulation. However, psychophysical evidence for functional integration of audiovisual motion has been ambiguous. In this study we measure perception of an audiovisual form of biological motion, tap dancing. The results show that the audio tap information interacts with visual motion information, but only when in synchrony, demonstrating a functional combination of audiovisual information in a natural task. The advantage of multimodal combination was better than the optimal maximum likelihood prediction.

Zimmermann, E. & Lappe, M. (2009). Mislocalization of flashed and stationary visual stimuli after adaptation of reactive and scanning saccades,J Neurosci, 35 (29), 11055-11064. PDF

When we look around and register the location of visual objects, our oculomotor system continuously prepares targets for saccadic eye movements. The preparation of saccade targets may be directly involved in the perception of object location because modification of saccade amplitude by saccade adaptation leads to a distortion of the visual localization of briefly flashed spatial probes. Here, we investigated effects of adaptation on the localization of continuously visible objects. We compared adaptation-induced mislocalization of probes that were present for 20 ms during the saccade preparation period and of probes that were present for >1 s before saccade initiation. We studied the mislocalization of these probes for two different saccade types, reactive saccades to a suddenly appearing target and scanning saccades in the self-paced viewing of a stationary scene. Adaptation of reactive saccades induced mislocalization of flashed probes. Adaptation of scanning saccades induced in addition also mislocalization of stationary objects. The mislocalization occurred in the absence of visual landmarks and must therefore originate from the change in saccade motor parameters. After adaptation of one type of saccade, the saccade amplitude change and the mislocalization transferred only weakly to the other saccade type. Mislocalization of flashed and stationary probes thus followed the selectivity of saccade adaptation. Since the generation and adaptation of reactive and scanning saccades are known to involve partially different brain mechanisms, our results suggest that visual localization of objects in space is linked to saccade targeting at multiple sites in the brain.

Guzzetta, A., Tinelli, F., Del Viva, M. M., Bancale, A., Arrighi, R., Pascale, R. R., et al. (2009). Motion perception in preterm children: role of prematurity and brain damage,Neuroreport, 15 (20), 1339-1343. PDF

We tested 26 school-aged children born preterm at a gestational age below 34 weeks, 13 with and 13 without periventricular brain damage, with four different visual stimuli assessing perception of pure global motion (optic flow), with some form information (segregated translational motion) and form-defined static stimuli. Results were compared with a group of age-matched healthy term-born controls. Preterm children with brain damage showed significantly lower sensitivities relative to full-term controls in all four tests, whereas those without brain damage were significantly worse than controls only for the pure motion stimuli. Furthermore, when form information was embedded in the stimulus, preterm children with brain lesions scored significantly worse than those without lesions. These results suggest that in preterm children dorsal stream-related functions are impaired irrespective of the presence of brain damage, whereas deficits of the ventral stream are more related to the presence of periventricular brain damage.

Pei, F., Baldassi, S., Procida, G., Igliozzi, R., Tancredi, R., Muratori, F., et al. (2009). Neural correlates of texture and contour integration in children with autism spectrum disorders,Vision Res, 16 (49), 2140-2150. PDF

In this study, we have used an electrophysiological paradigm to investigate the neural correlates of the visual integration of local signals across space to generate global percepts in a group of low functioning autistic kids. We have analyzed the amplitude of key harmonics of the Visual Evoked Potentials (VEPs) recorded while participants observed orientation-based texture and contour stimuli, forming coherent global patterns, alternating with visual patterns in which the same number of local elements were randomly oriented in order to loose any globally organized feature. Comparing the results of the clinical sample with those obtained in an age-matched control group, we have observed that in the texture conditions the 1st and 3rd harmonics, containing signature of global form processing (Norcia, Pei, Bonneh, Hou, Sampath, & Pettet, 2005), were present in the control group, while in the experimental group only the 1st harmonic was present. In the Contour condition the 1st harmonic was not present for both groups while the 3rd harmonic was significantly present in the control group but absent in the group with autism. Moreover, the amount of organization required to elicit significant 1st harmonic response in the texture condition was higher in the clinical group. The present results bring additional support to the idea that texture and contour processing are supported by independent mechanisms in normal vision. Autistic vision would thus be characterized by a preserved, perhaps weaker texture mechanism, possibly mediated by feedback interactions between visual areas, and by a disfunction of the mechanism supporting contour processing, possibly mediated by long-range intra-cortical connections. Within this framework, the residual ability to detect contours shown in psychophysical studies could be due to the contribution of the texture mechanism to contour processing.

Burr, D. C., Baldassi, S., Morrone, M. C. & Verghese, P. (2009). Pooling and segmenting motion signals,Vision Res, 10 (49), 1065-1072. PDF

Humans are extremely sensitive to visual motion, largely because local motion signals can be integrated over a large spatial region. On the other hand, summation is often not advantageous, for example when segmenting a moving stimulus against a stationary or oppositely moving background. In this study we show that the spatial extent of motion integration is not compulsory, but is subject to voluntary attentional control. Measurements of motion coherence sensitivity with summation and search paradigms showed that human observers can combine motion signals from cued regions or patches in an optimal manner, even when the regions are quite distinct and remote from each other. Further measurements of contrast sensitivity reinforce previous studies showing that motion integration is preceded by a local analysis akin to contrast thresholding (or intrinsic uncertainty). The results were well modelled by two standard signal-detection-theory models.

Baldassi, S., Pei, F., Megna, N., Recupero, G., Viespoli, M., Igliozzi, R., et al. (2009). Search superiority in autism within, but not outside the crowding regime,Vision Res, 16 (49), 2151-2156. PDF

Visual cognition of observers with autism spectrum disorder (ASD) seems to show an unbalance between the complementary functions of integration and segregation. This study uses visual search and crowding paradigms to probe the relative ability of children with autism, compared to normal developments children, to extract individual targets from cluttered backgrounds both within and outside the crowding regime. The data show that standard search follows the same pattern in the ASD and control groups with a strong effect of the set size that is substantially weakened by cueing the target location with a synchronous spatial cue. On the other hand, the crowding effect of eight flankers surrounding a small peripheral target is virtually absent in the clinical sample, indicating a superior ability to segregate cluttered visual items. This data, along with evidence of an impairment to the neural system for binding contours in ASD, bring additional support to the general idea of a shift of the trade-off between integration and segregation toward the latter. More specifically, they show that when discriminability is balanced across conditions, an advantage in odd-man out tasks is evident in ASD observers only within the crowding regime, when binding mechanism might get compulsorily triggered in normal observers.

Cicchini, G. M. & Morrone, M. C. (2009). Shifts in spatial attention affect the perceived duration of events,J Vis, 1 (9), 9 1-13. PDF

We investigated the relationship between attention and perceived duration of visual events with a double-task paradigm. The primary task was to discriminate the size change of a 2 degree circle presented 10 degrees left, right, above, or below fixation; the secondary task was to judge the temporal separation (from 133 ms to 633 ms) of two equiluminant horizontal bars (10 deg x 2 deg) briefly flashed 12 degrees above or below fixation. The stimulus onset asynchrony (SOA) between primary and secondary task ranged from -1300 ms to +1000 ms. Temporal intervals in proximity of the onset of the primary task stimuli were perceived strongly compressed by up to 40%. The effect was proportional to the size of the interval with a maximum effect at 100 ms SOA. Control experiments show that neither primary-task difficulty, nor the type of primary task discrimination (form or motion, or equiluminant or luminance contrast) nor spatial congruence between primary and secondary task alter the effect. Interestingly, the compression occurred only when the intervals are marked by bars presented in separated spatial locations: when the interval is marked by two bars flashed in the same spatial position no temporal distortion was found. These data indicate that attention can alter perceived duration when the brain has to compare the passage of time at two different spatial positions, corroborating earlier findings that mechanisms of time perception may monitor separately the various spatial locations possibly at high level of analysis.

Binda, P., Cicchini, G. M., Burr, D. C. & Morrone, M. C. (2009). Spatiotemporal distortions of visual perception at the time of saccades,J Neurosci, 42 (29), 13147-13157. PDF

Both space and time are grossly distorted during saccades. Here we show that the two distortions are strongly linked, and that both could be a consequence of the transient remapping mechanisms that affect visual neurons perisaccadically. We measured perisaccadic spatial and temporal distortions simultaneously by asking subjects to report both the perceived spatial location of a perisaccadic vertical bar (relative to a remembered ruler), and its perceived timing (relative to two sounds straddling the bar). During fixation and well before or after saccades, bars were localized veridically in space and in time. In different epochs of the perisaccadic interval, temporal perception was subject to different biases. At about the time of the saccadic onset, bars were temporally mislocalized 50-100 ms later than their actual presentation and spatially mislocalized toward the saccadic target. Importantly, the magnitude of the temporal distortions co-varied with the spatial localization bias and the two phenomena had similar dynamics. Within a brief period about 50 ms before saccadic onset, stimuli were perceived with shorter latencies than at other delays relative to saccadic onset, suggesting that the perceived passage of time transiently inverted its direction. Based on this result we could predict the inversion of perceived temporal order for two briefly flashed visual stimuli. We developed a model that simulates the perisaccadic transient change of neuronal receptive fields predicting well the reported temporal distortions. The key aspects of the model are the dynamics of the "remapped" activity and the use of decoder operators that are optimal during fixation, but are not updated perisaccadically.

Burr, D., Silva, O., Cicchini, G. M., Banks, M. S. & Morrone, M. C. (2009). Temporal mechanisms of multimodal binding,Proc Biol Sci, 1663 (276), 1761-1769. PDF

The simultaneity of signals from different senses-such as vision and audition-is a useful cue for determining whether those signals arose from one environmental source or from more than one. To understand better the sensory mechanisms for assessing simultaneity, we measured the discrimination thresholds for time intervals marked by auditory, visual or auditory-visual stimuli, as a function of the base interval. For all conditions, both unimodal and cross-modal, the thresholds followed a characteristic 'dipper function' in which the lowest thresholds occurred when discriminating against a non-zero interval. The base interval yielding the lowest threshold was roughly equal to the threshold for discriminating asynchronous from synchronous presentations. Those lowest thresholds occurred at approximately 5, 15 and 75 ms for auditory, visual and auditory-visual stimuli, respectively. Thus, the mechanisms mediating performance with cross-modal stimuli are considerably slower than the mechanisms mediating performance within a particular sense. We developed a simple model with temporal filters of different time constants and showed that the model produces discrimination functions similar to the ones we observed in humans. Both for processing within a single sense, and for processing across senses, temporal perception is affected by the properties of temporal filters, the outputs of which are used to estimate time offsets, correlations between signals, and more.

Thompson, P. & Burr, D. (2009). Visual aftereffects,Curr Biol, 1 (19), R11-14. PDF

Burr, D. & Ross, J. (2008). A visual sense of number,Curr Biol, 6 (18), 425-428. PDF

Evidence exists for a nonverbal capacity for the apprehension of number, in humans [1] (including infants [2, 3]) and in other primates [4-6]. Here, we show that perceived numerosity is susceptible to adaptation, like primary visual properties of a scene, such as color, contrast, size, and speed. Apparent numerosity was decreased by adaptation to large numbers of dots and increased by adaptation to small numbers, the effect depending entirely on the numerosity of the adaptor, not on contrast, size, orientation, or pixel density, and occurring with very low adaptor contrasts. We suggest that the visual system has the capacity to estimate numerosity and that it is an independent primary visual property, not reducible to others like spatial frequency or density of texture [7].

Del Viva, M. M. & Gori, M. (2008). Anti-Glass patterns and real motion perception: same or different mechanisms?,J Vis, 2 (8), 1 1-15. PDF

A sequence of anti-Glass patterns, composed by dot pairs with opposite luminance polarity, elicits a clear perception of motion in the direction of the white dot of the pair. This effect can be reversed by introducing a delay in the presentation of white dots, suggesting a faster processing of light dots as a cause of the motion signal (M. M. Del Viva, M. Gori, & D. C. Burr, 2006). If this hypothesis is correct, anti-Glass patterns should interact with real motion signals. In this study, we compare the motion induced by these stimuli to test whether they are analyzed by the same motion mechanism. We found that motion induced by anti-Glass patterns annuls real motion, when they are presented simultaneously in the same display and moving in opposite directions. By lowering the contrast of one of them, motion toward the stimulus with higher contrast prevails. We also found sub-threshold summation of motion induced by anti-Glass patterns and real motion, when presented simultaneously and moving in the same direction. These findings indicate that anti-Glass patterns and moving stimuli are processed by the same, contrast-dependent motion mechanism and lend further support to the proposed explanation of the effect.

Perna, A., Tosetti, M., Montanaro, D. & Morrone, M. C. (2008). BOLD response to spatial phase congruency in human brain,J Vis, 10 (8), 15 11-15. PDF

Human psychophysical observations, computational models, and the selectivity of neurons in primary visual cortex all suggest that an early step in visual processing is the detection of features such as lines and edges. However, previous fMRI experiments investigating the responses of early visual areas to phase coherence have led to apparently discordant results. We studied the human brain BOLD responses to structured periodic band-pass images of matched amplitude spectrum but of different phase spectra, arranged to create three distinct types of stimuli: pure edges; pure lines (matched global and local energy to the edges, but different phase); and random noise (random phase spectrum, hence no salient features, and a different spatial distribution of local energy from the lines and edges stimuli). Alternation of lines against edges did not activate primary visual cortex, but did activate two higher order visual areas. Alternation of these lines or edges against the random stimulus produced a strong activity in many visual areas, including primary visual cortex. Interestingly, the BOLD activity was higher for the edges and lines than for the random stimuli for a wide range of stimulus contrasts, indicating the presence of non-linear gain modulation in the cell response. These results show that phase congruency is coded at the level of primary visual cortex. We show that a stage of response gain modulation can explain our present and previous fMRI discordant results.

Cicchini, G. M., Valsecchi, M. & De'Sperati, C. (2008). Head movements modulate visual responsiveness in the absence of gaze shifts,Neuroreport, 8 (19), 831-834. PDF

Visuospatial attention is strongly associated with saccades. Given that gaze shifts are often accomplished by combined eye-head movements, attention may also be coupled to head movements. We showed that simply turning the head without shifting the gaze is sufficient to cause a transient unbalance in responding to a visual stimulus. Manual responses to a stimulus flashed shortly before the onset of a horizontal head movement were faster in congruent trials, when the head moved towards the stimulus, than in incongruent trials, when the head moved away from the stimulus. These effects are similar to those observed for saccades. We take this as evidence for a tight link between visuospatial attention and head movements, even when the gaze does not shift.

Morrone, M. C., Guzzetta, A., Tinelli, F., Tosetti, M., Del Viva, M., Montanaro, D., et al. (2008). Inversion of perceived direction of motion caused by spatial undersampling in two children with periventricular leukomalacia,J Cogn Neurosci, 6 (20), 1094-1106. PDF

We report here two cases of two young diplegic patients with cystic periventricular leukomalacia who systematically, and with high sensitivity, perceive translational motion of a random-dot display in the opposite direction. The apparent inversion was specific for translation motion: Rotation and expansion motion were perceived correctly, with normal sensitivity. It was also specific for random-dot patterns, not occurring with gratings. For the one patient that we were able to test extensively, contrast sensitivity for static stimuli was normal, but was very low for direction discrimination at high spatial frequencies and all temporal frequencies. His optokinetic nystagmus movements were normal but he was unable to track a single translating target, indicating a perceptual origin of the tracking deficit. The severe deficit for motion perception was also evident in the seminatural situation of a driving simulation video game. The perceptual deficit for translational motion was reinforced by functional magnetic resonance imaging studies. Translational motion elicited no response in the MT complex, although it did produce a strong response in many visual areas when contrasted with blank stimuli. However, radial and rotational motion produced a normal pattern of activation in a subregion of the MT complex. These data reinforce the existent evidence for independent cortical processing for translational, and circular or radial flow motion, and further suggest that the two systems have different vulnerability and plasticity to prenatal damage. They also highlight the complexity of visual motion perception, and how the delicate balance of neural activity can lead to paradoxical effects such as consistent misperception of the direction of motion. We advance a possible explanation of a reduced spatial sampling of the motion stimuli and report a simple model that simulates well the experimental results.

Guzzetta, F., Cioni, G., Mercuri, E., Fazzi, E., Biagioni, E., Veggiotti, P., et al. (2008). Neurodevelopmental evolution of West syndrome: a 2-year prospective study,Eur J Paediatr Neurol, 5 (12), 387-397. PDF

OBJECTIVE: The aim of this study was to evaluate the epileptic and developmental evolution in infants with West syndrome. METHODS: A prospective study of 21 infants was performed, with a follow-up at 2 years. Serial assessment included long-term EEG monitoring, visual and auditory evaluation and assessment of neurodevelopment. RESULTS: Neurosensory and developmental impairments at the spasm onset were transitory in seven cases, including four cryptogenic forms. In all other cases, there was a progressive worsening in neurosensory and developmental impairments. The epileptic evolution was generally better: in 11 of the 16 infants without seizures at outcome, spasms had already disappeared by 2 months after disease onset. Statistic analysis of results showed a correlation between neurosensory impairment and development throughout the whole follow-up. In addition, visual function at T1 resulted significant predictor of developmental outcome. Among the epileptic features, disorganization of slow sleep was an unfavorable prognostic factor. CONCLUSION: Some forms of West syndrome are confirmed to have a benign evolution: among them there are not only cryptogenic cases but also symptomatic ones without significant neurodevelopmental impairment. Abnormalities of sleep organization, expression of the pervasive epileptic disorder, seem to play a role in determining a developmental deterioration. Neurosensory impairment since the onset of the disease could be a relevant cause of the developmental disorder.

Gheri, C. & Baldassi, S. (2008). Non-linear integration of crowded orientation signals,Vision Res, 22 (48), 2352-2358. PDF

Crowding of oriented signals has been explained as linear, compulsory averaging of the signals from target and flankers [Parkes, L., Lund, J., Angelucci, A., Solomon, J. A., & Morgan, M. (2001). Compulsory averaging of crowded orientation signals in human vision. Nature Neuroscience, 4(7), 739-744]. On the other hand, a comparable search task with sparse stimuli is well modeled by a 'Signed-Max' rule that integrates non-linearly local tilt estimates [Baldassi, S., & Verghese, P. (2002). Comparing integration rules in visual search. Journal of Vision, 2(8), 559-570], as reflected by the bimodality of the distributions of reported tilts in a magnitude matching task [Baldassi, S., Megna, N., & Burr, D. C. (2006). Visual clutter causes high-magnitude errors. PLoS Biology, 4(3), e56]. This study compares the two models in the context of crowding by using a magnitude matching task, to measure distributions of perceived target angles and a localization task, to probe the degree of access to local information. Response distributions were bimodal, implying uncertainty, only in the presence of abutting flankers. Localization of the target is relatively preserved but it quantitatively falls in between the predictions of the two models, possibly suggesting local averaging followed by a max operation. This challenges the notion of global averaging and suggests some conscious access to local orientation estimates.

Tinelli, F., Pei, F., Guzzetta, A., Bancale, A., Mazzotti, S., Baldassi, S., et al. (2008). The assessment of visual acuity in children with periventricular damage: a comparison of behavioural and electrophysiological techniques,Vision Res, 10 (48), 1233-1241. PDF

It has been controversial whether electrophysiology offers better precision than behavioural techniques in measuring visual acuity in children with brain damage. We investigated the concordance between sweep VEPs and Acuity Cards (AC) in 29 children with periventricular leukomalacia (PVL), the most common type of brain damage in preterm infants. An overall good correlation was shown but with relatively better behavioural acuity values. VEP/AC ratio was significantly correlated to corpus callosum posterior thinning. We propose that this result reflects the efficacy of the compensatory mechanisms following early brain damage which may differentially affect the two methods.

Ross, J. & Burr, D. (2008). The knowing visual self,Trends Cogn Sci, 10 (12), 363-364. PDF

Like all information-processing systems, biological visual systems are limited by internal and external noise; but this noise never actually impinges on our conscious perception. An article recently published in the Journal of Vision suggests that, at least for orientation judgments, the visual system has access to its own noisiness and sets thresholds accordingly. This could well be a general principle in perception, with important and wide ranging consequences.

Gori, M., Del Viva, M., Sandini, G. & Burr, D. C. (2008). Young children do not integrate visual and haptic form information,Curr Biol, 9 (18), 694-698. PDF

Several studies have shown that adults integrate visual and haptic information (and information from other modalities) in a statistically optimal fashion, weighting each sense according to its reliability [1, 2]. When does this capacity for crossmodal integration develop? Here, we show that prior to 8 years of age, integration of visual and haptic spatial information is far from optimal, with either vision or touch dominating totally, even in conditions in which the dominant sense is far less precise than the other (assessed by discrimination thresholds). For size discrimination, haptic information dominates in determining both perceived size and discrimination thresholds, whereas for orientation discrimination, vision dominates. By 8-10 years, the integration becomes statistically optimal, like adults. We suggest that during development, perceptual systems require constant recalibration, for which cross-sensory comparison is important. Using one sense to calibrate the other precludes useful combination of the two sources.

Pellicano, E., Jeffery, L., Burr, D. & Rhodes, G. (2007). Abnormal adaptive face-coding mechanisms in children with autism spectrum disorder,Curr Biol, 17 (17), 1508-1512. PDF

In low-level vision, exquisite sensitivity to variation in luminance is achieved by adaptive mechanisms that adjust neural sensitivity to the prevailing luminance level. In high-level vision, adaptive mechanisms contribute to our remarkable ability to distinguish thousands of similar faces [1]. A clear example of this sort of adaptive coding is the face-identity aftereffect [2, 3, 4, 5], in which adaptation to a particular face biases perception toward the opposite identity. Here we investigated face adaptation in children with autism spectrum disorder (ASD) by asking them to discriminate between two face identities, with and without prior adaptation to opposite-identity faces. The ASD group discriminated the identities with the same precision as did the age- and ability-matched control group, showing that face identification per se was unimpaired. However, children with ASD showed significantly less adaptation than did their typical peers, with the amount of adaptation correlating significantly with current symptomatology, and face aftereffects of children with elevated symptoms only one third those of controls. These results show that although children with ASD can learn a simple discrimination between two identities, adaptive face-coding mechanisms are severely compromised, offering a new explanation for previously reported face-perception difficulties [6, 7, 8] and possibly for some of the core social deficits in ASD [9, 10].

Binda, P., Bruno, A., Burr, D. C. & Morrone, M. C. (2007). Fusion of visual and auditory stimuli during saccades: a Bayesian explanation for perisaccadic distortions,J Neurosci, 32 (27), 8525-8532. PDF

Brief stimuli presented near the onset of saccades are grossly mislocalized in space. In this study, we investigated whether the Bayesian hypothesis of optimal sensory fusion could account for the mislocalization. We required subjects to localize visual, auditory, and audiovisual stimuli at the time of saccades (compared with an earlier presented target). During fixation, vision dominates and spatially "captures" the auditory stimulus (the ventriloquist effect). But for perisaccadic presentations, auditory localization becomes more important, so the mislocalized visual stimulus is seen closer to its veridical position. The precision of the bimodal localization (as measured by localization thresholds or just-noticeable difference) was better than either the visual or acoustic stimulus presented in isolation. Both the perceived position of the bimodal stimuli and the improved precision were well predicted by assuming statistically optimal Bayesian-like combination of visual and auditory signals. Furthermore, the time course of localization was well predicted by the Bayesian approach. We present a detailed model that simulates the time-course data, assuming that perceived position is given by the sum of retinal position and a sluggish noisy eye-position signal, obtained by integrating optimally the output of two populations of neural activity: one centered at the current point of gaze, the other centered at the future point of gaze.

Bruno, A. & Morrone, M. C. (2007). Influence of saccadic adaptation on spatial localization: comparison of verbal and pointing reports,J Vis, 5 (7), 16 11-13. PDF

Under conditions of short-term saccadic adaptation, stimuli presented long before saccadic onset are perceptually mislocalized in space. Here we study whether saccadic adaptation can also affect localization of objects by pointing. We measured localization performance during fixation and after normal saccades and adapted saccades, for a bar presented well before a saccadic eye movement, for both pointing and verbal localization, under open-loop conditions generated by a transient dark period about 300 ms after the presentation of the bar. During fixation and normal saccade, localization performance for verbal report was veridical, while for pointing there was an overestimation of the target eccentricity respect to gaze, in agreement with the idea of separate representations of space for action and perception. During saccadic adaptation, there was a significant shift of both pointing and verbal report localization in the direction of adaptation with similar spatial selectivity for both tasks. These results indicate that saccadic adaptation induces a similar re-calibration of the action map as well as of the perceptual map, suggesting a common site of operation in the transformation from eye-centered to gaze-centered coordinates.

Caputo, R., Tinelli, F., Bancale, A., Campa, L., Frosini, R., Guzzetta, A., et al. (2007). Motor coordination in children with congenital strabismus: effects of late surgery,Eur J Paediatr Neurol, 5 (11), 285-291. PDF

BACKGROUND: Strabismus is one of the most common visual disorders in infancy. While there is a great attention on the effects of the timing of surgery as to the development of binocular vision, little is known about the possible influence of congenital strabismus on perceptual-motor and more generally, on neuromotor development. AIMS: Aim of this study was to investigate perceptual-motor and motor coordination abilities of 19 children with essential congenital esotropia who underwent a late surgery (after 4 years), compared to 23 age-matched controls. METHODS: Children were tested using the Movement Assessment Battery for Children (Movement ABC) that were performed both 1-week before surgery (T1) and about 3 months (+/-2 weeks) after surgery (T2). RESULTS AND CONCLUSIONS: At T1, abnormal or borderline results were found in more than half of the children with strabismus, as opposed to only about 17% of the controls. At T2 none of the children showed abnormal Movement ABC total scores and there was no difference in global scores between the study and the control group. The two groups also did not show any significant difference in individual items of the movement ABC with the exception of those assessing ball skills. Our results suggest that surgical correction of strabismus, even when performed after the 4th year of life, appears to be effective in improving perceptual-motor and motor function.

Burr, D., Tozzi, A. & Morrone, M. C. (2007). Neural mechanisms for timing visual events are spatially selective in real-world coordinates,Nat Neurosci, 4 (10), 423-425. PDF

It is generally assumed that perceptual events are timed by a centralized supramodal clock. This study challenges this notion in humans by providing clear evidence that visual events of subsecond duration are timed by visual neural mechanisms with spatially circumscribed receptive fields, localized in real-world, rather than retinal, coordinates.

d'Avossa, G., Tosetti, M., Crespi, S., Biagi, L., Burr, D. C. & Morrone, M. C. (2007). Spatiotopic selectivity of BOLD responses to visual motion in human area MT,Nat Neurosci, 2 (10), 249-255. PDF

Many neurons in the monkey visual extrastriate cortex have receptive fields that are affected by gaze direction. In humans, psychophysical studies suggest that motion signals may be encoded in a spatiotopic fashion. Here we use functional magnetic resonance imaging to study spatial selectivity in the human middle temporal cortex (area MT or V5), an area that is clearly implicated in motion perception. The results show that the response of MT is modulated by gaze direction, generating a spatial selectivity based on screen rather than retinal coordinates. This area could be the neurophysiological substrate of the spatiotopic representation of motion signals.

Ciaramelli, E., Leo, F., Del Viva, M. M., Burr, D. C. & Ladavas, E. (2007). The contribution of prefrontal cortex to global perception,Exp Brain Res, 3 (181), 427-434. PDF

Recent research suggests a role of top-down modulatory signals on perceptual processing, particularly for the integration of local elementary information to form a global holistic percept. In this study we investigated whether prefrontal cortex may be instrumental in this top-down modulation in humans. We measured detection thresholds for perceiving a circle defined by a closed chain of grating patches in 6 patients with prefrontal lesions, 4 control patients with temporal lesions and 17 healthy control subjects. Performance of patients with prefrontal lesions was worse than that of patients with temporal lesions and normal controls when the patterns were sparse, requiring integration across relatively extensive regions of space, but similar to the control groups for denser patterns. The results clearly implicate the prefrontal cortex in the process of integrating elementary features into a holistic global percept, when the elements do not form a "pop-out" display.

Tozzi, A., Morrone, M. C. & Burr, D. C. (2007). The effect of optokinetic nystagmus on the perceived position of briefly flashed targets,Vision Res, 6 (47), 861-868. PDF

Stimuli flashed briefly around the time of an impending saccade are mislocalized in the direction of the saccade and also compressed towards the saccadic target. Similarly, targets flashed during pursuit eye movements are mislocalized in the direction of pursuit. Here, we investigate the effects of optokinetic nystagmus (OKN) on visual localization. Subjects passively viewed a wide-field drifting grating that elicited strong OKN, comprising the characteristic slow-phase tracking movement interspersed with corrected "saccade-like" fast-phase movements. Subjects reported the apparent position of salient bars flashed briefly at various positions on the screen. In general, bars were misperceived in the direction of the slow-phase tracking movement. Bars flashed around the onset of the fast-phase movements were subject to much less mislocalization, pointing to a competing shift in the direction of the fast-phase, as occurs with saccades. However, as distinct from saccades, there was no evidence for spatial compression around the time of the corrective fast-phase OKN. The results suggest that OKN cause perceptual mislocalizations similar to those of smooth pursuit and saccades, but there are some differences in the nature of the mislocalizations, pointing to different perceptual mechanisms associated with the different types of eye movements.

Perna, A. & Morrone, M. C. (2007). The lowest spatial frequency channel determines brightness perception,Vision Res, 10 (47), 1282-1291. PDF

This study investigates the role played by individual spatial scales in determining the apparent brightness of greyscale patterns. We measured the perceived difference in brightness across an edge in the presence of notch filtering and high-pass filtering for two stimulus configurations, one that elicits the perception of transparency and one that appears opaque. For both stimulus configurations, the apparent brightness of the surfaces delimited by the border decreased monotonically with progressive (ideal) high-pass filtering, with a critical cut-off at 1 c/deg. Using two octave ideal notch filtering, the maximum detrimental effect on apparent brightness was observed at about 1c/deg. Critical frequencies for apparent brightness did not vary with contrast, viewing distance, or surface size, suggesting that apparent brightness is determined by the channel tuned at 1 c/deg. Modelling the data with the local energy model [Morrone, M. C., & Burr, D. C. (1988). Feature detection in human vision: a phase dependent energy model. Proceedings of the Royal Society (London), B235, 221-245] at 1c/deg confirmed the suggestion that this channel mediates apparent brightness for both opaque and transparent borders, with no need for pooling or integration across spatial channels.

Chirimuuta, M., Burr, D. & Morrone, M. C. (2007). The role of perceptual learning on modality-specific visual attentional effects,Vision Res, 1 (47), 60-70. PDF

Morrone et al. [Morrone, M. C., Denti, V., & Spinelli, D. (2002). Color and luminance contrasts attract independent attention. Current Biology, 12, 1134-1137] reported that the detrimental effect on contrast discrimination thresholds of performing a concomitant task is modality specific: performing a secondary luminance task has no effect on colour contrast thresholds, and vice versa. Here we confirm this result with a novel task involving learning of spatial position, and go on to show that it is not specific to the cardinal colour axes: secondary tasks with red-green stimuli impede performance on a blue-yellow task and vice versa. We further show that the attentional effect can be abolished with continued training over 2-4 training days (2-20 training sessions), and that the effect of learning is transferable to new target positions. Given the finding of transference, we discuss the possibility that V4 is a site of plasticity for both stimulus types, and that the separation is due to a luminance-colour separation within this cortical area.

Del Viva, M. M. & Morrone, M. C. (2006). A feature-tracking model simulates the motion direction bias induced by phase congruency,J Vis, 3 (6), 179-195. PDF

Here we report a new motion illusion where the prevailing motion direction is strongly influenced by the relative phase of the harmonic components of the stimulus. The basic stimulus is the sum of three sinusoidal contrast-reversing gratings: the first, the third, and the fifth harmonic of two square wave gratings that drift in opposite direction. The phase of one of the fifth components was kept constant at 180 deg, whereas the phase of the other fifth harmonic was varied over the range 0-150 deg. For each phase value of the fifth harmonic, the motion was strongly biased toward its direction, corresponding to the direction with stronger phase congruency between the three harmonics. The strength of the prevailing motion was assessed by measuring motion direction discrimination thresholds, by varying the contrast of the third and the fifth harmonics plaid pattern. Results show that the contrast of high harmonics had to be increased by more than a factor of 10, to achieve a balance of motion for phase differences greater than 60 deg between the 2 fifth harmonics. We also measured the dependence on the absolute phase of harmonic components and found that it is not an important parameter, excluding the possibility that local luminance cues could be mediating the effect. A feature-tracking model based on previous work is proposed to simulate the data. The model computes local energy function from a pair of space-time separable front stage filters and applies a battery of directional second stage mechanisms. It is able to simulate quantitatively the phase congruency dependence illusion and the insensitivity to overall phase. Other energy models based on directional filters fail to simulate the phase congruency dependency effect.

Baranello, G., Rando, T., Bancale, A., D'Acunto, M. G., Epifanio, R., Frisone, M. F., et al. (2006). Auditory attention at the onset of West syndrome: correlation with EEG patterns and visual function,Brain Dev, 5 (28), 293-299. PDF

At the onset of West syndrome a specific impairment of visual function has been clearly demonstrated, while other aspects of sensorial development, and in particular of the auditory function, have been less studied. The aim of this study was to evaluate auditory function and orienting responses at the onset of West syndrome, and to relate the results with EEG patterns, visual function and neurodevelopmental competence. A prospective multicentric study was performed on 25 successively enrolled infants with West syndrome; all the patients underwent a full clinical assessment, including MRI and video-EEG, visual function and auditory orienting responses (AORs) as well as Griffiths' developmental scales. The whole assessment performed at the onset of spasms (T0) was repeated after two months (T1). AORs resulted significantly impaired both at T0 and T1. At the onset of spasms a highly significant relationship of auditory attention with visual function and neurodevelopmental competence was shown in both cryptogenic and symptomatic forms, but it was no longer present after two months. Our results may suggest a possible pervasive effect of the epileptic disorder on sensory processing, associated to a deficit of neurodevelopment. Although we failed to show a significant correlation between auditory orienting responses and EEG patterns, some evidence seems to support at least partially an influence of the epileptic disorder per se on the genesis of the sensorial impairment. A longer follow up and a larger cohort will be useful for a better clarification of these findings.

Burr, D. & Alais, D. (2006). Combining visual and auditory information,Prog Brain Res, (155), 243-258. PDF

Robust perception requires that information from by our five different senses be combined at some central level to produce a single unified percept of the world. Recent theory and evidence from many laboratories suggests that the combination does not occur in a rigid, hardwired fashion, but follows flexible situation-dependent rules that allow information to be combined with maximal efficiency. In this review we discuss recent evidence from our laboratories investigating how information from auditory and visual modalities is combined. The results support the notion of Bayesian combination. We also examine temporal alignment of auditory and visual signals, and show that perceived simultaneity does not depend solely on neural latencies, but involves active processes that compensate, for example, for the physical delay introduced by the relatively slow speed of sound. Finally, we go on to show that although visual and auditory information is combined to maximize efficiency, attentional resources for the two modalities are largely independent.

Alais, D., Lorenceau, J., Arrighi, R. & Cass, J. (2006). Contour interactions between pairs of Gabors engaged in binocular rivalry reveal a map of the association field,Vision Res, 8-9 (46), 1473-1487. PDF

A psychophysical study was conducted to investigate contour interactions (the 'association field'). Two Gabor patches were presented to one eye, with random-dot patches in corresponding locations of the other eye so as to produce binocular rivalry. Perceptual alternations of the two rivalry processes were monitored continuously by observers and the two time series were cross-correlated. The Gabors were oriented collinearly, obliquely, or orthogonally, and spatial separation was varied. A parallel condition was also included. Correlation between the rivalry processes strongly depended on separation and relative orientation. Correlations between adjacent collinear Gabors was near-perfect and reduced with spatial separation and as relative orientation departed from collinear. Importantly, variations in cross-correlation did not alter the rivalry processes (average dominance duration, and therefore alternation rate, was constant across conditions). Instead, synchronisation of rivalry oscillations accounts for the correlation variations: rivalry alternations were highly synchronised when contour interactions were strong and were poorly synchronised when contour interactions were weak. The level of synchrony between these two stochastic processes, in depending on separation and relative orientation, effectively reveals a map of the association field. These association fields are not greatly affected by contrast, and can be demonstrated between contours that are presented to separate hemispheres.

Bruno, A., Brambati, S. M., Perani, D. & Morrone, M. C. (2006). Development of saccadic suppression in children,J Neurophysiol, 3 (96), 1011-1017. PDF

We measured saccadic suppression in adolescent children and young adults using spatially curtailed low spatial frequency stimuli. For both groups, sensitivity for color-modulated stimuli was unchanged during saccades. Sensitivity for luminance-modulated stimuli was greatly reduced during saccades in both groups but far more for adolescents than for young adults. Adults' suppression was on average a factor of about 3, whereas that for the adolescent group was closer to a factor of 10. The specificity of the suppression to luminance-modulated stimuli excludes generic explanations such as task difficulty and attention. We suggest that the enhanced suppression in adolescents results from the immaturity of the ocular-motor system at that age.

Burr, D. & Morrone, C. (2006). Perception: transient disruptions to neural space-time,Curr Biol, 19 (16), R847-849. PDF

How vision operates efficiently in the face of continuous shifts of gaze remains poorly understood. Recent studies show that saccades cause dramatic, but transient, changes in the spatial and also temporal tuning of cells in many visual areas, which may underly the perceptual compression of space and time, and serve to counteract the effects of the saccades and maintain visual stability.

Arrighi, R., Alais, D. & Burr, D. (2006). Perceptual synchrony of audiovisual streams for natural and artificial motion sequences,J Vis, 3 (6), 260-268. PDF

We investigated the conditions necessary for perceptual simultaneity of visual and auditory stimuli under natural conditions: video sequences of conga drumming at various rhythms. Under most conditions, the auditory stream needs to be delayed for sight and sound to be perceived simultaneously. The size of delay for maximum perceived simultaneity varied inversely with drumming tempo, from about 100 ms at 1 Hz to 30 ms at 4 Hz. Random drumming motion produced similar results, with higher random tempos requiring less delay. Video sequences of disk stimuli moving along a motion profile matched to the drummer produced near-identical results. When the disks oscillated at constant speed rather than following "biological" speed variations, the delays necessary for perceptual synchrony were systematically less. The results are discussed in terms of real-world constraints for perceptual synchrony and possible neural mechanisms.

Burr, D., McKee, S. & Morrone, C. M. (2006). Resolution for spatial segregation and spatial localization by motion signals,Vision Res, 6-7 (46), 932-939. PDF

We investigated two types of spatial resolution for perceiving motion-defined contours: grating acuity, the capacity to discriminate alternating stripes of opposed motion from transparent bi-directional motion; and alignment acuity, the capacity to localize the position of motion-defined edges with respect to stationary markers. For both tasks the stimuli were random noise patterns, low-pass filtered in the spatial dimension parallel to the motion. Both grating and alignment resolution varied systematically with spatial frequency cutoff and speed. Best performance for grating resolution was about 10 c/deg (for unfiltered patterns moving at 1-4 deg/s), corresponding to a stripe resolution of about 3'. Grating resolution corresponds well to estimates of smallest receptive field size of motion units under these conditions, suggesting that opposing signals from units with small receptive fields (probably located in V1) are contrasted efficiently to define edges. Alignment resolution was about 2' at best, under similar conditions. Whereas alignment judgment based on luminance-defined edges is typically 3-10 times better than resolution, alignment based on motion-defined edges is only 1.1-1.5 times better, suggesting motion contours are less effectively encoded than luminance contours.

Alais, D., Morrone, C. & Burr, D. (2006). Separate attentional resources for vision and audition,Proc Biol Sci, 1592 (273), 1339-1345. PDF

Current models of attention, typically claim that vision and audition are limited by a common attentional resource which means that visual performance should be adversely affected by a concurrent auditory task and vice versa. Here, we test this implication by measuring auditory (pitch) and visual (contrast) thresholds in conjunction with cross-modal secondary tasks and find that no such interference occurs. Visual contrast discrimination thresholds were unaffected by a concurrent chord or pitch discrimination, and pitch-discrimination thresholds were virtually unaffected by a concurrent visual search or contrast discrimination task. However, if the dual tasks were presented within the same modality, thresholds were raised by a factor of between two (for visual discrimination) and four (for auditory discrimination). These results suggest that at least for low-level tasks such as discriminations of pitch and contrast, each sensory modality is under separate attentional control, rather than being limited by a supramodal attentional resource. This has implications for current theories of attention as well as for the use of multi-sensory media for efficient informational transmission.

Burr, D. & Ross, J. (2006). The effects of opposite-polarity dipoles on the detection of Glass patterns,Vision Res, 6-7 (46), 1139-1144. PDF

Glass patterns--randomly positioned coherently orientated dipoles--create a strong sensation of oriented spatial structure. On the other hand, coherently oriented dipoles comprising dots of opposite polarity ("anti-Glass" patterns) have no distinct spatial structure and are very hard to distinguish from random noise. Although anti-Glass patterns have no obvious spatial structure themselves, their presence can destroy the structure created by Glass patterns. We measured the strength of this effect for both static and dynamic Glass patterns, and showed that anti-Glass patterns can raise thresholds for Glass patterns by a factor of 2-4, increasing with density. The dependence on density suggests that the interactions occur at a local level. When the Glass and anti-Glass dipoles were confined to alternate strips (in translational and circular Glass patterns), the detrimental effect occurred for stripe widths less than about 1.5 degrees, but had little effect for larger stripe widths, reinforcing the suggestion that the interaction occurred over a limited spatial extent. The extent of spatial interaction was much less than that for spatial summation of these patterns, at least 30 degrees under matched experimental conditions. The results suggest two stages of analysis for Glass patterns, an early stage of limited spatial extent where orientation is extracted, and a later stage that sums these orientation signals.

Burr, D. & Morrone, C. (2006). Time perception: space-time in the brain,Curr Biol, 5 (16), R171-173. PDF

Baldassi, S., Megna, N. & Burr, D. C. (2006). Visual clutter causes high-magnitude errors,PLoS Biol, 3 (4), e56. PDF

Perceptual decisions are often made in cluttered environments, where a target may be confounded with competing "distractor" stimuli. Although many studies and theoretical treatments have highlighted the effect of distractors on performance, it remains unclear how they affect the quality of perceptual decisions. Here we show that perceptual clutter leads not only to an increase in judgment errors, but also to an increase in perceived signal strength and decision confidence on erroneous trials. Observers reported simultaneously the direction and magnitude of the tilt of a target grating presented either alone, or together with vertical distractor stimuli. When presented in isolation, observers perceived isolated targets as only slightly tilted on error trials, and had little confidence in their decision. When the target was embedded in distractors, however, they perceived it to be strongly tilted on error trials, and had high confidence of their (erroneous) decisions. The results are well explained by assuming that the observers' internal representation of stimulus orientation arises from a nonlinear combination of the outputs of independent noise-perturbed front-end detectors. The implication that erroneous perceptual decisions in cluttered environments are made with high confidence has many potential practical consequences, and may be extendable to decision-making in general.