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Head movements modulate visual responsiveness in the absence of gaze shifts,Neuroreport, 8 (19), 831-834.

Head movements modulate visual responsiveness in the absence of gaze shifts,Neuroreport, 8 (19), 831-834.

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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.

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A visual sense of number,Curr Biol, 6 (18), 425-428.

A visual sense of number,Curr Biol, 6 (18), 425-428.

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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].

Anti-Glass patterns and real motion perception: same or different mechanisms?,J Vis, 2 (8), 1 1-15.

Anti-Glass patterns and real motion perception: same or different mechanisms?,J Vis, 2 (8), 1 1-15. 

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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.

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BOLD response to spatial phase congruency in human brain,J Vis, 10 (8), 15 11-15.

BOLD response to spatial phase congruency in human brain,J Vis, 10 (8), 15 11-15.

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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.

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Inversion of perceived direction of motion caused by spatial undersampling in two children with periventricular leukomalacia,J Cogn Neurosci, 6 (20), 1094-1106.

Inversion of perceived direction of motion caused by spatial undersampling in two children with periventricular leukomalacia,J Cogn Neurosci, 6 (20), 1094-1106. 

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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.

Neurodevelopmental evolution of West syndrome: a 2-year prospective study,Eur J Paediatr Neurol, 5 (12), 387-397.

Neurodevelopmental evolution of West syndrome: a 2-year prospective study,Eur J Paediatr Neurol, 5 (12), 387-397.

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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.

Non-linear integration of crowded orientation signals,Vision Res, 22 (48), 2352-2358.

Non-linear integration of crowded orientation signals,Vision Res, 22 (48), 2352-2358.

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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.

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The assessment of visual acuity in children with periventricular damage: a comparison of behavioural and electrophysiological techniques,Vision Res, 10 (48), 1233-1241.

The assessment of visual acuity in children with periventricular damage: a comparison of behavioural and electrophysiological techniques,Vision Res, 10 (48), 1233-1241. 

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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.

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The knowing visual self,Trends Cogn Sci, 10 (12), 363-364.

The knowing visual self,Trends Cogn Sci, 10 (12), 363-364.

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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.

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Young children do not integrate visual and haptic form information,Curr Biol, 9 (18), 694-698.

Young children do not integrate visual and haptic form information,Curr Biol, 9 (18), 694-698.

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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.