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Underestimation of perceived number at the time of saccades,Vision Res, 1 (51), 34-42.

Underestimation of perceived number at the time of saccades,Vision Res, 1 (51), 34-42. 

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Saccadic eye movements produce transient distortions in both space and time. Mounting evidence suggests that space and time perception are linked, and associated with the perception of another important perceptual attribute, numerosity. Here we investigate the effect of saccades on the perceived numerosity of briefly presented arrays of visual elements. We report a systematic underestimation of numerosity for stimuli flashed just before or during saccades, of about 35% of the reference numerosity. The bias is observed only for relatively large arrays of visual elements, in line with the notion that a distinct perceptual mechanism is involved with enumeration of small numerosities in the ‘subitizing’ range. This study provides further evidence for the notion that space, time and number share common neural representations, all affected by saccades.

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Spatiotemporal profile of peri-saccadic contrast sensitivity,J Vis, 14 (11).

Spatiotemporal profile of peri-saccadic contrast sensitivity,J Vis, 14 (11).

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Sensitivity to luminance contrast is reduced just before and during saccades (saccadic suppression), whereas sensitivity to color contrast is unimpaired peri-saccadically and enhanced post-saccadically. The exact spatiotemporal map of these perceptual effects is as yet unknown. Here, we measured detection thresholds for briefly flashed Gaussian blobs modulated in either luminance or chromatic contrast, displayed at a range of eccentricities. Sensitivity to luminance contrast was reduced peri-saccadically by a scaling factor, which was almost constant across retinal space. Saccadic suppression followed a similar time course across all tested eccentricities and was maximal shortly after the saccade onset. Sensitivity to chromatic contrast was enhanced post-saccadically at all tested locations. The enhancement was not specifically linked to the execution of saccades, as it was also observed following a displacement of retinal images comparable to that caused by a saccade. We conclude that luminance and chromatic contrast sensitivities are subject to distinct modulations at the time of saccades, resulting from independent neural processes.

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Brief periods of monocular deprivation disrupt ocular balance in human adult visual cortex, Curr Biol. 2011 Jul 26;21(14):R538-9.

Brief periods of monocular deprivation disrupt ocular balance in human adult visual cortex, Curr Biol. 2011 Jul 26;21(14):R538-9.

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Neuroplasticity is a fundamental property of the developing mammalian visual system, with residual potential in adult human cortex [1]. A short period of abnormal visual experience (such as occlusion of one eye) before closure of the critical period has dramatic and permanent neural consequences, reshaping visual cortical organization in favour of the non-deprived eye [2,3]. We used binocular rivalry [4] – a sensitive probe of neural competition – to demonstrate that adult human visual cortex retains a surprisingly high degree of neural plasticity, with important perceptual consequences. We report that 150 minutes of monocular deprivation strongly affects the dynamics of binocular rivalry, unexpectedly causing the deprived eye to prevail in conscious perception twice as much as the non-deprived eye, with significant effects for up to 90 minutes. Apparent contrast of stimuli presented to the deprived eye was also increased, suggesting that the deprivation acts by up-regulation of cortical gain-control mechanisms of the deprived eye. The results suggest that adult visual cortex retains a good deal of plasticity that could be important in reaction to sensory loss.

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Spatiotopic selectivity of adaptation-based compression of event duration, J Vis, 2 (11), 21; author reply 21a.

Spatiotopic selectivity of adaptation-based compression of event duration, J Vis, 2 (11), 21; author reply 21a.

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A. Bruno, I. Ayhan, and A. Johnston (2010) have recently challenged our report of spatiotopic selectivity for adaptation of event time (D. Burr, A. Tozzi, & M. C. Morrone, 2007) and also our claim that retinotopic adaptation of event time depends on perceived speed. To assist the reader judge this issue, we present here a mass of data accumulated in our laboratories over the last few years, all confirming our original conclusions. We also point out that where Bruno et al. made experimental measurements (rather than relying on theoretical reasoning), they too find clearly significant spatiotopically tuned adaptation-based compression of event time but of lower magnitude to ours. We speculate on the reasons for the differences in magnitude

Acoustic cues to visual detection: a classification image study,J Vis, 6 (11)

Acoustic cues to visual detection: a classification image study,J Vis, 6 (11)

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A non-informative sound is known to improve contrast detection thresholds for a synchronous visual target (M. Lippert, N. K. Logothetis, & C. Kayser, 2007). We investigated the spatio-temporal characteristics of the mechanisms underlying this crossmodal effect by using a classification image paradigm specifically suited to investigate perceptual templates across both space and time (P. Neri & D. J. Heeger, 2002). A bright bar was embedded in 2D (space-time) dynamic noise and observers were asked to detect its presence in both unimodal (only visual) and bimodal (audio-visual) conditions. Classification image analysis was performed and the 1st and 2nd order kernels were derived. Our results show that the cross-modal facilitation of detection consists in a reduction of activity of the early mechanisms elicited by the onset of the stimulation and not directly involved in the identification of the target. In fact, the sound sharpens the 2nd order kernels (involved in target detection) by suppressing the activation preceding the target, whereas it does not influence the 1st order kernels. These data suggest that the sound affects some non-linear process involved with the detection of a visual stimulus by, decreasing the activity of contrast energy filters temporally uncorrelated with the target, hence reducing temporal uncertainty.

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Adaptation Affects Both High and Low (Subitized) Numbers Under Conditions of High Attentional Load,Seeing and Perceiving, (24), 141-150.

Adaptation Affects Both High and Low (Subitized) Numbers Under Conditions of High Attentional Load,Seeing and Perceiving, (24), 141-150.

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It has recently been reported that, like most sensory systems, numerosity is subject to adaptation. However, the effect seemed to be limited to numerosity estimation outside the subitizing range. In this study we show that low numbers, clearly in the subitizing range, are adaptable under conditions of high attentional load. These results support the idea that numerosity is detected by a perceptual mechanism that operates over the entire range of numbers, supplemented by an attention-based system for small numbers (subitizing).

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Applying the Helmholtz illusion to fashion: horizontal stripes won’t make you look fatter,Iperception, 1 (2), 69-76.

Applying the Helmholtz illusion to fashion: horizontal stripes won’t make you look fatter,Iperception, 1 (2), 69-76.

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A square composed of horizontal lines appears taller and narrower than an identical square made up of vertical lines. Reporting this illusion, Hermann von Helmholtz noted that such illusions, in which filled space seems to be larger than unfilled space, were common in everyday life, adding the observation that ladies’ frocks with horizontal stripes make the figure look taller. As this assertion runs counter to modern popular belief, we have investigated whether vertical or horizontal stripes on clothing should make the wearer appear taller or fatter. We find that a rectangle of vertical stripes needs to be extended by 7.1% vertically to match the height of a square of horizontal stripes and that a rectangle of horizontal stripes must be made 4.5% wider than a square of vertical stripes to match its perceived width. This illusion holds when the horizontal or vertical lines are on the dress of a line drawing of a woman. We have examined the claim that these effects apply only for 2-dimensional figures in an experiment with 3-D cylinders and find no support for the notion that horizontal lines would be ‘fattening’ on clothes. Significantly, the illusion persists when the horizontal or vertical lines are on pictures of a real half-body mannequin viewed stereoscopically. All the evidence supports Helmholtz’s original assertion.

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Cross-Sensory Facilitation Reveals Neural Interactions between Visual and Tactile Motion in Humans,Front Psychol, (2), 55.

Cross-Sensory Facilitation Reveals Neural Interactions between Visual and Tactile Motion in Humans,Front Psychol, (2), 55.

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Many recent studies show that the human brain integrates information across the different senses and that stimuli of one sensory modality can enhance the perception of other modalities. Here we study the processes that mediate cross-modal facilitation and summation between visual and tactile motion. We find that while summation produced a generic, non-specific improvement of thresholds, probably reflecting higher-order interaction of decision signals, facilitation reveals a strong, direction-specific interaction, which we believe reflects sensory interactions. We measured visual and tactile velocity discrimination thresholds over a wide range of base velocities and conditions. Thresholds for both visual and tactile stimuli showed the characteristic “dipper function,” with the minimum thresholds occurring at a given “pedestal speed.” When visual and tactile coherent stimuli were combined (summation condition) the thresholds for these multisensory stimuli also showed a “dipper function” with the minimum thresholds occurring in a similar range to that for unisensory signals. However, the improvement of multisensory thresholds was weak and not directionally specific, well predicted by the maximum-likelihood estimation model (agreeing with previous research). A different technique (facilitation) did, however, reveal direction-specific enhancement. Adding a non-informative “pedestal” motion stimulus in one sensory modality (vision or touch) selectively lowered thresholds in the other, by the same amount as pedestals in the same modality. Facilitation did not occur for neutral stimuli like sounds (that would also have reduced temporal uncertainty), nor for motion in opposite direction, even in blocked trials where the subjects knew that the motion was in the opposite direction showing that the facilitation was not under subject control. Cross-sensory facilitation is strong evidence for functionally relevant cross-sensory integration at early levels of sensory processing.

Eye position effects in oculomotor plasticity and visual localization,J Neurosci, 20 (31), 7341-7348.

Eye position effects in oculomotor plasticity and visual localization,J Neurosci, 20 (31), 7341-7348.

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For visual localization to remain accurate across changes of gaze, a signal representing the position of the eye in the orbita is needed to code spatial locations in a reference frame that is independent of retinal displacements. Here we report evidence that the localization of visual objects in space is coded in an extraretinal reference frame. In human subjects, we used outward saccadic adaptation, which can be induced artificially by a systematic displacement of the saccade target. This form of oculomotor plasticity is accompanied by changes in spatial perception, thus highlighting the relevance of saccade metrics for visual localization. We tested the reference frame of outward adaptation for reactive and scanning saccades and visual localization. For scanning saccades, adaptation magnitude was drastically reduced at positions distant from the adapted eye position. Changes in visual localization showed a very similar modulation of eye position. These results suggest that scanning saccade adaptation is encoded in a nonretinotopic reference frame. Eye position effects for reactive saccade adaptation were smaller, and the induced mislocalization did not vary significantly between eye positions. The different modulation of reactive and scanning saccade adaptation supports the idea that oculomotor plasticity can occur at multiple sites in the brain. The findings are also consistent with previous evidence for a stronger influence of scanning saccade adaptation on the visual localization of objects in space.

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Greater Sparing of Visual Search Abilities in Children After Congenital Rather Than Acquired Focal Brain Damage,Neurorehabil Neural Repair.

Greater Sparing of Visual Search Abilities in Children After Congenital Rather Than Acquired Focal Brain Damage,Neurorehabil Neural Repair.

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BACKGROUND: Visual search refers to the capacity of an individual to find a target among simultaneously presented distracters and is based on visual abilities such as a fast visual processing and an accurate control of ballistic eye movements (saccades) that guide the fovea to the target location. OBJECTIVE: In adults, visual field defects caused by brain damage are often associated with visual search disorders; in children, little is known about the effects of early brain lesions on visual search abilities. METHODS: To test the presence of visual search defects and to investigate the role of cortical plasticity after early brain lesions, 29 children with congenital or acquired cerebral lesions, with and without visual field defects, underwent a visual search test battery. RESULTS: The children with acquired lesions and visual field defects had longer reaction times (RTs) in the contralesional visual field compared with the ipsilesional, whereas those with congenital lesions and visual field defects did not have differences in RTs between the contralateral and ipsilateral visual fields and had a visual search pattern similar to children without a visual field defect. CONCLUSIONS: These findings support the hypothesis of more effective mechanisms of functional compensation and reorganization of the visual system in children with very early brain lesions, as opposed to those with later damage.