Long Integration Time for Accelerating and Decelerating Visual, Tactile and Visuo-tactile Stimuli, Multisensory Research, 1-2 (26), 53-68.

The human visual system is good at discriminating speed but not acceleration. However, as speed is seldom constant, it is important to be able to extract speed in conditions of acceleration and deceleration. We measured visual, tactile and bimodal speed-matching over a wide range of accelerations and decelerations in a 2IFC procedure. Both visual and tactile stimuli were generated on physical wheels etched with a sinusoidal profile. During different experimental sessions the wheels could be seen, or touched, or both. Comparisons between different unimodal and bimodal matched speeds revealed similar integration times for the two modalities, in both cases around one second, suggesting that it occurs at a relatively high level of processing. Bimodal precision of speed discrimination was better than unimodal discrimination, as predicted by the maximum likelihood model of optimal integration.

Spatiotopic neural representations develop slowly across saccades,Curr Biol, 5 (23), R193-194.

One of the long-standing unsolved mysteries of visual neuroscience is how the world remains apparently stable in the face of continuous movements of eyes, head and body. Many factors seem to contribute to this stability, including rapid updating mechanisms that temporarily remap the visual input to compensate for the impending saccade [1]. However, there is also a growing body of evidence pointing to more long-lasting spatiotopic neural representations, which remain solid in external rather than retinal coordinates [2-6]. In this study, we show that these spatiotopic representations take hundreds of milliseconds to build up robustly.

The New Visual Neuroscience: MIT Press.

The “motion silencing” illusion results from global motion and crowding,J Vis, 5 (13).

Suchow and Alvarez (2011) recently devised a striking illusion, where objects changing in color, luminance, size, or shape appear to stop changing when they move. They refer to the illusion as “motion silencing of awareness to visual change.” Here we present evidence that the illusion results from two perceptual processes: global motion and crowding. We adapted Suchow and Alvarez’s stimulus to three concentric rings of dots, a central ring of “target dots” flanked on either side by similarly moving flanker dots. Subjects had to identify in which of two presentations the target dots were continuously changing (sinusoidally) in size, as distinct from the other interval in which size was constant. The results show: (a) Motion silencing depends on target speed, with a threshold around 0.2 rotations per second (corresponding to about 10 degrees /s linear motion). (b) Silencing depends on both target-flanker spacing and eccentricity, with critical spacing about half eccentricity, consistent with Bouma’s law. (c) The critical spacing was independent of stimulus size, again consistent with Bouma’s law. (d) Critical spacing depended strongly on contrast polarity. All results imply that the “motion silencing” illusion may result from crowding.

A mechanism for detecting coincidence of auditory and visual spatial signals,Multisens Res, 4 (26), 333-345.

Information about the world is captured by our separate senses, and must be integrated to yield a unified representation. This raises the issue of which signals should be integrated and which should remain separate, as inappropriate integration will lead to misrepresentation and distortions. One strong cue suggesting that separate signals arise from a single source is coincidence, in space and in time. We measured increment thresholds for discriminating spatial intervals defined by pairs of simultaneously presented targets, one flash and one auditory sound, for various separations. We report a ‘dipper function’, in which thresholds follow a ‘U-shaped’ curve, with thresholds initially decreasing with spatial interval, and then increasing for larger separations. The presence of a dip in the audiovisual increment-discrimination function is evidence that the auditory and visual signals both input to a common mechanism encoding spatial separation, and a simple filter model with a sigmoidal transduction function simulated the results well. The function of an audiovisual spatial filter may be to detect coincidence, a fundamental cue guiding whether to integrate or segregate.

Contextual effects in interval-duration judgements in vision, audition and touch,Exp Brain Res.

We examined the effect of temporal context on discrimination of intervals marked by auditory, visual and tactile stimuli. Subjects were asked to compare the duration of the interval immediately preceded by an irrelevant “distractor” stimulus with an interval with no distractor. For short interval durations, the presence of the distractor affected greatly the apparent duration of the test stimulus: short distractors caused the test interval to appear shorter and vice versa. For very short reference durations (< o =100 ms), the contextual effects were large, changing perceived duration by up to a factor of two. The effect of distractors reduced steadily for longer reference durations, to zero effect for durations greater than 500 ms. We found similar results for intervals defined by visual flashes, auditory tones and brief finger vibrations, all falling to zero effect at 500 ms. Under appropriate conditions, there were strong cross-modal interactions, particularly from audition to vision. We also measured the Weber fractions for duration discrimination and showed that under the conditions of this experiment, Weber fractions decreased steadily with duration, following a square-root law, similarly for all three modalities. The magnitude of the effect of the distractors on apparent duration correlated well with Weber fraction, showing that when duration discrimination was relatively more precise, the context dependency was less. The results were well fit by a simple Bayesian model combining noisy estimates of duration with the action of a resonance-like mechanism that tended to regularize the sound sequence intervals.

Erratum to: Contextual effects in interval-duration judgements in vision, audition and touch,Exp Brain Res.

Visual sustained attention and numerosity sensitivity correlate with math achievement in children,J Exp Child Psychol, 2 (116), 380-391.

In this study, we investigated in school-age children the relationship among mathematical performance, the perception of numerosity (discrimination and mapping to number line), and sustained visual attention. The results (on 68 children between 8 and 11years of age) show that attention and numerosity perception predict math scores but not reading performance. Even after controlling for several variables, including age, gender, nonverbal IQ, and reading accuracy, attention remained correlated with math skills and numerosity discrimination. These findings support previous reports showing the interrelationship between visual attention and both numerosity perception and math performance. It also suggests that attentional deficits may be implicated in disturbances such as developmental dyscalculia.

Optimal Multimodal Integration in Spatial Localization,J Neurosci 33(35):14259-14268.

Saccadic eye movements facilitate rapid and efficient exploration of visual scenes, but also pose serious challenges to establishing reliable spatial representations. This process presumably depends on extraretinal information about eye position, but it is still unclear whether afferent or efferent signals are implicated and how these signals are combined with the visual input. Using a novel gaze-contingent search paradigm with highly controlled retinal stimulation, we examined the performance of human observers in locating a previously fixated target after a variable number of saccades, a task that generates contrasting predictions for different updating mechanisms.Weshow that while localization accuracy is unaffected by saccades, localization precision deteriorates nonlinearly, revealing a statistically optimal combination of retinal and extraretinal signals. These results provide direct evidence for optimal multimodal integration in the updating of spatial representations and elucidate the contributions of corollary discharge signals and eye proprioception.

Spatial position information accumulates steadily over time, J Neurosci 33(47):18396-18401.

One of the more enduring mysteries of neuroscience is how the visual system constructs robust maps of the world that remain stable in the face of frequent eye-movements. Here we show that encoding the position of objects in external space is a relatively slow process, building up over hundreds of milliseconds. We display targets to which human subjects saccade after a variable preview duration. As they saccade, the target is displaced leftwards or rightwards, and subjects report the displacement direction. When subjects saccade to targets without delay, sensitivity is poor: but if the target is viewed for 300-500 ms before saccading, sensitivity is similar to that during fixation with a strong visual mask to dampen transients. These results suggest that the poor displacement thresholds usually observed in the “saccadic suppression of displacement” paradigm do not reflect the action of special mechanisms conferring saccadic stability, but the fact that the target has had insufficient time to be encoded in memory. Under more natural conditions, trans-saccadic displacement detection is as good as in fixation, when the displacement transients are masked.