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Biomedical subjects

Ladan Shams

Publications and source records attributed to Ladan Shams.

8 recordsLinked to original sources

Sound facilitates visual learning.

Numerous studies show that practice can result in performance improvements on low-level visual perceptual tasks [1-5]. However, such learning is characteristically difficult and slow, requiring many days of training [6-8]. Here, we show that a multisensory audiovisual training procedure facilitates visual learning and results in significantly faster learning than unisensory visual training. We trained one group of subjects with an audiovisual motion-detection task and a second group with a visual motion-detection task, and compared performance on trials containing only visual signals across ten days of training. Whereas observers in both groups showed improvements of visual sensitivity with training, subjects trained with multisensory stimuli showed significantly more learning both within and across training sessions. These benefits of multisensory training are particularly surprising given that the learning of visual motion stimuli is generally thought to be mediated by low-level visual brain areas [6, 9, 10]. Although crossmodal interactions are ubiquitous in human perceptual processing [11-13], the contribution of crossmodal information to perceptual learning has not been studied previously. Our results show that multisensory interactions can be exploited to yield more efficient learning of sensory information and suggest that multisensory training programs would be most effective for the acquisition of new skills.

Acoustic Stimulation↗

Sound-induced flash illusion as an optimal percept.

Recently, it has been shown that visual perception can be radically altered by signals of other modalities. For example, when a single flash is accompanied by multiple auditory beeps, it is often perceived as multiple flashes. This effect is known as the sound-induced flash illusion. In order to investigate the principles underlying this illusion, we developed an ideal observer (derived using Bayes' rule), and compared human judgements with those of the ideal observer for this task. The human observer's performance was highly consistent with that of the ideal observer in all conditions ranging from no interaction, to partial integration, to complete integration, suggesting that the rule used by the nervous system to decide when and how to combine auditory and visual signals is statistically optimal. Our findings show that the sound-induced flash illusion is an epiphenomenon of this general, statistically optimal strategy.

Acoustic Stimulation↗

Touch-induced visual illusion.

Although vision is considered the dominant modality, recent studies demonstrate the influence of other modalities on visual perception. For example, in the sound-induced flash illusion, two auditory stimuli cause one visual flash to be perceived as two. We report an extension of the sound-induced flash illusion to the tactile-visual domain, yielding the touch-induced flash illusion. Observers reported seeing two flashes on the majority of trials when a single flash was presented concurrently with two task-irrelevant brief tactile stimuli. Somatosensory stimulation changed the sensitivity (d') of detecting visual stimuli, which suggests that the observed effect is at least partly due to perceptual interactions. Together with other recent findings, these results challenge the notion that the processing of visual information is independent of activity in other modalities.

Analysis of Variance↗

Early modulation of visual cortex by sound: an MEG study.

Sound can alter visual perception. This has been recently demonstrated by a strong illusion in which a single flash is perceived as multiple flashes when accompanied by multiple brief sounds. While psychophysical findings on this sound-induced flash illusion indicate that the modulations of visual percept by sound occur at a perceptual processing level, it remains unclear at what level of perceptual processing these interactions occur and what mechanisms mediate them. Here we investigated these questions using MEG. We found modulation of activity in occipital and parietal scalp locations, when comparing illusion trials with visual-alone and auditory-alone trials. This modulation occurred as early as 35-65 ms from the onset of the visual stimulus. Activity was also modulated in the occipital and parietal areas as well as anterior areas at a later ( approximately 150 ms post-stimulus) onset. No significant interactions were observed in occipital and parietal areas in trials in which illusion was not perceived. These results indicate that the auditory alteration of visual perception as reflected by the illusion is associated with modulation of activity in visual cortex. The early onset of these modulations suggests that a feed-forward or lateral circuitry is at least partially involved in these interactions.

Acoustic Stimulation↗

Sound-induced illusory flash perception: role of gamma band responses.

In the recently discovered sound-induced illusory flash phenomenon, a single flash accompanied with two auditory beeps is perceived as two flashes in a majority of trials. Here we asked what the neural substrates distinguishing illusion and no-illusion (i.e. perception of single flash) percepts are under identical stimulus configuration. Wavelet based method was used to analyze gamma band (> 30 Hz) responses in the event-related potential (ERP) signals recorded over visual cortical regions. We found: (i) significantly higher oscillatory and induced gamma band responses in illusion than in no-illusion trials, and (ii) significant supra-additive audio-visual interactions only in illusion trials. These results provide a clear neurophysiological correlate to the perception of illusion. Furthermore, the results suggest that auditory stimuli modulate cortical processing of visual stimuli, and the flash illusion (qualitative alteration of visual percept) only takes place when this modulation exceeds some critical threshold for the registration of conscious awareness.

Acoustic Stimulation↗

Acquisition of visual shape primitives.

Shape primitives have long been proposed as components for object models in the visual system, and account for a considerable body of behavioral findings. While a large amount of effort has been devoted to the study of detection of these parts in the scenes, no research has been undertaken simulating the acquisition of these representations. We present a model which suggests how the shape primitives may be learned by experience in a self-organized fashion. This model offers the first successful unsupervised learning of shape primitives which are as complex as object parts and can serve as intermediate representations for various objects. The algorithm uses synthetic gray-level objects, each composed of several parts (primitives or else), and shape primitives emerge as a result of partial matches between several objects. Our algorithm does not use any a priori knowledge about any attributes of the patterns to be learned; and the recurrence of these visual patterns in various objects is the only basis for their emergence as new features.

Algorithms↗

The role of complex cells in object recognition.

Primate's primary visual cortex (V1) is dominated by complex cells. This choice of nature seems puzzling, as complex cells are insensitive to spatial phase--information which is generally believed to be essential for perceptual characterization and recognition of images. Modeling complex cells as Gabor wavelet magnitudes, we have mathematically and empirically examined the information content of their responses. Our results show that in spite of phase insensitivity of individual complex cell responses, population responses contain sufficient information to capture the perceptual essence of images. A complex cell type representation seems to be not only sufficiently discriminating for object identification, but also--due to its inherent ambiguities--robust to changes in background, lighting, and small deformations.

Form Perception↗

Visual illusion induced by sound.

We present the first cross-modal modification of visual perception which involves a phenomenological change in the quality-as opposed to a small, gradual, or quantitative change-of the percept of a non-ambiguous visual stimulus. We report a visual illusion which is induced by sound: when a single flash of light is accompanied by multiple auditory beeps, the single flash is perceived as multiple flashes. We present two experiments as well as several observations which establish that this alteration of the visual percept is due to cross-modal perceptual interactions as opposed to cognitive, attentional, or other origins. The results of the second experiment also reveal that the temporal window of these audio-visual interactions is approximately 100 ms.

Acoustic Stimulation↗