Developmental dyslexia affects 5%-10% of the population, resulting in poor spelling and reading skills. While there are well-documented differences in the way dyslexics process low-level visual and auditory stimuli, it is mostly unknown whether there are similar differences in audiovisual multisensory processes. Here, we investigated audiovisual integration using the redundant target effect (RTE) paradigm. Some conditions demonstrating audiovisual integration appear to depend upon magnocellular pathways, and dyslexia has been associated with deficits in this pathway; so, we postulated that developmental dyslexics ("dyslexics" hereafter) would show differences in audiovisual integration compared with controls. Reaction times (RTs) to multisensory stimuli were compared with predictions from Miller's race model. Dyslexics showed difficulty shifting their attention between modalities; but such "sluggish attention shifting" (SAS) appeared only when dyslexics shifted their attention from the visual to the auditory modality. These results suggest that dyslexics distribute their crossmodal attention resources differently from controls, causing different patterns in multisensory responses compared to controls. From this, we propose that dyslexia training programs should take into account the asymmetric shifts of crossmodal attention.

Recent studies have challenged the traditional notion of modality-dedicated cortical systems by showing that audition and touch evoke responses in the same sensory brain regions. While much of this work has focused on somatosensory responses in auditory regions, fewer studies have investigated sound responses and representations in somatosensory regions. In this functional magnetic resonance imaging (fMRI) study, we measured BOLD signal changes in participants performing an auditory frequency discrimination task and characterized activation patterns related to stimulus frequency using both univariate and multivariate analysis approaches. Outside of bilateral temporal lobe regions, we observed robust and frequency-specific responses to auditory stimulation in classically defined somatosensory areas. Moreover, using representational similarity analysis to define the relationships between multi-voxel activation patterns for all sound pairs, we found clear similarity patterns for auditory responses in the parietal lobe that correlated significantly with perceptual similarity judgments. Our results demonstrate that auditory frequency representations can be distributed over brain regions traditionally considered to be dedicated to somatosensation. The broad distribution of auditory and tactile responses over parietal and temporal regions reveals a number of candidate brain areas that could support general temporal frequency processing and mediate the extensive and robust perceptual interactions between audition and touch.

Our ability to process temporal frequency information by touch underlies our capacity to perceive and discriminate surface textures. Auditory signals, which also provide extensive temporal frequency information, can systematically alter the perception of vibrations on the hand. How auditory signals shape tactile processing is unclear; perceptual interactions between contemporaneous sounds and vibrations are consistent with multiple neural mechanisms. Here we used a crossmodal adaptation paradigm, which separated auditory and tactile stimulation in time, to test the hypothesis that tactile frequency perception depends on neural circuits that also process auditory frequency. We reasoned that auditory adaptation effects would transfer to touch only if signals from both senses converge on common representations. We found that auditory adaptation can improve tactile frequency discrimination thresholds. This occurred only when adaptor and test frequencies overlapped. In contrast, auditory adaptation did not influence tactile intensity judgments. Thus auditory adaptation enhances touch in a frequency- and feature-specific manner. A simple network model in which tactile frequency information is decoded from sensory neurons that are susceptible to auditory adaptation recapitulates these behavioral results. Our results imply that the neural circuits supporting tactile frequency perception also process auditory signals. This finding is consistent with the notion of supramodal operators performing canonical operations, like temporal frequency processing, regardless of input modality. Auditory signals can influence the tactile perception of temporal frequency. Multiple neural mechanisms could account for the perceptual interactions between contemporaneous auditory and tactile signals. Using a crossmodal adaptation paradigm, we found that auditory adaptation causes frequency- and feature-specific improvements in tactile perception. This crossmodal transfer of aftereffects between audition and touch implies that tactile frequency perception relies on neural circuits that also process auditory frequency.

Recent studies have proposed that some cross-modal illusions might be expressed in what were previously thought of as sensory-specific brain areas. Therefore, one interesting question is whether auditory-driven visual illusory percepts respond to manipulations of low-level visual attributes (such as luminance or chromatic contrast) in the same way as their nonillusory analogs. Here, we addressed this question using the double flash illusion (DFI), whereby one brief flash can be perceived as two when combined with two beeps presented in rapid succession. Our results showed that the perception of two illusory flashes depended on luminance contrast, just as the temporal resolution for two real flashes did. Specifically we found that the higher the luminance contrast, the stronger the DFI. Such a pattern seems to contradict what would be predicted from a maximum likelihood estimation perspective, and can be explained by considering that low-level visual stimulus attributes similarly modulate the perception of sound-induced visual phenomena and "real" visual percepts. This finding provides psychophysical support for the involvement of sensory-specific brain areas in the expression of the DFI. On the other hand, the addition of chromatic contrast failed to produce a change in the strength of the DFI despite it improved visual sensitivity to real flashes. The null impact of chromaticity on the cross-modal illusion might suggest a weaker interaction of the parvocellular visual pathway with the auditory system for cross-modal illusions.