Functional interactions between auditory and visual systems in the human brain: evidence from neurotypical development and temporary sensory deprivation model

Our understanding of the sensory brain functioning has been remarkably influenced by the multisensory research perspective. Traditional models of senses functioning indicated that sensory inputs interact only after unisensory processing has occurred. Conversely, in the last two decades, evidence has accumulated showing that interactions across the senses occur since the earliest stages of sensory processing. In turn, sensory areas which have traditionally been considered sensory specific, and thus constrained to analyze only one type of sensory input, are now viewed as hubs, whose functioning and development can be influenced by the processing of the other senses. In this work, we investigated aspects of the interactions between auditory and visual modalities. In two separate studies, I employed complementary perspectives to characterize (i) on the one hand the activity of primary visual cortex (V1) during auditory processing, and (ii) on the other hand assess the influence of early visual input on the functional development of the auditory pathway. In the first study we took advantage of auditory modeling of natural and synthetically derived sounds and employed functional magnetic resonance imaging (fMRI) to measure cortical activity in the occipital and temporal areas, early stages of visual and acoustic processing, respectively. In the second study we investigated subcortical responses through auditory evoked potential (auditory brainstem responses, ABR) in children with an atypical development of the visual system. Results of the first study suggested that specific sound characteristics are represented in V1 activity. In fact, a low-level acoustic feature – the sound amplitude modulation – was clearly mapped in the striate cortex. This finding supported and expanded current knowledge on the multimodal role of primary visual cortex. In the second study, an influence of early visual experience on the development of subcortical auditory pathway emerged. That is, early visual inputs were found to possibly be a prerequisite for the full development of subcortical auditory responses, ultimately suggesting the presence of a crossmodal sensitive period. These findings strengthen the current view that auditory and visual systems are bounded, with a high degree of specificity, since earliest hierarchical stages of sensory processing, and since earliest developmental phases.