Exploring competition within uni- and multi-sensory perception using fMRI in awake monkeys – CompeSense

Recent years have brought keen awareness that exploring perception solely through isolated unisensory processes provides a tremendously impoverished view of both brain and behavior. Multimodal interactions are ubiquitous in human perception, and evidence is currently accumulating that cross-modal interplay occur in many brain areas, even in areas underlying early processing stages and previously viewed as sensory-specific. Multisensory integration often yields behavioral benefits such as more accurate or faster responses. Hence, it is generally thought to aid perception by creating a single unitary percept. This view is supported by human brain imaging studies as well as by monkey recording studies showing that multisensory coding occurs at the single neuron level. However, multisensory interplay can also lead to competition among, rather than integration of, perceptions, with deleterious behavioral consequences in the form, for example, of sensory dominance of one modality at the expense of another. The present project focuses on this less studied aspect of perception. The aim is to identify the areas of the primate brain related to unimodal (visual or tactile) and crossmodal (visuo-tactile) sensory competition and to determine their specific function. This issue will be addressed using functional Magnetic Resonance Imaging (fMRI) in the awake monkey. This technique is ideally suited to bridge the gap between fMRI results in humans and neurophysiological data in monkeys. It will enable us, in addition, to perform selective reversible lesions of fMRI-identified foci of activation that will establish causal links between specific frontal or parietal cortical areas and competitive phenomena. The paradigm designed for monkeys is inspired from our own human studies showing that the clinical phenomenon called extinction, which constitutes a striking pathological example of how stimuli can compete for attention, does possess a physiological counterpart in normal subjects. Three versions of the same task will be used to assess unimodal and crossmodal extinction-like effect. In all versions, monkeys will lie in a sphinx position in a horizontal bore magnet while facing a visual display comprising a central fixation dot and three response squares. For unimodal visual 'extinction', a stimulus will be presented either on the right side of the central dot, or on the left side, or both. The animal will signal detection by making a saccade to the right, left, or central response squares, respectively. To test unimodal, tactile sensory competition, the procedure will be identical except that left, right or double tactile stimulation of the hand will be delivered, rather than visual stimuli. Finally, cross-modal 'extinction' will be tested by presenting double visual-tactile stimulation versus single, either tactile or visual stimulus. Beyond increasing our knowledge of the neural bases of multisensory perception by focusing on competitive, rather than integrative phenomena, the findings may prove useful for rehabilitation of disabilities such as neglect and extinction, learning disabilities, or speech disorders.