The Co - occurrence of Multisensory Facilitation and Cross - modal Conflict in the 1 Human Brain

28 Perceptual objects often comprise a visual and auditory signature which arrives 29 simultaneously through distinct sensory channels, and cross-modal features are linked by 30 virtue of being attributed to a specific object. Continued exposure to cross-modal events 31 sets up expectations about what a given object most likely "sounds" like, and vice versa, 32 thereby facilitating object detection and recognition. The binding of familiar auditory 33 and visual signatures is referred to as semantic, multisensory integration. While 34 integration of semantically-related cross-modal features is behaviorally advantageous, 35 situations of sensory dominance of one modality at the expense of another, impairs 36 performance. In the present study, magnetoencephalography recordings of semantically37 related cross-modal and unimodal stimuli captured the spatiotemporal patterns underlying 38 multisensory processing at multiple stages. At early stages, 100ms after stimulus onset, 39 posterior parietal brain regions responded preferentially to cross-modal stimuli 40 irrespective of task instructions or the degree of semantic relatedness between the 41 auditory and visual components. As participants were required to classify cross-modal 42 stimuli into semantic categories, activity in superior temporal and posterior cingulate 43 cortices increased between 200 and 400ms. As task instructions changed to incorporate 44 cross-modal conflict, a process whereby auditory and visual components of cross-modal 45 stimuli were compared to estimate their degree of congruence, multisensory processes 46 were captured in parahippocampal, dorsomedial, and orbitofrontal cortices 100 and 47 400ms after stimulus onset. Our results suggest that multisensory facilitation is 48 associated with posterior parietal activity as early as 100ms after stimulus onset. 49 However, as participants are required to evaluate cross-modal stimuli based on their 50 semantic category or their degree of congruence, multisensory processes extend in 51 cingulate, temporal, and prefrontal cortices. 52 . 53 Diaconescu, Alain, McIntosh, 2011 1