Inverse effectiveness and the superadditivity of multisensory interactions in auditory association areas of the awake macaque monkey.

Monica N O' Connell, Cognitive Neuroscience and Schizophrenia Program Nathan S. Kline Institute for Psychiatric Research

Abstract
Prior anatomical and physiological studies have defined the posterior auditory cortices as sites of multisensory convergence and interaction. This study examined the effects of stimulus intensity on unimodal responses and multisensory interactions in caudomedial (CM) auditory cortex and the adjacent retroinsular (RI) area. We sampled laminar field potential and multiunit activity profiles with linear array multielectrodes from both areas in awake monkeys. We focused on the timing, laminar profile and quality (super-or sub-additive) of bimodal interactions. Auditory (A) stimuli consisted of binaural 60 dB clicks. Somatosensory (S) stimuli consisted of mild electrical pulses applied bilaterally to the median nerve at the wrist; recordings concentrated on the hand representations in CM and RI. We compared bimodal A-S response with the sum of the A and S unimodal responses using t-tests to determine the timing and laminar distribution of super- or sub-additive multisensory interactions. Super-additive interactions in CM were biased toward the super and infragranular layers and occurred in the 20-80ms time range post-stimulus. In RI super-additivity mainly occurred in the same time frame and throughout all the cortical layers. Manipulation of stimulus intensity revealed increase in super-additivity near threshold (“Inverse Effectiveness”) in most of CM but not RI.

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