More than just the sum of its parts: High-density electrical mapping of the inverse effectiveness principle in humans

Daniel Senkowski, The Cognitive Neurophysiology Laboratory, Nathan S. Kline Institute for Psychiatric Research

Abstract
The well-known principle of inverse effectiveness (IE) posits that the amplitude of a multisensory integration effect will be inversely related to the strength of the unisensory responses. Evidence for IE comes primarily from animal studies, while IE in humans has not been extensively investigated yet. Here we used high-density electrical mapping to examine multisensory audio-visual (AV) integration effects as a function of stimulus intensity by changing the contrast and the sound level of basic stimuli (gratings and tones). A continuous stream of unisensory auditory (A), unisensory visual (V) and multisensory AV of low, middle, and high stimulus intensity was centrally presented while subjects were instructed to make a speeded response to any stimulus presentation. Multisensory integration effects were tested by the additive model where ERPs to multisensory stimuli are directly compared with the linear summation of the ERPs to the respective unisensory stimuli. As predicted by the principle of IE, early (<100 ms) super-additive (AV>A+V) integration effects were found for the low and middle stimulus intensities. In contrast, multisensory integration effects for the high stimulus intensity level showed a sub-additive (AV

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