A Symmetric Approach Elucidates Multisensory Information Integration

Abstract

Recent advances in neuronal multisensory integration suggest that the five senses do not exist in isolation of each other. Perception, cognition and action are integrated at very early levels of central processing, in a densely-coupled system equipped with multisensory interactions occurring at all temporal and spatial stages. In such a novel framework, a concept from the far-flung branch of topology, namely the Borsuk-Ulam theorem, comes into play. The theorem states that when two opposite points on a sphere are projected onto a circumference, they give rise to a single point containing their matching description. Here we show that the theorem applies also to multisensory integration: two environmental stimuli from different sensory modalities display similar features when mapped into cortical neurons. Topological tools not only shed new light on questions concerning the functional architecture of mind and the nature of mental states, but also provide an empirically assessable methodology. We argue that the Borsuk-Ulam theorem is a general principle underlying nervous multisensory integration, resulting in a framework that has the potential to be operationalized.Keywords: multimodal; heteromodal; integration; neuron; brain; Borsuk-Ulam theorem; antipodal "A color is a physical object when we consider its dependence upon its luminous source; regarding, however, its dependence upon the retina, it becomes a psychological object, a sensation. Not the subject, but the direction of our investigation, is different in the two domains" (Mach 1885).Current advances in human neurosciences shed new light on questions concerning the status of the mental and its relation to the physical. Multisensory neurons (we will also term them "heteromodal" or "multimodal") receiving convergent inputs from multiple sensory modalities (independent sources of information called "cues") integrate information from the five different senses [1]. When cues are available, combining them facilitates the detection of salient events and reduces perceptual uncertainty, in order to improve reactions to immediate dangers and to rapidly varying events [2]. Multisensory neurons are able as well to carry out complex brain activities, such as object categorization [3]. Heteromodal integration displays complicated temporal patterns: absent in the superior colliculus of newborn's brain, it arises in the earlier weeks/months of postnatal life [4,5]. As time goes on, neurons develop their capacity to engage in multisensory integration, which determines whether stimuli are to be integrated or treated as independent events. Multisensory integration's development is due to early sensory experience, extensive experience with heteromodal cues and, above all, maturation of cooperative interactions between superior colliculus and cortex [6,7]. The ability to engage in multisensory integration specifically requires cortical influences [8,9]: without the help of cortical activity, neurons become responsive to multiple sensory modalities, but are unable to ...

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