Introduction to the special issue on multimodality of early sensory processing: Early visual maps flexibly encode multimodal space

As living organisms, we have the capability to explore our environments through different senses, each making use of specialized organs and return- ing unique information. This is relayed to a set of cortical areas, each of which appears to be specialized for processing information from a single sense — hence the definition of ‘unisensory’ areas. Many models assume that primary unisensory cortices passively reproduce information from each sensory organ; these then project to associative areas, which actively combine multisensory signals with each other and with cognitive stances. By the same token, the textbook view holds that sensory cortices undergo plastic changes only within a limited ‘critical period’; their function and architecture should remain stable and unchangeable thereafter. This model has led to many fundamental dis- coveries on the architecture of the sensory systems (e.g., oriented receptive fields, binocularity, topographic maps, to name just the best known). How- ever, a growing body of evidence calls for a review of this conceptual scheme. Based on single-cell recordings from non-human primates, fMRI in humans, psychophysics, and sensory deprivation studies, early sensory areas are losing their status of fixed readouts of receptor activity; they are turning into func- tional nodes in a network of brain areas that flexibly adapts to the statistics of the input and the behavioral goals. This special issue in Multisensory Research aims to cover three such lines of evidence: suggesting that (1) the flexibility of spatial representations, (2) adult plasticity and (3) multimodality, are not properties of associative areas alone, but may depend on the primary visual cortex V1