**1. Introduction**

The brain's ability to generate a rich and unambiguous representation of the world requires the multimodal integration of sensory signals often co-occurring in time and space. A crucial issue is how the brain integrates the separate elements of an object perceived through individual sensory channels (vision, audition, touch, etc.) in order not only to improve detection and discrimination and evaluate crossmodal congruency, but also to form a unified percept. Over the last decades, a growing number of studies have challenged the traditional view of the sensory neocortex as a parcellation of highly specialized primary areas, each being exclusively dedicated to the integration and processing of a unique sensory modality. An ancillary conception is that signals conveyed through unisensory streams mainly converge and interact in higher-order association regions of the temporal and parietal cortex in which multisensory integration culminates. Accordingly, these cortical regions tuned to integrate increasingly complex sensory signals send divergent projections back to early sensory areas to exert modulatory feedback on their constituent neurons. However, this hierarchical model of multisensory integration has been reappraised in view of accumulating evidence over recent decades that primary sensory cortical areas are anatomically and functionally interconnected. There is increasing awareness that multisensory integration starts in lower sensory areas, presumably via thalamo-cortical and direct cortico-cortical connections [1–3] (macaque; ferret) ([4, 5], for reviews). Hence, it is of primary interest to unravel how heteromodal inputs interplaying with the dominant modality in primary sensory areas may contribute to improving perception. In addition, a major issue is to determine how crossmodal plasticity subserves functional compensation and behavioral recovery after the loss or impairment of a sensory organ, or following cortical damage. This review chapter has a double focus, firstly on the interplay between primary sensory cortices in normal condition, and secondly on crossmodal plasticity operating in primary and higher-order cortical areas following sensory loss.
