Cross-modal plasticity for the spatial processing of sounds in visually deprived subjects.

Until only a few decades ago, researchers still considered sensory cortices to be fixed or "hardwired," with specific cortical regions solely dedicated to the processing of selective sensory inputs. But recent evidences have shown that the brain can rewire itself, showing an impressive range of cross-modal plasticity. Visual deprivation is one of the rare human models that allow us to explore the role of experience-dependent plasticity of a sensory cortex deprived of its natural inputs. The objective of this paper is to describe recent results regarding the spatial processing of sounds in blind subjects. These studies suggest that blind individuals may demonstrate exceptional abilities in auditory spatial processing and that such enhanced performances may be intrinsically linked to the recruitment of occipital areas deprived of their normal visual inputs. Such results highlight the brain's remarkable ability to rewire its components to compensate for the challenging neurological condition that is visual deprivation. Moreover, we shall discuss that such cross-modal recruitment may, to some extent, follow organizational principles similar to the functional topography of the region observed in the sighted. Even if such recruitment is especially present in individuals having lost their sight in early infancy, occipital regions also show impressive plastic properties when vision is lost at a later age. This observation will be related to recent results demonstrating that occipital regions play a more important role than previously expected in the spatial processing of sounds, even in sighted subjects. Putative physiological mechanisms underlying such cross-modal recruitment will then be discussed. All these results have important implications for understanding the role of visual experience in shaping the development of occipital regions and may guide the implementation of rehabilitative methods such as sensory substitution or neural implants.