Chapter 15 - Crossmodal plasticity in sensory loss
Introduction
While most humans can rely on several sensory systems to appropriately interact with the environment, some individuals are born without one or more senses while others may lose one or more senses during their lifetime. Still, persons with sensory loss are often able to live independently and can achieve an impressive degree of accomplishments. In fact, there is a plethora of reports (though often anecdotic) of persons with a sensory loss demonstrating extraordinary abilities with one or several of their remaining senses, with the large number of successful blind musicians being the most prominent example. Going back several decades, Diderot, in his “Lettre sur les aveugles” (Diderot, 1749), reported the famous case of a blind mathematician who could recognize fake from real money coins just by touching them. Similarly, William James explained blind individuals’ remarkable ability to navigate through their environment without colliding with obstacles as resulting from a form of “facial perception” (James, 1890). At first glance, such performance may seem somewhat “supranormal.” However, over the past decades, we have acquired extensive knowledge on compensatory and adaptive changes in primarily unaffected senses occurring after sensory loss and have a better understanding as to how and why they occur.
The substantial literature on such compensatory mechanisms that are observed in the blind has often attributed these enhancements to some form of “crossmodal plasticity.” Crossmodal plasticity generally refers to the adaptive reorganization of neurons to integrate the function of a new sensory modality following the loss of another. In fact, such crossmodal plasticity appears to at least partly explain many extraordinary abilities observed in persons with sensory loss.
In the following sections, we provide an overview of crossmodal plastic changes that follow sensory loss. We specifically focus on three major topics, that is, blindness, deafness, and loss of chemical senses and how these states affect the other sensory systems.
Section snippets
Behavioral reorganization in blindness
It has long been debated whether blind individuals have perceptual advantages or disadvantages in processing information received via the intact modalities. The fundamental question has been whether the lack of vision disrupts the proper development of nonvisual skills or if, in contrast, blindness enables above-normal performance in the preserved modalities. Even if several studies support the notion that vision may be required to adequately calibrate other sensory modalities (Axelrod, 1959,
Deafness
The previous section provided evidence as to why the study of blind individuals constitutes an excellent model of the adaptability of the human brain, and how its plastic properties can in turn influence behavior and often improve sensory and cognitive abilities in these individuals. While crossmodal plasticity has been less extensively studied in the deaf, with the advent of small and efficient cochlear implants, it will become more and more important to understand crossmodal plasticity in
Anosmia, ageusia, loss of trigeminal chemosensation
The chemical senses, that is, smell, taste, and the chemosensory trigeminal system, have obtained considerably less attention when compared to vision or audition. As opposed to physical senses, such as vision, audition, and touch, they allow us to experience our chemical environment via the interaction of substances with sensory organs, mostly, but not exclusively (Lindemann, 1996), via ligand–receptor interactions (Alimohammadi and Silver, 2000, Buck and Axel, 1991). Together, the three
Conclusion
Loss of a sensory system has vast consequences for the affected person and his interactions with environment. Here, we have outlined how sensory loss leads to changes in primarily unaffected sensory systems. This crossmodal plasticity shows in a fascinating way how the brain copes with sensory deprivation. Only the proper understanding of the mechanisms of crossmodal plasticity will allow us to develop tools to help persons with sensory loss to better experience the world with the unaffected
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