The plasticity of the mammalian brain – that is, its ability to adapt to environmental situations by changing its connectivity – is one of its most outstanding properties, distinguishing it from most other computational devices. This plasticity is perhaps most striking in the sensory systems that provide input to the brain. The plasticity of sensory systems in higher centers of the brain, such as the cerebral cortex, is the basis for its adaptability to the environment. During individual development, neural plasticity is greater than during adulthood, which is necessitated by the growth of the organism and the need of the brain to get programmed. Although sensory plasticity tapers off in adulthood, it does not cease completely. This chapter deals with the behavioral, anatomical, and physiological plasticity in animals and humans that grow up blind. I discuss plasticity in the somatosensory and auditory systems of visually deprived cats, mice, and humans. Evidence for crossmodal plasticity was acquired using single-unit neurophysiology and neuroanatomy in animal models of early blindness and using imaging techniques in humans. The data support a concept of developmental plasticity whereby major sensory processing modules in the cortex are set up without the influence of sensory experience, but the sensory modality that drives them depends on sensory experience.
Expansion of Whisker-Barrel System in Early-Blind Animals
In rodents, the facial vibrissae, or whiskers, provide one of the most important sources of information to the brain. This is underscored by the fact that rodents possess a special representation in their somatosensory cortex known as barrels that can be visualized with various anatomical and histochemical techniques (Van der Loos and Woolsey, 1973). The barrel cortex shows pronounced intramodal plasticity: when one of the whiskers is removed, the corresponding barrel shrinks. However, this plasticity of the whisker-barrel system is apparent even when sensory deprivation is exerted in a different sensory modality, such as the visual: mice that are reared blind from birth with a binocular enucleation develop significantly longer vibrissae and, correspondingly, an expanded barrel field (Rauschecker et al., 1992; Fig. 8.1). This may be interpreted by increased usage of the whiskers, which leads to not only use-dependent expansion of their central representation but also a hypertrophy of the peripheral sense organ itself.