Neocortical Plasticity

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[A fundamental question in philosophy, psychology, and neuroscience concerns the development of our perceptions of the world. Are our perceptions completely determined by the intrinsic brain circuitry and its development, independent of experience (nativism), or are our perceptions and the underlying neural circuitry determined by experience (empiricism)? A truly major contribution of neuroscience in the twentieth century was the resolution of this issue, based largely on study of the development of the mammalian visual system. As might be expected the truth lies somewhere in between.

This extraordinary story is told in the third entry in this series, on theD evelopment of theV isualS ystem. In brief, the basic wiring diagram of the mammalian visual system is specified by genetic and developmental factors. However, normal visual stimulation (i.e., experience) is necessary for the system to develop normally. Thus, occluding one eye in developing mammals (most work has been done on cats and monkeys) at a particular critical period markedly alters the normal wiring diagram in the primary visual system, resulting in markedly impaired vision. Patterned visual stimulation must be temporally synchronized (simultaneous) between the two eyes and must also be spatially synchronous for normal binocular vision to develop. This synchronized pattern leads to changes in the strength of the connections between active neurons, a phenomenon called synaptic plasticity.

In addition to the extraordinary degree of plasticity seen in the development of the visual system, theA dultV isualC ortexis also capable of a remarkable degree of adaptation and reorganization. Thus, infusing neuromodulatory and growth factors in the visual cortex of adult mammals can reinstate the plasticity in ocular dominance characteristics. Retinal lesions result in a wide range of biochemical changes in the deafferented visual cortex. Perhaps more important, the adult visual cortex exhibits marked synaptic plasticity: processes of LONG-TERM POTENTIATION (LTP) and LONG-TERM DEPRESSION (LTD). The McColloch effect is an example of perceptual plasticity that persists for up to twenty-four hours; after exposure to colored gratings, black and white gratings appear to be limited in complementary colors.

Another system that exhibits dramatic neocortical plasticity is theS omatosensoryC ortex, which consists of a number of processing areas with representations or "maps"of the body surface. Experience can alter the organization of these maps dramatically. Thus, amputation of digits or limbs in monkeys and humans results in marked changes in somatosensory cortex such that the maps of the adjacent intact body parts expand. Similarly, particular types of training (e.g., in tactile discriminations or motor skills) can result in expansion of the relevant regions of somatosensory cortex in both monkeys and humans. These experience-dependent alterations in somatosensory cortex appear to involve alterations in both the cortex itself and the relays of sensory information in the brain stem and thalamus.

TheM otorC ortexalso exhibits marked plasticity. Training in particular motor skills can alter to some degree the organization of motor representation in the motor area of the neocortex. However extensive repetition of a particular movement does not necessarily result in expansion of representation of the movements in the motor cortex. There is evidence that processes of LTP and LTD are involved in motor learning. The cerebellum is also much involved in the learning of motor skills.]