Plasticity: Cortical Reorganization

Cortical Reorganization

Plasticity in the nervous system can also lead to larger-scale changes in the brain’s architecture. In one study, investigators trained monkeys to respond in one way if they heard a certain musical pitch and in another way if they heard a slightly different pitch (Recanzone, Schreiner, & Merzenich, 1993). We know from other evidence that—just as in humans—the monkeys’ projection areas for sounds are organized in maps; different sites on the monkey’s cortex are responsive to different frequencies of sound. After train-ing, though, the map of the monkey’s auditory projection area was reorganized, so that much more cortical area was now devoted to the frequencies emphasized during training.

Can the same plasticity be demonstrated in humans? One research team used neu-roimaging to examine the somatosensory projection areas in a group of highly trained musicians, all of whom played string instruments; a comparison group consisted of nonmusicians (Elbert, Pantev, Wienbruch, Rockstroh, & Taub, 1995). The results showed that in the musicians’ brains, more cortical area was dedicated to the represen-tation of input from the fingers—suggesting that because of their instrumental training, the musicians’ brains had been reorganized to devote more tissue to skills essential for their playing.

A related result comes from a study of London cabdrivers. These drivers need sophis-ticated navigation skills to find their way around London, and they become more and more skillful as they gain experience. This skill, in turn, is reflected directly in their brain structure: Studies show that these cabdrivers have enlarged hippocampi—and the hippocampus, you’ll recall, is a brain structure crucial for navigation. Further, the more years of cab-driving experience an individual had, the greater the degree of hip-pocampal enlargement (E. Maguire et al., 2000).

Even more evidence comes from research with the blind. In one study, investigators used neuroimaging to compare the brain activity in blind and sighted research partici-pants who were exploring a surface with their fingertips (Sadato et al., 1996). The sighted participants showed the expected pattern of increased activity in somato-sensory areas as they felt the target surface. In contrast, during this task the blind participants showed increased activity in the visual cortex. Apparently, for these individ-uals, this brain area had taken on a new job. No longer occupied with visual informa-tion, this area of cortex had shifted to the entirely new task of processing information from the fingertips (also see Kauffman et al., 2002).

Thus it seems that the brain is plastic both at the microscopic level, where it involves changes in how neurons communicate with each other, and at a much grander level. If a person receives a lot of practice in a task, more brain tissue is recruited for the task—presumably because the tissue has been “reassigned” from some other task. Likewise, sensory cortex that was initially sensitive to one modality can apparently be reassigned to an entirely different modality.