Use, disuse, and growth of the brain.

It is well known that across species, the relative size of the cortical area representing a particular sensory surface is proportional to how important that sense is for the animal. Furthermore, we are commonly aware of the observation that the loss of one sense, such as sight, appears to lead to an increase in sensitivity of the remaining senses, although the physiological basis for this is not entirely clear. Now, several studies, including that of Zheng and Purves (11), have suggested that the cortical area devoted to a particular sensory system can be modulated by neuronal activity during development. The fact that use, or disuse, of a sensory organ can lead to significant changes in its area of representation in the developing cortex is intriguing and calls for further investigations aimed at understanding the functional significance and the mechanisms underlying these changes. What remains to be determined is whether enhanced "growth" also means enhanced performance by that sensory system and, if so, whether this is the result of selective changes in neuronal connectivity and/or synaptic efficacy. It is too early to tell, but, whatever the outcome, it is refreshing to consider neuronal growth in the light of enhanced neural activity, in parallel to the results of activity deprivation, to which we are more accustomed.