Basic principles of cortical plasticity & tonotopy

From early development to adulthood, the brain is continuously shaped by the interactions of genetic and environmental factors. In neurobiology, changes in the anatomical and physiological properties of the central nervous system in response to altered sensory input are described as “plastic” changes (Willot, 1996). Neural plasticity allows thus the brain to adapt to continuous changes in our environment. The tonotopy, the fact that frequencies are coded along a frequency axis in the auditory system, can be altered through plastic mechanisms. This may be illustrated by two examples. Early exposure to white noise covering all frequencies disrupts the tonotopic representation of the auditory cortex and degrades the frequency selectivity of cortical neurons in animals (Zhang et al., 2002). In contrast, the cortical area that is activated by piano tones is significantly enlarged in musicians compared to control subjects who have never played instruments (Pantev et al., 1998). This increase I is probably correlated to the age at which the musician started to practice. Plastic properties of the auditory pathway can thus lead to an under-representation or an over-representation of certain frequencies in the cortical tonotopic maps. These alterations are strongly related to the way inputs are modified and are more dramatic when subjects are younger.