Plasticity And Reorganization

In the past several years, much research has revealed that the functional characteristics of the auditory cortex are not necessarily static but instead may be influenced by environmental experience. It has been shown that many properties of cortical neurons may change as a function of such experience, and that reorganization may occur when there is a change in the normal sensory input.

The most dramatic changes in auditory cortex organization are evident when there is loss of a sensory modality, such as occurs in blind or deaf individuals. In experimental animals, it has been shown that early loss of vision entails an increase in auditory-sensitive neurons in some cortical areas. Also, some of these neurons respond with a higher degree of accuracy to sounds in specific spatial positions than do similar neurons in normal animals. This latter observation is most likely responsible for the behavioral advantage that is sometimes seen in blind humans and other species for navigating in space with the use of auditory cues.

Reorganization of auditory cortex may also occur when it is deprived of its normal afferent input due to cochlear or other damage to the periphery of the auditory system. For example, if the afferent input from a restricted area of the basilar membrane is disrupted, the tonotopic organization of the cortex is also altered, with frequencies corresponding to the damaged region diminishing, while adjacent tonal regions increase their representation. Damage to one ear also changes the response properties of neurons that are normally sensitive to binaural cues.

In cases of complete deafness, on the other hand, there is evidence from both human and animal studies that auditory cortical regions may respond to visual cues, although the nature ofthe processes involved has not been worked out. However, it has been shown in some animal species that when optic nerve inputs are rerouted to the medial geniculate nucleus during early phases of development, auditory cortical areas begin to show some of the structural and functional characteristics of visual cortex, implying that these features are driven by the nature of the stimulation that is received rather than by epigenetic factors exclusively.

Other evidence of plasticity in the auditory cortex derives from studies examining changes associated with learning and conditioning paradigms. For example, a tone stimulus results in greater cortical activation when it has been conditioned to precede an air puff to the eye than when it is presented in isolation. This increased activity disappears once the tone is no longer paired with the air puff and it is specific to the frequency used for conditioning. Such findings indicate that auditory cortical activity reflects not only the characteristics of the stimulus but also its associative value and hence the importance of the stimulus in a given situation.

In monkeys, cortical tonotopic organization may be influenced by training the animal to respond to a particular frequency, which induces an increase in the representation of that frequency in the tonotopic map at the expense of adjacent regions. The degree of expansion correlates with behavioral performance. These findings indicate that far from being fixed, neural response properties are dynamic and subject to considerable influences based on environmental input. These types of modifications are likely to be important for speech perception as well; infants begin to respond differentially to the phonemes of the language around them by 6 months of age, which is likely related to learning-induced changes in auditory cortical processing. Changes to cortical organization as a consequence of experience have also been noted in studies of musically trained individuals, who show a higher amplitude of cortical magnetic response to piano tones compared to pure tones, whereas musically untrained subjects do not. This type of finding tends to be strongest in individuals whose training started early in life, indicating that plasticity is greatest during development, although it may still occur in the adult organism.

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