Maps of frequency and space in the Mammalian auditory system: Generation and plasticity

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Abstract
A basic organisational feature of the major sensory systems is the existence of topographically organised representations (or “maps”) of various stimulus features in the brain. Some of these maps reflect orderly connections between the receptor epithelium and the brain, whereas others are derived computationally. In the auditory system, frequency maps in cortical and subcortical structures reflect orderly connections, whereas maps of spatial location must be derived computationally. One of the major cues on which behavioural localisation of auditory stimuli depends is provided by interaural intensity differences (IIDs). In one region of the brain, a topographic organisation of IID sensitivity based on different types of binaural interaction has been demonstrated and has been shown to be the basis of a map of auditory space. Topographic maps based on orderly connections with the receptor epithelium have generally been thought not to be modifiable by experience in adult animals. However, the frequency map in auditory cortex of adult animals with unilateral restricted cochlear lesions exhibits plasticity, in that the region deprived of its normal input by the lesion is not silent but is partially or wholly occupied by expanded representations of adjacent frequencies. Analogous forms of plasticity have been described in somatosensory and visual cortex, and plasticity in such maps as a consequence of altered input therefore appears to be a general feature of sensory systems.