Injury-induced reorganization of frequency maps in adult auditory cortex: the role of unmasking of normally-inhibited inputs.

Restricted cochlear lesions in adult animals, causing partial deafness, result in a reorganization of primary auditory cortex (AI) such that the region deprived of its normal input by the lesion is occupied by an expanded representation of peri-lesion cochlear regions, and hence of peri-lesion frequencies. One possible mechanism underlying the change in frequency responsiveness involved in such reorganization is that inputs to the cortical neurons at frequencies at and near their "new" post-lesion characteristic frequencies (CFs) are normally present but suppressed by inhibition, and are "unmasked" by the effects of the lesion. Evidence in support of this explanation is provided by two-tone forward-masking experiments which reveal that many AI neurons receive surround inhibitory input. When input to such neurons at their CF is reduced by an intense temporary-threshold-shift (TTS)-inducing stimulus, the response areas of some neurons expand into the region of their inhibitory surrounds, the effect that would be expected if unmasking were involved in cortical reorganization. In other neurons, however, response areas contracted after the TTS-inducing stimulation. Although unmasking of normally-inhibited inputs is likely to contribute to auditory cortical reorganization, the immediate unmasking that is seen in visual and somatosensory systems is unlikely to play a major role in auditory cortical reorganization, as no evidence of immediate unmasking was seen following acute cochlear lesions in guinea pigs.