Neuronal responses in cat primary auditory cortex to electrical cochlear stimulation. III. Activation patterns in...

The effects of auditory deprivation on the spatial distribution of cortical response thresholds to electrical stimulation of the adult cat cochlea were evaluated. Threshold distributions for single- and multiple-unit responses from the middle cortical layers were obtained on the ectosylvian gyrus in three groups of animals: adult, acutely implanted animals ("acute group"); adult animals, 2 wk after deafening and implantation ("short-term group"); adult, neonatally deafened animals ("long-term group") implanted after 2-5 years of deafness. For all three groups, we observed similar patterns of circumscribed regions of low response thresholds in the region of primary auditory cortex (AI). A dorsal and a ventral region of low response thresholds were found separated by a narrow, anterior-posterior strip of elevated thresholds. The two low-threshold regions in the acute and the short-term group were arranged cochleotopically. This was reflected in a systematic shift of the cortical locations with minimum thresholds as a function of cochlear position of the radial and monopolar stimulation electrodes. By contrast, the long-term deafened animals maintained only weak or no signs of cochleotopicity. In some cases of this group, significant deviations from a simple tri-partition of the dorsoventral axis of AI was observed. Analysis of the spatial extent of the low-threshold regions revealed that the activated area in acute cases was significantly smaller than the long- and the short-term cases for both dorsal and ventral AI. There were no significant differences in the rostrocaudal extent of activation between long- and short-term deafening, although the total activated area in the short-term cases was larger than in long-term deafened animals. The width of the narrow high-threshold ridge that separated the dorsal and ventral low-threshold regions was the widest for the acute cases and the narrowest for the short-term deafened animals. The findings of relative large differences in cortical response distributions between the acute and short-term animals suggests that the effects observed in long-term deafened animals are not solely a consequence of loss of peripheral innervation density. The effects may reflect electrode-specific effects or reorganizational changes based on factors such as differences in excitatory and inhibitory balance.