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The Journal of Neurophysiology Vol. 82 No. 6 December 1999, pp. 3506-3526
Copyright ©1999 by the American Physiological Society
1Epstein Laboratory, Coleman Laboratory, Department of Otolaryngology, University of California, San Francisco 94143-0732; and 2Communicative Disorders Department, San Francisco State University, San Francisco, California 94132
Raggio, Marcia W. and
Christoph E. Schreiner.
Neuronal Responses in Cat Primary Auditory Cortex to Electrical
Cochlear Stimulation. III. Activation Patterns in Short- and Long-Term
Deafness. J. Neurophysiol. 82: 3506-3526, 1999. 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.
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