R. Rajan et al., EFFECT OF UNILATERAL PARTIAL COCHLEAR LESIONS IN ADULT CATS ON THE REPRESENTATION OF LESIONED AND UNLESIONED COCHLEAS IN PRIMARY AUDITORY-CORTEX, Journal of comparative neurology, 338(1), 1993, pp. 17-49
We examined the effect of unilateral restricted cochlear lesions in ad
ult cats on the topographic representations (''maps'') of the lesioned
and unlesioned cochleas in the primary auditory cortex (AI) contralat
eral to the lesioned cochlea. Frequency (tonotopic) maps were derived
by conventional multineuron mapping procedures in anesthetized animals
. In confirmation of a study in adult guinea pigs (Robertson and Irvin
e [1989] J. Comp. Neurol. 282:456-471), we found that 2-11 months afte
r the unilateral cochlear lesion the map of the lesioned cochlea in th
e contralateral Al was altered so that the AI region in which frequenc
ies with lesion-induced elevations in cochlear neural sensitivity woul
d have been represented was occupied by an enlarged representation of
lesion-edge frequencies (i.e., frequencies adjacent to those with elev
ated cochlear neural sensitivity). Along the tonotopic axis of AI the
total representation of lesion-edge frequencies could extend up to alm
ost-equal-to 2.6 mm rostral to the area of normal representation of th
ese frequencies. There was no topographic order within this enlarged r
epresentation. Examination of threshold sensitivity at the characteris
tic frequency (CF, frequency to which the neurons were most sensitive)
in the reorganized regions of the map of the lesioned cochlea establi
shed that the changes in the map reflected a plastic reorganization ra
ther than simply reflecting the residue of prelesion input. In contras
t to the changed in the map of the lesioned contralateral cochlea, the
map of the unlesioned ipsilateral cochlea, did not differ from those
in normal animals. Thus, in contrast to the normal very good congruenc
y between ipsilateral and contralateral Al maps, in the lesioned anima
ls ipsilateral and contralateral maps differed in the region of Al in
which there had been a reorganization of the map of the lesioned cochl
ea. Outside the region of contralateral map reorganization, ipsilatera
l and contralateral AI maps remained congruent within normal limits. T
he difference between the two maps in the region of contralateral map
reorganization suggested, in light of the physiology of binaural inter
actions in the auditory pathway, that the cortical reorganization refl
ected subcortical changes. Finally, response properties of neuronal cl
usters within the reorganized map of the lesioned cochlea were compare
d to normative data with respect to threshold sensitivity at CF, the s
ize of frequency ''response areas,'' and response latencies. In the ma
jority of cases, CF thresholds were similar to normative data. The fre
quency ''response areas'' were slightly less sharply tuned than normal
, but not significantly. Response latencies were significantly shorter
than normal in three animals and significantly longer in one animal.
(C) 1993 Wiley-Liss, Inc.