EFFECT OF UNILATERAL PARTIAL COCHLEAR LESIONS IN ADULT CATS ON THE REPRESENTATION OF LESIONED AND UNLESIONED COCHLEAS IN PRIMARY AUDITORY-CORTEX

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.