EFFECTS OF CHRONIC COCHLEAR DAMAGE ON THRESHOLD AND FREQUENCY TUNING OF NEURONS IN AI AUDITORY-CORTEX

We describe the effects of long-term cochlear lesions on the frequency response properties of AI cortical neurons in the cat. Young animals were treated with amikacin to produce bilateral, basal to mid-turn coc hlear lesions. After 12-24 months the response properties of single ne urons or small unit clusters in primary auditory cortex were recorded in anesthetized animals. Responses to stimulus frequency and intensity were mapped in detail and frequency threshold curves (FTCs)and Q(10dB ) values were derived. Subsequent to recording experiments, scanning e lectron microscopy of the sensory epithelium was used to characterize the degree and extent of the cochlear damage. In normal control animal s, Q(10dB) values were, on average, lower than those derived by others from cochlear nerve fibre recordings in the same species. In amikacin -treated animals, deterioration was evident in the threshold and tunin g properties of cortical neurons, particularly in those cells whose in put originated in damaged cochlear regions. Often, neurons associated with 'normal' cochlear areas (as assessed by scanning microscopy) also had poor frequency tuning compared with controls. As an animal model of sensorineural hearing loss, we consider the cat with long-term coch lear lesions to be more appropriate than animals with acute or short-t erm pathology. We also suggest that in making physiological-psychophys ical correlations, neural responses from the central auditory system ( e.g. cortex) should perhaps be given more consideration than data deri ved at the cochlear level.