Single-neuron labeling and chronic cochlear pathology. I. Threshold shift and characteristic-frequency shift.

Iontophoretic injections of horseradish peroxidase were used to label single auditory-nerve fibers in cats with permanent, noise-induced or drug-induced threshold shifts. Using these labeling techniques, it was possible to estimate the degree to which the characteristic frequency (CF) of a neuron shifts as its threshold shifts in cases of chronic cochlear pathology. The original CF of an abnormal neuron was estimated by measuring the location of its peripheral terminal along the cochlear spiral and comparing that to data from normal animals on the relation between CF and cochlear location [12]. Data from 25 labeled neurons with estimated threshold shifts varying from 0 to 70 dB suggest that, at least for units with original CF above 1 kHz, the measured CF shifts down as the threshold rises. The largest CF shifts (0.26 to 0.66 octave) were seen in units contacting inner hair cells within 1 mm of cochlear regions in which the entire organ of Corti (including supporting cells) was destroyed. For the abnormal units located more than 1 mm from such regions, the discrepancy between the CF predicted on the basis of fiber location and the CF measured from the tuning curve was less than 0.18 octave even though threshold shifts were comparable in the two groups. Units with the largest CF shifts showed abnormally low slopes on the portion of the tuning curve immediately above CF. This observation suggested a means of identifying abnormal units with significant CF shifts by simple inspection of the tuning curve. The possibility that the degree, and even the direction, of the CF shift varies from high- to low-CF units is discussed.