Keele Research Repository

The main aim of this study was to investigate the threshold and tuning properties of cochlear nerve fibres in kanamycin treated guinea pigs having various degrees of hair cell loss restricted to outer hair calls (OHCs).
The frequency threshold curves (FTCs) of fibres originating in regions of total OHC loss had thresholds elevated 40-50 dB above normal, and a broadening of their 10 dB bandwidths, on average, to five times greater than normal. On the assumption that the inner hair cells (iHCs) in the regions of OHC loss are unaffected by the kanamycin, the results suggest that the normal low threshold, sharply tuned properties of cochlear fibres depend in some way on the integrity of the OHCs. Because the majority, if not all, of these fibres originate at the IHCs, some form of interaction between outer and inner hair cells and /or their innervations is implicated.
Also investigated was the relationship between the elevation of minimum thresholds of FTCs and their 10 dB bandwidths. For fibres with characteristic frequencies (CFs) above 2 kHz, the relationship was non-linear such that the 10 dB bandwidth was substantially increased only for threshold elevation greater than 30-40 dB. For lower CFs, 10 dB bandwidths increased hand in hand with minimum theshold elevation.
The physiological findings are discussed in relation to possible mechanisms of sharp cochlear tuning, and also to possible correlates in psychophysical studies of deafness of cochlear origin.
Cochlear action potentials (CAPS) were also recorded from normal and kanamycin treated GPs. Latency and amplitude: intensity functions were measured ( to empirically test theories of how they reflect underlying cochlear fibre activity) and also CAP thresholds to tone pip stimuli. The latter thresholds correlated well with the minimum thresholds of corresponding cochlear fibres over a frequency range of 1-40 kHz, for both normal and pathological cochleas.