Changes in cochlear sensitivity do not alter relative thresholds of different spontaneous rate categories of primary auditory nerve fibres.

The CF thresholds of three different spontaneous firing rate categories of single auditory nerve fibres were measured in guinea pigs with widely differing cochlear sensitivities. The CF thresholds for all fibres were compared to the CAP thresholds at the same frequencies. The relationship between single fibre CF threshold and CAP thresholds for the three different fibre categories remained unchanged despite the very wide range of absolute CAP thresholds. The widely ranging CAP thresholds in these animals were probably due to varying degrees of outer hair cell pathology and these data therefore support the notion that in normal animals, the origin of differences in threshold between fibres with different spontaneous rates, lies in the properties of the inner hair cells and/or their synapses with afferent dendrites.