Auditory nerve fibre activity in the tokay gecko

Summary

Extracellular recording from single auditory nerve fibres in the lizard,Gekko gecko, revealed interesting patterns of evoked and spontaneous activity. The fibres are sensitive and sharply tuned (Figs. 1 and 3) and have characteristic frequencies (CFs) from 0.15 to 5 kHz (Fig. 2). Responses to pure tone bursts fall into three main categories with different frequency distributions (Figs. 5, 6 and 7): 1) tonic; such fibres have CFs<0.7 kHz; 2) phasic, restricted to CFs>0.7 kHz; 3) intermediate, with an intermediate frequency range: 0.4–2 kHz. As there is also some intensity dependence to these response patterns, fibres were classified according to their responses to tones at 20 dB (above threshold). Within small samples of fibres drawn from the three categories, all were susceptible to two-tone suppression (Fig. 10) and responded to a broad-band Tokay call (Fig. 11). Possible correlations between these categories and populations of cochlear hair cells that differ with respect to orientation and tectorial connections are discussed. A few fibres (10 in 410) were seen to respond to both onset and offset of certain pure tones (Fig. 9).

Interspike interval histograms revealed that fibres with CFs≧0.5 kHz have spontaneous discharge patterns comparable to those that have been described in other species, despite low discharge rates (Figs. 12 and 13). However, fibres with CFs≦0.5 kHz tend to fire spontaneously at intervals close to the CF period or integral multiples thereof (Fig. 13). We propose that this spontaneous discharge pattern may reflect ‘ringing’ of hair cells at their CFs and so may provide indirect evidence that hair cell membranes possess a tuned system of conductances.