Peripheral auditory processing in the bobtail lizard Tiliqua rugosa III. Patterns of spontaneous and tone-evoked nerve-fibre activity

1. 70% of primary afferent fibres in the bobtail lizard were irregularly spontaneously active, with rates between 0.1 and 123.7 spikes/s (mean of 31.2 spikes/s). The rate distribution is bimodal. Mid-frequency fibres with lower thresholds tend to have higher spontaneous rates.

2. About one third of all fibres show a more prominent mode in their inter-spike interval histogram than is to be expected from a quasi-Poisson distribution. Preferred intervals in the spontaneous discharge patterns were not normally seen; this is presumably due to the lack of exclusive innervation of hair cells.

3. Primary fibres mostly responded to sound with a discharge rate increase. The form of the peri-stimulustime histogram varies between fibres; three different response types are described. Fibres of low characteristic frequency (CF up to 0.65 kHz) show a characteristic change in their response pattern with stimulation frequency; it is suggested that primary suppression plays an important role in shaping the very phasic response to tones at the fibres' upper frequency range. The responses of higher-CF fibres (CF 0.55–4 kHz) are independent of stimulation frequency. About one third of them shows a primary-like discharge pattern. The majority, however, responds with a chopper-like discharge pattern and there is evidence that this discharge, as in mammalian cochlear-nucleus stellate cells, originates from temporal summation.

4. The systematically-varying response pattern typical for low-CF fibres introduces special difficulties in defining the extent of the excitatory tuning curve and of the two-tone rate suppression area. The influence of threshold criteria on the overlap seen between both curves is discussed.