Time course of rate responses to two-tone stimuli in auditory nerve fibres in the guinea pig.

ABSTRACT

Peri-stimulus time histograms (PSTHs) were constructed from responses of auditory nerve fibres in anaesthetized guinea pigs. Acoustic stimuli consisted of pure tones, presented either as tone bursts, or in two-tone combinations in which a gated test tone was superimposed on a continuous excitatory tone at characteristic frequency (CF). The majority of the sample of fibres displayed two-tone rate suppression (2TRS). The suppression was either a monotonic or a non-monotonic function of the level of the superimposed test tone. Monotonic suppression of CF-driven rate occurred only for test tones at frequencies higher than CF, presented at levels up to the maximum available (approx. 100 dB SPL). For test tones below CF, 2TRS initially increased, then reverted towards excitation for higher levels of the test tone. Three levels were identified in non-monotonic, two-tone rate functions; (1) the threshold for rate suppression, (2) the maximally suppressing level and (3) the level (referred to as the balance point) at which average firing rate was restored to the background, CF-driven rate. PSTHs for two-tone responses obtained for test tone levels between the maximally-suppressing level and the balance point typically showed brief decrements (notches) in spike rate, at the onset and following the offset of the test tone. The latency, depth and duration of notches, however, depended on the level of the test tone, in a different manner for onset and offset. In some cases, without overt rate excitation above the probe-driven rate, the offset notch became more pronounced and of extended duration with increased level of the test tone, suggestive of adaptation to the test tone. Two-tone responses, in which rate exceeded the background, CF-driven rate, in general were preceded by a reduced onset notch and were followed by a longer-lasting depression of the background spike rate, typical of post-excitatory depression. Relative to responses obtained to the test tones presented alone, excitatory two-tone responses were of lower rate and were delayed by the onset notch. Onset notches sometimes preceded rate excitation in responses to single tones. Some features of the time course of rate suppression and excitation displayed in PSTHs for responses to one and two-tone stimuli seem inconsistent with current models of 2TRS.