Very rapid adaptation in the guinea pig auditory nerve

Guinea pig auditory ganglion cell responses to 100-ms duration tone bursts were recorded over a range of stimulus intensities. The responses, recorded in the form of peristimulus/poststimulus time histograms, were analysed by reduction into two phases. The first phase was a rapid exponential adaptation from an initial onset response; the second was a more gradual reduction in the firing rate which, over the 100 ms duration of the stimulus, appeared to be a linear function of time. The first, rapid, phase was nonlinear in its response to changes in stimulus intensity, exhibiting a change in amplitude and having a time constant which decreased with increasing intensity. Individual units were consistent in the magnitude and time course of this phase. The second phase was also nonlinear with intensity, and was far more variable from unit to unit. With the recording parameters employed it was not possible to determine whether the effect of intensity on the second phase was an effect on the magnitude or time course, or both. Stimulus termination responses were also analysed, and typically were of one of two forms. If, at any particular stimulus intensity, the unit under study showed little sign of the slower adaptation then the termination response was a simple depression of activity (perhaps to zero) which recovered with an exponential time constant of about 25 ms, independent of intensity. If, however, the peristimulus responses showed a significant amount of the slow adaptation then the termination responses also exhibited a second, slower, phase of recovery. This was modelled over the recording epoch as a linear function of time. The magnitude of the slow offset response also increased with intensity faster than did the average firing rate.