Forward masking of auditory nerve fiber responses

Responses of single fibers were obtained from the auditory nerve of chinchillas. Tone-burst stimuli consisted of a masking stimulus followed by a probe stimulus. Forward masking of a fiber's response is defined as a reduction in the magnitude of the probe-evoked response caused by the addition of the masking stimulus. The recovery of probe response magnitude as a function of the time interval between masker offset and probe onset (ΔT) follows an exponential time course. A relationship between the time course or magnitude of poststimulus recovery and the characteristic frequency (CF) of a fiber was not detected. The iso-forward masking contour near the threshold of the masking effect across masker frequencies approximates a fiber's frequency threshold curve (FTC). In other words, forward masking tuning curves are essentially the same as frequency threshold curves. The frequency dependence of forward masking is compared to that of two-tone suppression. Tonal stimuli outside the boundaries of a fiber's FTC that produce two-tone suppression are ineffective forward maskers. Certain frequency/intensity combinations within the FTC may produce both suppresion and forward masking and tones within the remaining area of the FTC produce no suppression but are effective forward maskers. Both the time course and magnitude of the forward masking effect are dependent on the discharge rate evoked by the masker regardless of the masker's absolute level or spectral content. An increase in masker-evoked excitation causes an increase in time constant and a greater reduction in probe response magnitude, r(d). The function relating r(d) to masker level parallels the firing rate/masker level function up to 40 dB above response threshold. A decrease in masker duration from 100 ms leads to a decrease in both r(d) and the time constant of recovery. There is no difference between the 100 and 200 ms duration conditions. Forward masking in single fibers is related to the period of poststimulus recovery of spontaneous activity, a component of a fiber's response pattern to the masker, and this component is tentatively identified as a period of recovery from short-term adaptation.