Auditory neurophonic responses to amplitude-modulated tones: transfer functions and forward masking

Two auditory neurophonic responses - one recorded from the scalp (frequency following response or FFR) and one from the auditory nerve (auditory nerve neurophonic or ANN) - were obtained following stimulation of the cat cochlea with amplitude-modulated (AM) high-frequency tones. The carrier frequencies varied between 2 and 30 kHz. The modulation frequencies varied between 400 and 3000 Hz. The AM responses were compared with pure-tone neurophonic responses. The AM response waveforms were found to have a similar spectral composition, similar rates of adaptation, and similar rates of recovery from forward masking as the comparable pure-tone responses. As with the pure-tone neurophonics, an unmodulated masking stimulus can produce prolonged depression of the probe response. The amount and duration of this depression is dependent upon the level and frequency of the masker. The frequency dependence of the depression is demonstrated by forward masked tuning curves which indicate that the AM responses arise from fiber populations which have restricted characteristic frequency distributions centered on the carrier frequency. Response amplitude as a function of stimulus level (I/O) functions, response amplitude as a function of carrier frequency (carrier transfer functions or CTF) and response amplitude as a function of modulation frequency (modulation transfer functions or MTF) were also measured. It was found that the I/O functions were saturating monotonic functions of stimulus intensity, CTFs were flat for carrier frequencies from 6 to 30 kHz, and MTFs were flat for modulation frequencies from 100 to 1500 Hz. These results are compared with similar data for single units and compound action potentials.