Envelope-following response and modulation transfer function in the dolphin's auditory system.

Potentials following the envelopes of sinusoidally amplitude-modulated tones (envelope response, EFR) were recorded from the head surface in bottle-nosed dolphins. EFR appeared at modulation rates from 300 to 3400 Hz. EFR amplitude was higher at rates from 500 to 1400 Hz with peaks at 600 and 1000 Hz and troughs at 700-850, 1200, and 2000 Hz; at rates above 1700 Hz it fell steeply. EFR dependence on modulation depth was linear except at the highest response amplitudes, which made it possible to obtain the modulation transfer function (MTF). EFR appears to be generated by several sources. One source had a latency of about 4 ms and followed modulation rates up to 1700 Hz, while another had a latency of 2 ms and followed modulation rates up to 3.4 kHz. The latencies of both sources coincided with those of waves of the auditory brainstem response (ABR). Comparison of MTF with the ABR spectrum had shown that several MTF peaks and troughs reflected the ABR spectrum. The latencies of the two sources were consistent with origins in the midbrain and auditory nerve, respectively.