Auditory evoked responses to amplitude modulated stimuli consisting of multiple envelope components

Steady-state auditory-evoked potentials were recorded noninvasively from alert bottlenosed dolphins, Tursiops truncates, using suction cup electrodes placed on the scalp surface. Responses were elicited using continuous acoustic signals consisting of 2, 3, or 4 tones with lowest frequency at 1000 Hz or 5000 Hz, and having a maximum frequency separation of 171 Hz. Due to the interaction of the stimulus tones, the stimulus waveform was comprised of 1 to 6 dominant temporal envelope components. Evoked responses were averaged in the time domain and Fourier transformed for analysis. The spectrum of the averaged evoked potential contained peaks at Fourier components corresponding to all stimulus envelope frequencies. Thus, scalp potentials, representing the synchronized discharge of large neuronal assemblies, followed the low-frequency temporal envelope of the stimulating waveform whether comprised of 1, 3, or 6 dominant envelope components; this envelope following response (EFR) was the dependent variable in all experiments.