AUDITORY-EVOKED RESPONSES TO AMPLITUDE-MODULATED STIMULI CONSISTING OF MULTIPLE ENVELOPE COMPONENTS

Steady-state auditory-evoked potentials were recorded noninvasively fr om alert bottlenosed dolphins, Tursiops truncatus, using suction cup e lectrodes placed on the scalp surface. Responses were elicited using c ontinuous acoustic signals consisting of 2, 3, or 4 tones with lowest frequency at 1000 Hz or 5000 Hz, and having a maximum frequency separa tion of 171 Hz. Due to the interaction of the stimulus tones, the stim ulus waveform was comprised of 1 to 6 dominant temporal envelope compo nents. Evoked responses were averaged in the time domain and Fourier t ransformed for analysis. The spectrum of the averaged evoked potential contained peaks at Fourier components corresponding to all stimulus e nvelope frequencies. Thus, scalp potentials, representing the synchron ized discharge of large neuronal assemblies, followed the low-frequenc y temporal envelope of the stimulating waveform whether comprised of 1 , 3, or 6 dominant envelope components; this envelope following respon se (EFR) was the dependent variable in all experiments.