AUDITORY-EVOKED RESPONSES TO RHYTHMIC SOUND PULSES IN DOLPHINS

The ability of auditory evoked potentials to follow sound pulse (click or pip) rate was studied in bottlenosed dolphins. Sound pulses were p resented in 20-ms rhythmic trains separated by 80-ms pauses. Rhythmic click or pip trains evoked a quasi-sustained response consisting of a sequence of auditory brainstem responses. This was designated as the r ate-following response. Rate following response peak-to-peak amplitude dependence on sound pulse rate was almost flat up to 200 s(-1) then d isplayed a few peaks and valleys superimposed on a low-pass filtering function with a cut-off frequency of 1700 s(-1) at a 0.1-amplitude lev el. Peaks and valleys of the function corresponded to the pattern of t he single auditory brain stem response spectrum; the low-pass cut-off frequency nias below the auditory brain stem response spectrum bandwid th. Rate-following response frequency composition (magnitudes of the f undamental and harmonics) corresponded to the auditory brain stem resp onse frequency spectrum except for lower fundamental magnitudes at fre quencies above 1700 Hz. These regularities were similar for both click and pip trains. The rate-following response to steady-state rhythmic stimulation was similar to the rate-following response evoked by short trains except for a slight amplitude decrease with the rate increase above 10 s(-1). The latter effect is attributed to a long-term rate-de pendent adaptation in conditions of the steady-state pulse stimulation .