COMPARISON OF THE ENVELOPE FOLLOWING RESPONSE IN THE MONGOLIAN GERBIL USING 2-TONE AND SINUSOIDALLY AMPLITUDE-MODULATED TONES

Two-tone (TT) and sinusoidally amplitude-modulated (SAM) signals, although differing in spectra, are both periodic; the period corresponds to the difference between the two frequencies (f(2,1) = f(2) - f(1)) in the former and to the frequency of the modulation tone (f(mod)) in the latter. Here the results of a study comparing the steady-state electrophysiologic responses to TT and SAM stimuli recorded from Nembutal-anesthetized Mongolian gerbils are reported. In the first experiment a modulation rate transfer function (MRTF) was obtained for each stimulus type by setting the SAM carrier frequency (f(c)) and f(1) of the TT signal at the same frequency while f(mod) and f(2,1) were covaried. MRTFs were obtained for f(1)s and f(c)s of 1, 3, and 5 kHz with envelopes which varied between 50 and 500 Hz in 50-Hz increments. Stimuli were presented at 75 dB peak sound-pressure level (pSPL). Responses to the two stimulus types yielded MRTFs which were very similar and generally low pass in shape. In the second experiment responses to the TT and SAM signals were recorded in the presence of a continuous interfering tone of 85-dB pSPL which was varied between 650 Hz and 3 kHz. In these experiments a maximum reduction in the response to the TT and SAM signals, measured at f(2,1) and f(mod) as well as at f(c) and f(1), occurred within a narrow frequency band above the frequency of the probe carrier and a broader region of reduced response extending to higher frequencies. This reduction in response was asymmetrical, spreading more to high than to low frequencies. The similarity of both MRTFs and interference response patterns supports the view that the envelope following responses to TT and SAM stimuli are manifestations of the same nonlinear phenomena.