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

Two-tone (TT) and sinusoidally amplitude-modulated (SAM) signals, alth ough differing in spectra, are both periodic; the period corresponds t o 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 th e latter. Here the results of a study comparing the steady-state elect rophysiologic responses to TT and SAM stimuli recorded from Nembutal-a nesthetized Mongolian gerbils are reported. In the first experiment a modulation rate transfer function (MRTF) was obtained for each stimulu s 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. MR TFs were obtained for f(1)s and f(c)s of 1, 3, and 5 kHz with envelope s 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 generall y 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 sig nals, measured at f(2,1) and f(mod) as well as at f(c) and f(1), occur red within a narrow frequency band above the frequency of the probe ca rrier and a broader region of reduced response extending to higher fre quencies. This reduction in response was asymmetrical, spreading more to high than to low frequencies. The similarity of both MRTFs and inte rference response patterns supports the view that the envelope followi ng responses to TT and SAM stimuli are manifestations of the same nonl inear phenomena.