ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

2pPP67. Responses of cochlear nucleus units in the chinchilla to best frequency tones amplitude modulated by low-frequency, two-tone complexes.

Parmly Hear. Inst., Loyola Univ. Chicago, 6525 N. Sheridan Rd., Chicago, IL 60626

A tone that is amplitude modulated by two tones (f[sub mod1] and f[sub mod2]) contains no spectral energy at the envelope beat frequency (f[sub mod2]-f[sub mod1]) for either the stimulus waveform or the half-wave rectified waveform. Preliminary data showed that Fourier analysis of post-stimulus time histograms of chinchilla CN units recorded in response to BF tones that were amplitude modulated by two tones (f[sub mod1], f[sub mod2]<64 Hz) yields spectral peaks at f[sub mod2]-f[sub mod1] in addition to the peaks at f[sub mod1] and f[sub mod2] [Shofner and Sheft, 2410(A) (1993)]. The magnitudes of these spectral peaks are measures of synchrony at each frequency component. For all units, synchrony at f[sub mod1] and f[sub mod2] is greater than the synchrony at f[sub mod2]-f[sub mod1]. For a given unit, synchrony at f[sub mod1] and f[sub mod2] remains relatively constant as a function of overall level, whereas synchrony at f[sub mod2]-f[sub mod2] decreases as the level increases. Synchrony at f[sub mod1] and f[sub mod2] is largest in chopper units, intermediate in primarylike units, and smallest in onset units. This hierarchy differs from that described in the literature for CN units in response to BF tones modulated at best modulation frequencies. No hierarchy appears to exist for synchrony at f[sub mod2]-f[sub mod1]. [Work supported by a PPG from NIDCD.]