Two-tone suppression in the auditory nerve of the cat: Suppression contours

For frequencies less than 4 kHz, Fourier analysis of period histograms offers a convenient method for extracting information about auditory nerve fibers' responses to complex signals. In the present study, responsiveness to an excitatory tone (F1) was diminished by a suppressor tone (F2). In general, both tones were excitatory. Suppression magnitude was computed from the shift in the Fourier amplitude versus intensity function for F1 caused by the introduction of F2, in fibers whose characteristic frequencies (CF) were less than 6 kHz. In conditions where suppression magnitude was estimable by both Fourier and discharge-rate analyses, the two techniques produced similar values. “Suppression areas” were constructed for fixed-frequency F1 and variable-frequency, constant-intensity F1. For a given F1, suppression was always maximal at F2 = CF. This was true regardless of the position of F1 within the excitatory response area. Suppression contours obtained at different suppressor-tone intensities resembled the iso-intensity contours obtained in response area determinations, but the suppression contours were narrower than iso-intensity discharge rate contours obtained at equivalent intensities of F2. The results support the conclusions that (a) suppression exists throughout a fiber's response area, (b) suppression behavior is independent of overall discharge rate, and (c) suppression is a primary factor in determining which components in a complex signal will dominate a fiber's response. [Work supported by NIH.]