Neural correlates of the aural combination tone 2f1- f2

Physiological studies confirm the nineteenth-century view that the phenomenon of aural combination tones has important implications for the understanding of the auditory system. Responses of single fibers in the auditory nerve of anesthetized cats were studied for evidence of combination tones when the acoustic stimulus consisted of two sinusoidal tones. The stimulus frequencies were chosen so that the combination frequency 2f1- f2, where 1 < f2/f1< 2, was approximately equal to the characteristic frequency of the fiber. The major findings are: 1) responses locked in time to individual cycles of a reference combination tone (2f1- f2) were found for every fiber, 2) the time-locked response could be cancelled by adding a combination tone (2f1- f1) to the stimulus, 3) the stimulus frequencies could be chosen to obtain responses that were predominantly time locked to the reference combination tone, and 4) the rates of discharge for both single and two-tone stimuli show a similar dependence upon sound level. These characteristics of the neural responses are similar to those expected from acoustic stimuli that contain a combination tone (2f1- f2) of amplitude approximately proportional to the actual stimulus amplitude. Therefore, these findings do not reflect an overloading type of distortion but rather some normal operating property of the auditory system. The physiological findings bear a close relation to those of psychophysical experiments on combination tones.