Correlates of combination tones observed in the response of neurons in the anteroventral cochlear nucleus of the cat

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Abstract
Neurons in the anteroventral cochlear nucleus of the cat respond to combination tones of the forms f2−f1 and f1−n (f2−f1), where n is a small positive integer 1, 2, 3,.... The most easily observed combination tones are f2−f1 and 2f1−f2. In general, a combination tone is effective if three conditions are fulfilled: (1) the combination‐tone frequency must fall within the pure‐tone response area of the neuron; (2) the intensity levels of the primaries must be appropriate; and (3) the separation of the primary frequencies cannot be unduly large. For any form of combination tone, a combination‐tone response area could be plotted by fixing f1 at some level and varying f2 in small steps. The actual frequency of the combination tone could be determined from the timing of the discharges for all neurons whose discharges are phase locked. The combination‐tone response areas indicate that the response to a given form of combination tone is optimal when the combination frequency is at or near the best frequency of the neuron. Moreover, the combination‐tone response areas are similar in width to the pure‐tone response area of the neuron. These findings suggest that the neuron responds to combination tones as if such tones were actually delivered to the ear. It was further found that the combination‐tone frequencies that activate the neuron may be several octaves below the frequencies of the two primaries that produce the combination tone. Combination‐tone threshold and suprathreshold studies show that the combination‐tone level increases with decreasing separation of the two stimulus components. Levels of the combination tones expressed in equivalent decibels of SPL increase approximately proportionately with stimulus level (L1=L2) for both f2−f1 and 2f1−f2. Furthermore, for both these combination tones, the combination‐tone level as a function of L2 (L1 fixed) shows a peaked curve with the maximum occurring when L2 is about equal to L1. The types of combination tones encountered and the effects of variations in stimulus frequency are in accord with psychophysical findings. The effect of stimulus level on combination tone level is in agreement with psychophysical observations for 2f1−f2 but seems to disagree with most psychophysical data for f2−f1 at higher stimulus frequencies. The results suggest that a marked nonlinearity is involved in cochlear processing, at least in the 40‐ to 90‐dB‐SPL range. Subject Classification: [43]65.56, [43]65.42, [43]65.40.
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