The auditory evoked potential difference tone and cubic difference tone measured from the inferior colliculus of the chinchilla

Abstract

The auditory evoked potential f2-f1 difference tone (DT) and the 2 f1-f2 cubic difference tone (CDT) were recorded from electrodes implanted in the inferior colliculus in a group of chinchillas. The purpose of this study was to measure normative aspects of AEP distortion products in awake chinchillas, by comparing the DT and CDT under a variety of stimulus conditions. For experiment 1, f1 was held constant at 1998 Hz, while the f2/f1 ratio was varied from 1.05 to 1.50. Input-output functions were measured over a range of primary tone levels up to 80 dB SPL. The amplitude of the DT was greatest for the smallest f2/f1 ratio, and decreased systematically as f2/f1 ratio increased. DT amplitude was greater than CDT amplitude for all primary tone pairs. Experiment 2 was conducted to determine the effect of f1 frequency upon the DT and CDT for a constant f1-f2 difference frequency of 102 Hz (f1-999, 1998, 4999, and 9998 Hz). The DT input-output functions were overlapping for all f1 frequencies. For the CDT, amplitude decreased with increasing f1 frequency, which corresponded to an increase in CDT frequency. In experiment 3, the relationship between ear of stimulation and inferior colliculus recorded from was investigated. DT input-output functions (f1 = 1998 Hz, DT = 102 Hz) were measured for monaural contralateral, monaural ipsilateral, and dichotic stimulus conditions. DT amplitude was largest for the contralateral condition, followed by the ipsilateral condition. A smaller, dichotic component to the DT was observed as well.