Abstract:
Detection thresholds for pure-tone stimuli were measured in quiet and in the presence of octave-band noise presented simultaneously to the contralateral ear in a group of Japanese macaques. Subjects were trained to respond to the presence of 1-s sinusoids presented to the ipsilateral ear in a go/no-go procedure. All stimuli were presented through insert earphones. Pure-tone stimuli were 1-s in duration and ranged from 125 Hz to 16.0 kHz. Continuous noise stimuli were octave bands, centered at the ipsilateral test-tone frequency, presented at levels of 0 to 70 dB above threshold for the center frequency stimulus. While some variability was evident across subjects, both in the frequency distribution and magnitude (as a function of contralateral noise level) of threshold increases, all subjects exhibited an orderly increase in thresholds in the presence of contralateral noise. Maximum increases in threshold were evident between 1.0 and 4.0 kHz. Measurable threshold elevations were evident at noise levels as low as 20 dB above threshold and, in general, increased systematically with contralateral noise levels. Decreases in threshold were evident at low-noise levels. The characteristics of the measured threshold shifts are similar to efferent suppression effects which have been reported in the physiological literature. [Work supported by NIH.]