Temporal properties of two‐tone suppression: Phase‐locking of high characteristic frequency auditory‐nerve fibers to low‐frequency tones

Abstract

Responses of auditory‐nerve fibers with high characteristic frequency (greater than 5 kHz) to characteristic frequency (CF) tones can be suppressed by tones of much lower frequency. In such cases, the fiber responses are phase‐locked to the low‐frequency suppression tone, but not to the characteristic frequency tone. Such situations are ideal for studying the temporal properties of two‐tone suppression. In this series of experiments, period histograms synchronized to the low‐frequency suppressor are computed. As the sound pressure level of a low‐frequency suppressor is increased (in the presence of a CF tone of constant level), phase‐locking to the suppressor appears at levels below those which cause a decrease in average rate. The phase of the histograms is estimated by the phase of the fundamental component of the Fourier series for the histogram. Over the range of levels where two‐tone suppression occurs, the histogram phase shifts until at high levels the phase is the same as the phase in response to the suppressor tone presented alone. That is, parts of the cycle of the suppressor tone which are excitatory at high stimulus levels can cause suppression at lower levels. [Supported by Grant No. NS12112 from NIH.]