Responses of primary auditory fibers to combined noise and tonal stimuli.

Responses to tonal stimuli, with and without added noise of different bandwidths, were obtained from anesthetized cat auditory-nerve fibers using glass micropipettes. When low-pass noise with a cut-off frequency at least one octave below best (or characteristic) frequency was used, every fiber tested at high enough intensities showed a suppression of the tonal response. This suppression did not cause a general reduction of neural responsiveness to all sounds, but rather took the general form of a frequency-specific reduction in the effective intensity of the tonal stimuli. The suppression mechanism(s) involved thus adjust the sensitivity of these fibers to cover higher intensity ranges in the presence of noise. The frequency of the most severely affected tones was always at or near best frequency, in confirmation of previous work (Abbas, P.J. and Sachs, M.B. (1976): J. Acoust. Soc. Am. 59, 112-122; Kiang, N.Y.-S. and Moxon, E.C. (1974): J. Acoust. Soc. Am. 55, 620-630). The suppresson is a direct but highly nonlinear function of the intensity and bandwidth of the noise. The effects on tonal response of wide-band noise were more variable, sometimes causing suppression similar to that induced by the low-pass noise and sometimes causing only 'strong-signal capture' effects. A model of noise-induced suppression has been developed whereby each sound produces both an excitatory effect, sharply tuned at best frequency, and a suppressive effect, which also has its lowest threshold at best frequency but is more broadly tuned.