Suppression of Auditory-Nerve-Fiber Responses to Off-CF Tones

Auditory-nerve-fiber (ANF) rate-level functions for probe tones at the characteristic frequency (CF) and at off-CF frequencies were collected with and without an 80 dB SPL non-excitatory suppressor tone placed outside of the tuning curve on its high-frequency side. Suppression was quantified by the suppressor-induced shift to higher sound levels of these rate-level functions. Suppression generally increased with increases in probe frequency re. CF, but some fibers showed an increase followed by a decrease in suppression. For probe tones at and above CF, the suppression tended to increase as CF increased. Suppression was found throughout the tuning-curve tip so that for fibers with low CFs suppression was present over a broad range of probe frequencies. Overall, the data are consistent with the idea that as the probe-tone frequency increases above CF, the region along the cochlea where the probe response receives amplification moves basally and is therefore suppressed more by the fixed, high-side (i.e., basal) suppressor tone. Thus, for an AN fiber, the outer hairc cells (OHCs) that provide cochlear amplification for a CF tone are not the only OHCs that amplify the responses of that fiber. If the fiber is responding to tones above CF, than OHCs basal to the CF-amplifier OHCs are involved.