SENSITIVITY OF AUDITORY-NERVE FIBERS TO SPECTRAL NOTCHES

1. Listeners use direction-dependent spectral cues introduced by the t orso, head, and pinnae to localize the source of a sound in space. Amo ng the prominent direction-dependent spectral features in the free fie ld-to-eardrum transfer function are narrow regions of low acoustic ene rgy referred to as spectral notches. In this paper, we studied the sen sitivity of single auditory nerve fibers in the barbiturate-anesthetiz ed cat to broadband noise that had been filtered by a function whose s hape approximated natural notches in the free field-to-eardrum transfe r function. 2. Two experimental paradigms were employed. The first was the repeated presentation of a burst of broadband noise filtered by t he simulated-notch function. Center frequency of the notch was held co nstant at or around the fiber characteristic frequency (CF). We refer to this as a ''stationary'' notch stimulus. The second paradigm was th e repeated presentation of a broadband noise that was constructed from noise segments, each filtered by the simulated notch, whose CF was in cremented and then decremented in a systematic way. We refer to this a s a ''moving'' notch stimulus. Results from these two paradigms were c ompared with respect to notch detection. 3. Data were obtained from 16 1 single auditory nerve fibers having CFs ranging from 0.4 to 40 kHz. Most fibers studied had CFs >5 kHz, and they detected the presence of the spectral notch in an intensity- and frequency-dependent manner. Ea ch fiber responded vigorously to the presence of broadband noise. When the CF of the notch encroached on the response area of the fiber, the re was a demonstrable reduction in discharge rate. The greatest reduct ion in discharge rate occurred when the notch was centered at the fibe r's CF and when the level of the notch signal was some 15-55 dB above the fiber's noise threshold. There was close association in the freque ncy-intensity plane between the position of the most effective notch a nd the fiber's threshold tuning curve. 4. For high-spontaneous rate fi bers, a moving-notch stimulus, but not a stationary one, reduced the d ischarge below the spontaneous rate at and in the immediate vicinity o f the most effective notch frequency. This increases sensitivity to a spectral notch and suggests a mechanism by which localization ability is enhanced when there is relative motion between a sound source and t he head. 5. We conclude that information pertaining to the frequency a nd intensity of prominent spectral notches in the free field-to-eardru m transfer function is transmitted to the CNS in the discharge rate of an array of auditory nerve fibers.