Sensitivity of auditory nerve fibers to spectral notches

1. Listeners use direction-dependent spectral cues introduced by the torso, head, and pinnae to localize the source of a sound in space. Among the prominent direction-dependent spectral features in the free field-to-eardrum transfer function are narrow regions of low acoustic energy referred to as spectral notches. In this paper, we studied the sensitivity of single auditory nerve fibers in the barbiturate-anesthetized cat to broadband noise that had been filtered by a function whose shape approximated natural notches in the free field-to-eardrum transfer function. 2. Two experimental paradigms were employed. The first was the repeated presentation of a burst of broadband noise filtered by the simulated-notch function. Center frequency of the notch was held constant at or around the fiber characteristic frequency (CF). We refer to this as a "stationary" notch stimulus. The second paradigm was the repeated presentation of a broadband noise that was constructed from noise segments, each filtered by the simulated notch, whose CF was incremented and then decremented in a systematic way. We refer to this as a "moving" notch stimulus. Results from these two paradigms were compared with respect to notch detection. 3. Data were obtained from 161 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. Each fiber responded vigorously to the presence of broadband noise. When the CF of the notch encroached on the response area of the fiber, there was a demonstrable reduction in discharge rate. The greatest reduction in discharge rate occurred when the notch was centered at the fiber'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 frequency-intensity plane between the position of the most effective notch and the fiber's threshold tuning curve. 4. For high-spontaneous rate fibers, a moving-notch stimulus, but not a stationary one, reduced the discharge below the spontaneous rate at and in the immediate vicinity of 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 the head.(ABSTRACT TRUNCATED AT 400 WORDS)