Psychophysical assessment of the level-dependent representation of high-frequency spectral notches in the peripheral auditory system

Perception and coding of high-frequency spectral notches: potential implications for sound localization

The interaction of sound waves with the human pinna introduces high-frequency notches (5-10 kHz) in the stimulus spectrum that are thought to be useful for vertical sound localization. A common view is that these notches are encoded as rate profiles in the auditory nerve (AN). Here, we review previously published psychoacoustical evidence in humans and computer-model simulations of inner hair cell responses to noises with and without high-frequency spectral notches that dispute this view. We also present new recordings from guinea pig AN and "ideal observer" analyses of these recordings that suggest that discrimination between noises with and without high-frequency spectral notches is probably based on the information carried in the temporal pattern of AN discharges. The exact nature of the neural code involved remains nevertheless uncertain: computer model simulations suggest that high-frequency spectral notches are encoded in spike timing patterns that may be operant in the 4-7 kHz frequency regime, while "ideal observer" analysis of experimental neural responses suggest that an effective cue for high-frequency spectral discrimination may be based on sampling rates of spike arrivals of AN fibers using non-overlapping time binwidths of between 4 and 9 ms. Neural responses show that sensitivity to high-frequency notches is greatest for fibers with low and medium spontaneous rates than for fibers with high spontaneous rates. Based on this evidence, we conjecture that inter-subject variability at high-frequency spectral notch detection and, consequently, at vertical sound localization may partly reflect individual differences in the available number of functional medium- and low-spontaneous-rate fibers.

Sensitivity of the human auditory system to temporal fine structure at high frequencies

The frequency of sounds is coded partly by phase locking to the temporal fine structure (TFS) of the waveform evoked on the basilar membrane. On the basis of data obtained using sinusoids, it is usually assumed that in mammals, including humans, TFS information becomes unusable for frequencies above about 5000 Hz. Here, sensitivity to the TFS of complex sounds up to much higher frequencies is demonstrated. Subjects discriminated a harmonic complex tone, with a fundamental frequency F0, from a tone in which all harmonics were shifted upwards by the same amount in hertz. The phases of the components were selected randomly for every stimulus. Both tones had an envelope repetition rate equal to F0, but the tones differed in their TFS. To prevent discrimination based on spectral cues, the tones were passed through a fixed bandpass filter, centered at 14F0. A background noise was used to mask combination tones. Performance was well above chance for most subjects when F0 was 800 or 1000 Hz and all audible components were above 8000 Hz. Supplementary experiments confirmed that performance was not based on changes in the excitation pattern or on the discrimination of partially resolved components.