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Front Neurosci. 2014 May 27;8:112. doi: 10.3389/fnins.2014.00112. eCollection 2014.

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

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  • 1Klinikum rechts der Isar, Technische Universität München Munich, Germany.
  • 2Medical Research Council Institute of Hearing Research, University Park Nottingham, UK.
  • 3Departamento de Cirugía, Facultad de Medicina, Instituto de Neurociencias de Castilla y León, Instituto de Investigación Biomédica de Salamanca, Universidad de Salamanca Salamanca, Spain.


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.


HRTF; auditory nerve; head-related transfer function; phase-locking; rate profile; temporal profile

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