![]() In mammals, this has traditionally been interpreted as reflecting the travel times of the basilar membrane traveling wave to the places of different characteristic frequency (CF) along the cochlea (for review, see Ruggero, 1992). Group delays derived from the response phases of individual auditory nerve fibers contain a frequency-dependent component that increases from high to low frequency fibers. Although phase locking in the owl NM has been studied previously ( Sullivan and Konishi, 1984 Carr and Konishi, 1990), data were also recorded in the NM to compare both neuronal populations in the same individual owls.Ī second important aspect of the present study was the use of auditory nerve phase locking as a window on cochlear mechanisms of stimulus transduction and encoding. One important question relates to the putative improvement in the quality of phase locking in the NM over the auditory nerve input. ![]() The present study was undertaken to fill this significant gap and thus help establish the contributions of the auditory nerve to the temporal processing task. Although this circuit is in principle well established, very little is known about the phase-locking properties of auditory nerve fibers in the barn owl ( Sullivan and Konishi, 1984). ![]() ![]() A dedicated brainstem circuit then relays the inputs from both auditory nerves via axonal delay lines created by the Nucleus magnocellularis (NM) to neurons of the Nucleus laminaris, which perform binaural coincidence detection (for review, see Carr, 1993a). Auditory nerve fibers encode the ongoing temporal properties of the stimulus through phase locking to its sinusoidal components within individual narrowly tuned ranges of frequencies. The animal uses minute differences in the arrival times of sounds at its two ears to determine the azimuthal location of the sound source (for review, see Konishi, 1993). The barn owl has become an important model for the study of extremely fast temporal processing based on neural phase locking. Phase locking encodes the temporal structure of stimuli, from slow modulations to fine structure in the microsecond range in different sensory systems (for example, see Taniguchi and Ogawa, 1987 Surlykke et al., 1988 Heiligenberg, 1989 Wubbels, 1992 Carr, 1993b). Phase locking, i.e., the firing of neurons preferentially at a certain phase of an amplitude-modulated stimulus, is an important general mechanism in sensory physiology. This questions the common assumption that group delays reflect cochlear wave travel times. The response delays, or group delays, derived from the phase-versus-frequency functions of auditory nerve fibers were not consistent with the unusual spatial frequency representation in the owl cochlea. Phase locking in the NM was consistently inferior to that of auditory-nerve fibers at frequencies above 1 kHz, suggesting that the synapse plays a limiting role in temporal precision. Auditory nerve fibers showed an unusual step-like decline with a prominent plateau in the mid-frequency range (1.5–3 kHz), indicating that some specialization enables the owl to halt the deterioration and extend phase locking to frequencies up to 10 kHz, above the range commonly observed in other species. The quality of phase locking, expressed as vector strength, decreased with increasing frequency. Phase locking was regularly seen at sound pressure levels (SPL) below those eliciting an increase in spike rate, thus providing an additional cue for signal detection. For direct comparison in the same individuals, recordings were also obtained from the relevant next higher center, the nucleus magnocellularis (NM). This study examined the quality and intensity dependence of phase locking in single auditory nerve fibers of the barn owl to define the input to the known brainstem circuit for temporal processing. Phase locking has been shown to be the behaviorally relevant temporal code. The auditory system of the barn owl is an important model for temporal processing on a very fast time scale and for the neural mechanisms and circuitry underlying sound localization.
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