The periodic ringing of cochlear inputs sustains phase-based ITD coding for nonperiodic sounds

Evidence

A key feature of ITD sensitivity is how the observed microsecond delays may be created by neural elements that are both noisy and slow. There are two possibilities, illustrated by the example of a coincidence detector neuron that responds to a pure sine tone, presented to both ears at an ITD of 0 μs, or sound in front of the animal. One strategy requires equal conduction latencies from each ear to the coincidence detector, such that the inputs would arrive with equal latency and equal phase. The second strategy relies on relative delays and would simply require that inputs arrive at the same phase. Due to the periodicity of the sine wave, both scenarios would produce the same output. This coding strategy remains viable for non-periodic sounds because the tonotopic organization of the auditory system generates ringing properties of the inputs characteristic of the particular frequency (Carney et al. 1999 ; Recio et al. 1998 ; Wagner et al. 2005 ). … Previously we had shown that the inputs from the cochlear nuclei act as delay line inputs to form maps of ITD in the barn owl (Carr and Konishi 1990 ), but could not determine whether the delay lines had identical conduction latencies or were matched for phase only. — Dijk, Pim van, Physiology, Psychoacoustics and…, 2844

Categorization

• DEFINE: Distinguishes the two competing hypotheses for how microsecond ITD precision is achieved: matching absolute conduction latencies versus matching only relative phase delays.
• QUESTION: How does phase-only matching produce microsecond accuracy when the neural elements conveying the signal are noisy and slow? The question is resolved later in the chapter.