Interaural time difference maps derive from precise phase delays, not absolute latencies

Evidence

In general, changes in delay with depth were consistent with the NM axons acting as delay lines (Carr and Konishi 1990 ), and with modelled conduction velocities in McColgan et al. ( 2014 ). Ipsilateral recordings showed an increase in latency with recording depth in NL (open symbols in Fig. 2 ), while contralateral recordings (closed symbols) showed a decrease in latency with depth, consistent with the delay line geometry. The regression lines in Fig. 2c predicted velocities of +2.5 and -8.3 m/s, consistent with modelling results (McColgan et al. 2014 ) and measurements from previous studies (Carr and Konishi 1990 ). Nevertheless, we point out that while these depth-dependent changes in delay would be expected from multiple recordings from the same NM axon, they were not required for ITD sensitivity. Sensitivity to ITDs should be instead conferred by the differences in phase. Both extracellular neurophonic recordings and intracellular NM axonal recordings in NL revealed maps of ITD, created by precisely regulated conduction delays from intrinsically noisy elements. The microsecond precision needed to construct these maps of ITD appears to be achieved through precisely regulated phase delays, rather than by regulation of absolute latency. — Dijk, Pim van, Physiology, Psychoacoustics and…, 2857

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 ). — Dijk, Pim van, Physiology, Psychoacoustics and…, 2843

Categorization

• DEFINE: Clarifies the two distinct neural strategies by which coincidence detectors might encode interaural time differences: matched absolute latency vs. matched phase only.
• corroborated: 4 independent passages support this claim — Dijk, Pim van, Physiology, Psychoacoustics and…, 2857; Dijk, Pim van, Physiology, Psychoacoustics and…, 2843