Neural map of interaural phase difference in the owl's brainstem

Neurons of the barn owl's (Tyto alba) nucleus laminaris, the first site of binaural convergence, respond in a phase-locked fashion to a tone delivered to either ear. It may take longer to elicit phase-locked spikes from one ear than from the other. This disparity in delay differs from neuron to neuron and is independent of tonal frequency. In binaural stimulation, neurons respond best when sound in one ear leads that in the other by an amount equal to their delay disparities but opposite in sign. This condition causes simultaneous arrival of phase-locked spikes from the two sides. Laminaris neurons can thus be described as coincidence detectors. The phase of a tone-induced evoked potential, termed "neurophonic," varies systematically with position in nucleus laminaris. From dorsal to ventral within the nucleus, the phase delay of a contralaterally elicited potential decreases and that of its ipsilateral counterpart increases. Therefore, if the neurophonic delay is due to the delay of phase-locked spikes, an orderly representation of delay disparities is shown. Because they act as coincidence detectors, laminaris neurons should show selectivity for interaural phase difference based on their place in the nucleus. Thus, nucleus laminaris presumably measures and maps interaural phase differences by using the principles of delay lines and coincidence detection.