Abstract
Barn owls can localize their prey by ear. The physical cues for localization are the interaural time difference (ITD) for the horizontal plane and the interaural level difference (ILD) for the vertical plane. The owl's auditory system processes ITD and ILD in separate parallel pathways. Time is encoded by phase-locked spikes, which are action potentials that occur at particular phase angles of tonal stimuli. To detect ITDs, the owl uses special neural circuits consisting of coincidence detectors and axonal delay lines. Simultaneous arrival of spikes from the two sides causes coincidence detectors to fire. Axonal paths to different coincidence detectors are adjusted such that a coincidence occurs when the sum of acoustic and neural transmission delays is the same for left and right sides. Coincidence detectors respond not only to one particular ITDi and but also to ITDi ± nT, where n is an integer and T is period, because ITDi and ITDi ± nT indicate the same phase difference. This problem, called phase-ambiguity, is resolved in one part of the midbrain where different frequency bands converge on single neurons, endowing them with the ability to discriminate between ITDi and ITDi ± nT. These same neurons also respond selectively to combinations of ITD and ILD, resulting in the creation of spatial receptive fields. Analysis of stimulus induced postsynaptic potentials in these space-specific neurons show that potentials due to input from the ITD pathway and those due to input from the ILD pathway are multiplied. The space-specific neurons are not randomly distributed but organized into a map of auditory space. [Jpn J Physiol 54 Suppl:S2 (2004)]
