Cellular mechanisms for resolving phase ambiguity in the owl's inferior colliculus

Both mammals and birds use the interaural time difference (ITD) for localiz ation of sound in the horizontal plane. They may localize either real or ph antom sound sources, when the signal consists of a narrow frequency band. T his ambiguity does not occur with broadband signals. A plot of impulse rate s or amplitude of excitatory postsynaptic potentials against ITDs (ITD curv e) consists of peaks and troughs. In the external nucleus (ICX) of the owl' s inferior colliculus, ITD curves show multiple peaks when the signal is na rrow-band, such as tones. Of these peaks, one occurs at ITDi, which is inde pendent of frequency, and others at ITDi +/- T, where T is the tonal period . The ITD curve of the same neuron shows a large peak (main peak) at ITDi a nd no or small peaks (side peaks) at ITD +/- T, when the signal is broadban d. ITD curves for postsynaptic potentials indicate that ICX neurons integra te the results of binaural cross-correlation in different frequency bands. However, the difference between the main and side peaks is small. ICX neuro ns further enhance this difference in the process of converting membrane po tentials to impulse rates. Inhibition also appears to augment the differenc e between the main and side peaks.