Abstract
This report describes the binaural basis of the auditory space map in the optic tectum of the barn owl (Tyto alba). Single units were recorded extracellularly in ketamine-anesthetized birds. Unit tuning for interaural differences in timing and intensity of wideband noise was measured using digitally synthesized sound presented through earphones. Spatial receptive fields of the same units were measured with a free field sound source. Auditory units in the optic tectum are sharply tuned for both the azimuth and the elevation of a free field sound source. To determine the binaural cues that could be responsible for this spatial tuning, we measured in the ear canals the amplitude and phase spectra produced by a free field noise source and calculated from these measurements the interaural differences in time and intensity associated with each of 178 locations throughout the frontal hemisphere. For all frequencies, interaural time differences (ITDs) varied systematically and most strongly with source azimuth. The pattern of variation of interaural intensity differences (IIDs) depended on frequency. For low frequencies (below 4 kHz) IID varied primarily with source azimuth, whereas for high frequencies (above 5 kHz) IID varied primarily with source elevation. Tectal units were tuned for interaural differences in both time and intensity of dichotic stimuli. Changing either parameter away from the best value for the unit decreased the unit's response. The tuning of units to either parameter was sharp: the width of ITD tuning curves, measured at 50% of the maximum response with IID held constant (50% tuning width), ranged from 18 to 82 microsecs. The 50% tuning widths of IID tuning curves, measured with ITD held constant, ranged from 8 to 37 dB. For most units, tuning for ITD was largely independent of IID, and vice versa. A few units exhibited systematic shifts of the best ITD with changes in IID (or shifts of the best IID with changes in ITD); for these units, a change in the value of one parameter to favor one ear shifted the best value of the other parameter in favor of the same ear, i.e., in the direction opposite to that expected from “time-intensity trading.” Overall sound intensity had little or no effect on ITD tuning, but did increase the best IIDs of units tuned to nonzero IIDs. The tuning of units for ITD and IID changed systematically along different dimensions of the optic tectum to create coextensive, independent neurophysiological maps of ITD and IID.(ABSTRACT TRUNCATED AT 400 WORDS)
