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 Previous Article
Journal of Neuroscience, Vol 9, 2591-2605, Copyright © 1989 by Society for Neuroscience
Neural maps of interaural time and intensity differences in the optic tectum of the barn owl
JF Olsen, EI Knudsen and SD Esterly
Department of Neurobiology, Stanford University School of Medicine, California 94305.
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)
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