The Journal of Neuroscience, January 15, 2000, 20(2):862-877
Department of Neurobiology, Stanford University, Stanford,
California 94305-5125
Early auditory experience shapes the auditory spatial tuning of
neurons in the barn owl's optic tectum in a frequency-dependent manner. We examined the basis for this adaptive plasticity in terms of
changes in tuning for frequency-specific interaural time differences
(ITDs) and level differences (ILDs), the dominant sound localization
cues. We characterized broadband and narrowband ITD and ILD tuning in
normal owls and in owls raised with an acoustic filtering device in one
ear that caused frequency-dependent changes in sound timing and level.
In normal owls, units were tuned to frequency-specific ITD and ILD
values that matched those produced by sound sources located in their
visual receptive fields. In contrast, in device-reared owls, ITD tuning
at most sites was shifted from normal by ~55 µsec toward open-ear
leading for 4 kHz stimuli and 15 µsec toward the opposite-ear leading
for 8 kHz stimuli, reflecting the acoustic effects of the device. ILD tuning was shifted in the adaptive direction by ~3 dB for 4 kHz stimuli and 8 dB for 8 kHz stimuli, but these shifts were substantially smaller than expected based on the acoustic effects of the device. Most
sites also exhibited conspicuously abnormal frequency-response functions, including a strong dependence on stimulus ITD and a reduction of normally robust responses to 6 kHz stimuli. The results demonstrate that the response properties of high-order auditory neurons
in the optic tectum are adjusted during development to reflect the
influence of frequency-specific features of the binaural localization
cues experienced by the individual.
Key words: sound localization; hearing impairment; development; auditory plasticity; superior colliculus; Tyto alba