EXPERIENCE-DEPENDENT PLASTICITY IN THE INFERIOR COLLICULUS - A SITE FOR VISUAL CALIBRATION OF THE NEURAL REPRESENTATION OF AUDITORY SPACE IN THE BARN OWL

The optic tectum (homolog of the superior colliculus) contains mutuall y aligned neural maps of auditory and visual space. During development , the organization of the auditory map is guided by spatial informatio n provided by vision: barn owls raised wearing prismatic spectacles, w hich optically shift the visual field and the visual map in the optic tectum, develop an auditory map that is shifted by an approximately eq uivalent amount, such that alignment between the two maps is preserved (Knudsen and Brainard, 1991). In this study we investigated whether t his shift in the auditory map is intrinsic to the optic tectum or whet her it reflects plasticity at an earlier stage in the auditory pathway . Owls were raised wearing prismatic spectacles that displaced the vis ual field by 23-degrees to the left or right. This manipulation alters the normal correspondence between locations in the visual field and i nteraural time difference (ITD), the primary cue for the azimuth of a sound source. In normal owls and in owls with at least 150 d of prism experience, extracellular unit recordings were used to assess the repr esentations of ITD at anatomically and physiologically defined sites i n the optic tectum and in the two prior stages of the auditory pathway , the external and central nuclei of the inferior colliculus (ICx and ICc). In the optic tectum of normal owls, the values of ITD to which u nits responded most strongly (best ITDs) varied systematically with th e azimuths of unit visual receptive fields (VRFs). In the prism-reared owls, best ITDs were shifted from normal toward the values of ITD pro duced by sounds at the locations of the units' optically displaced VRF s. In the ICx of prism-reared owls, the representation of ITD also was shifted from normal, by an amount and in a direction that could compl etely account for the shift in ITD measured in the optic tectum. At so me sites in the ICx, the shift in ITD tuning was apparent within the f irst 7-8 msec of the response; shifted tuning at such short latencies argues that the altered representation of ITD in the ICx reflects plas ticity in the ascending auditory pathway, and is not the result of des cending activity from higher auditory centers. In the ICc, which immed iately precedes the ICx in the ascending pathway, the representation o f ITD was normal. The results indicate that the visual instruction of auditory spatial tuning of neurons in the optic tectum reflects plasti city at the level of the ICx, the site where the auditory map of space is first synthesized.