An instructive signal that calibrates the neural presentation of auditory space in the midbrain of the barn owl

When barn owls are raised wearing prismatic spectacles that horizontally displace the visual field, the auditory receptive fields of neurons in the optic tectum (OT) and external nucleus of the inferior colliculus (ICX) shift to compensate for the optical displacement of the field. This shift results from a change in the tuning of ICX neurons for interaural time difference (ITD). The experiments presented here examine how the visually-based instructive signal that guides these changes is generated and provided. In the experiments described in Chapter 2, I subjected owls to optical conditions that differed in the center of gaze and the visual periphery to determine whether the instructive signal is based on a topographic comparison between auditory and visual space maps or on a foveation-dependent visual assessment of the accuracy of auditory orienting responses. In 6 of 7 experiments, adaptive changes were as predicted by a topographic, template-based instructive signal. In the experiments described in Chapter 3, I used tracer injections to characterize a previously unknown projection from the OT to the ICX This projection is topographic and in register with the feed-forward projection from the ICX to the OT. Most of the tectal neurons that make up this projection extend dendrites superficially into the retino-recipient laminae and deep into the auditory recipient laminae. Thus, they are positioned to receive both visual and auditory information from particular locations in space, and could provide the topographic, template-based instructive signal to the ICX In the experiments described in Chapter 4, I made restricted, unilateral lesions in the portion of the optic tectum that represents frontal space. In 5 of 6 experiments, these lesions specifically eliminated plasticity in the portion of the map representing frontal space on the same side of the brain. Thus, the OT provides a signal that controls adaptive adjustment of the auditory space map in a location-specific manner. Taken together, these data are consistent with the hypothesis that the instructive signal that calibrates the auditory space map in the ICX is a retinotopic activity template provided by the topographic projection from the OT.