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Horizontal-disparity tuning of neurons in the visual forebrain of the behaving barn owl.

Stereovision plays a major role in depth perception of animals having frontally-oriented eyes, most notably primates, cats, and owls. Neuronal mechanisms of disparity sensitivity have only been investigated in anesthetized owls so far. In the current study, responses of 160 visual Wulst neurons to static random-dot stereograms (RDS) were recorded via radiotelemetry in awake, fixating barn owls. The majority of neurons (76%) discharged significantly as a function of horizontal disparity in RDS. The distribution of preferred disparities mirrored the behaviorally relevant range of horizontal disparities that owls can exploit for depth vision. Most tuning profiles displayed periodic modulation and could well be fitted with a Gabor function as expected if disparity detectors were implemented according to the disparity energy model. Corresponding to this observation, a continuum of tuning profiles was observed rather than discrete categories. To assess a possible clustering of neurons with similar disparity-tuning properties, single units, and multi-unit activity recorded at individual recording sites were compared. Only a minority of neurons were clustered according to their disparity-tuning properties, suggesting that neurons in the visual Wulst are not organized into columns by preferred disparity. To assess whether variable vergence eye movements influenced tuning data, we correlated tuning peak positions on a trial-by-trial basis for units that were recorded simultaneously. The general lack of significant correlation between single-trial peak positions of simultaneously recorded units indicated that vergence, if at all, had only a minor influence on the data. Our study emphasizes the significance of visual Wulst neurons in analyzing stereoscopic depth information and introduces the barn owl as a second model system to study stereopsis in awake, behaving animals.