Hierarchical processing of horizontal disparity information in the visual forebrain of behaving owls

According to their restricted receptive fields and input-filter characteris tics, disparity-sensitive neurons at early processing levels of the visual system perform rather ambiguous computations; they respond vigorously to di sparity in false-matched images and show multiple response peaks in their d isparity-tuning profiles. On the other hand, the perception of depth from b inocular disparity is reliable, thus raising the question as to where and h ow in the brain additional processing is accomplished leading toward behavi orally relevant disparity detection. To address this issue, tuning data dur ing stimulation with correlated and anticorrelated random-dot stereograms ( a-RDS) were obtained from 52 disparity-sensitive visual Wulst neurons in th ree behaving owls. From the disparity-tuning curves, several quantitative m easures were derived that allowed to determine the response ambiguity of a cell. A systematic decline of response ambiguities with increasing response latencies was observed. An increase in response latencies of neurons was c orrelated with a decrease of the strength of responses to a-RDS. Declining responses to a-RDS are expected for global detectors, because an owl was no t able to discriminate depth in psychophysical tests with a-RDS. In additio n, suppression of response side peaks was increased and disparity tuning wa s enhanced with growing response latencies. These results suggest a functio nal hierarchy of disparity processing in the owl's forebrain, leading from spatial filters to more global disparity detectors that may be able to solv e the correspondence problem. Nonlinear threshold operations and inhibition are proposed as candidate mechanisms to resolve coding ambiguities.