| Summary: | The detection of visual stimuli involves neurons which are selectively responsive to components of a visual scene. In the early stages of visual processing, it is commonly accepted that neurons respond to the changes in luminance associated with objects and object boundaries. However, recent experiments have demonstrated that some neurons can also respond to features which are not defined by luminance variations. These features are termed "second-order" because they require more complex processing, and neurons which respond to second-order features are necessarily nonlinear. === In this thesis, I undertook a three dimensional physiological characterization (i.e. tuning of orientation, spatial frequency and temporal frequency) of such nonlinear neurons in order to shed light on their processing capabilities. In particular we sought to address the following issues: (1) whether the temporal and spatial properties underlying second-order motion are similar to those underlying luminance based ("first-order") motion; (2) whether these properties remain constant using different types of second-order stimuli, suggesting that neurons' responses are invariant to the physical attributes comprising the stimulus; and (3) whether second-order processing is a cortical mechanism or can occur at an earlier stage of the visual system (e.g. in the lateral geniculate nucleus). Taken together these results have a dual function; they provide insight into the complex cellular processing of higher order features, and they provide a general framework for the generation of second-order models.
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