PROCESSING STREAMS IN EARLY VISION

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DESCRIPTION (Investigator's Abstract): Recent evidence indicates that the early stages in visual processing may be broken into several parallel streams that are specialized for the analysis of different visual attributes. The research proposed here will investigate the routing of information through these early processing streams using five stimulus media: luminance, color, texture, stereo and motion. In particular, the investigator is interested in the relative contributions of these early streams to the final determination of stimulus shape features (specifically size and orientation), motion and depth, and whether the measurement of shape, motion, and stereo emerges several times (repeated within each pathway) or only once in a common representation. Opposed stimuli, induction and adaptation paradigms will be used to evaluate the degree of independence of the analyses for each medium. Preliminary experiments suggest that a similar analysis of 2-D shape features is actually repeated for each medium. One reason for this duplication of analyses may be that pooling information from several media can improve the accuracy of contour localization particularly when luminance information is unreliable. A second reason is that using similar codes or image formats for each representation will facilitate their subsequent integration. On the other hand, another pilot experiment suggests that multiple, medium-specific analyses do not occur for motion. The situation may be similar for analyses of binocular disparity. If a medium is analyzed independently of other media for, say, orientation, the investigator will also look for independence of coding within that medium. For example, if the orientation of color-defined borders is analyzed independently of the orientation of borders defined in other media, it may be that the mechanisms responsible are selective for the orientation of any color border no matter which colors are involved -- that is, they may be specific to color but not selective for particular colors (no independent coding). Alternatively, they might be both color-selective as well as orientation-selective (independence). The same opposing induction and adaptation paradigms mentioned above will be used to evaluate the independence of analysis for different values within a medium. The results of these studies will be compared to a set of corresponding experiments on visual search for conjunctions. These visual search tasks will conjoin the same values (e.g. color and orientation, color and motion) as those tested in the opposing induction and adaptation experiments. The prediction is that independence in the opposing adaptation experiments will correlate with slow, serial search in the conjunction task. The explanation lies in the heterogeneity of the distractor groups. In addition to psychophysical tasks with normal observers, the investigators propose to test patients with known losses (in particular, cortical color blindness) to verify the routing of information. Where supported by the data, the investigators shall extend these tests as a measure of relative function of different visual pathways that may be diagnostic of particular diseases.