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This excludes many of the simple stimulus patterns , such as gratings and white noise , often used to study vision but rarely encountered in nature . " Ultraseek 's advanced statistical weighting allows you to just type in relevant words and phrases , and the system will provide the answer to your query in the top few documents ! To anticipate , we show that the simple fact that near surfaces can occlude more distant ones , but not vice versa , has profound consequences for the assignment of depth to whole regions . In these studies , subjects were presented with images of colorful , complex stimuli in four nearby locations of the upper right quadrant of the visual field while they maintained fixation .