Preattentive Processing as a Determinant of Global and Local Precedence.

Abstract

A visual stimulus can be described as having multiple levels of structure. Some are relatively global, some relatively local. An important question for investigators of visual form perception concerns the temporal order in which these levels are processed. Does the perception of an object start with global aspects and proceed toward local detail, or does it begin with fine details and build up to more global levels of structure? The answer to this question now seems to depend on the value of certain variables. For example, visual angle (Kinchla and Wolfe, 1979) and the quality of information at different levels of structure (Hoffman, 1980) are important factors. The two experiments in this dissertation show that early, preattentive processing is also a determinant of perceptual precedence. Two types of preattentive operations were studied: localization and figure-ground separation. It was predicted that localization would bias perception toward global precedence, while figure-ground separation would bias perception toward local precedence. In each case, the bias was thought to result from information accumulated during preattentive operations. For example, spatial localization was assumed to depend on a crude, global analysis of the visual input. This would give global information an early lead. Both experiments used a task and stimuli similar to those of Navon (1977). Stimuli were large characters composed of smaller characters. The task was a Stroop-type task in which subjects had to identify the character at one level of structure and ignore the other level. Reaction time was the dependent measure. In Experiment I, the subject's foreknowledge of the stimulus location was manipulated by cuing. As predicted, global precedence was greater than when there was spatial uncertainty. In Experiment II, the stimulus was embedded in a background of noise characters. In one condition, figure-ground separation was easy. In another condition, it was difficult. Results, though not completely in accord with predictions, showed that difficulty in figure-ground separation led to increased local precedence. In summary, this research demonstrates that preattentive processing is a powerful determinant of perceptual precedence. Stimulus localization may be partly responsible for several recent findings of global precedence.