Because of object constancy, the change of the size of an object usually means the change of the distance between the object and the observer. Comparing to receding objects, looming objects (such as predators, competitors, etc.) must be perceived and responded in priority. From the evolutionary perspective, expanding signals should be more important than contracting signals in survival for organisms. Consistent with this view, previous researches have demonstrated that organisms are more sensitive and show a cognitive bias to expanding signals, which is defined as expanding priority.
However, most stimuli used in previous studies were solid objects. When solid objects expand, their areas also expand, thus we can't exclude the influence of physical area change. As a result, in this thesis we used subjective contours as experimental stimuli whose physical areas keep constant even when they expand. We manipulated the distance between the four inducers of the subjective contour to change the perceived size of the stimulus.
Study 1 contained 3 experiments, which used subjective squares, normal squares and solid squares as experimental stimuli respectively. Subjects were asked to complete a dynamic visual searching task, in which they should judge whether there was a expanding stimulus in several contracting stimuli, or whether there was a contracting stimulus in several expanding stimuli. Similar search asymmetry were found for subjective squares and solid squares. That is, subjects can find out an expanding target in contracting distractors more accurately and quickly, compared to searching for a contracting target in expanding distractors. However, the expanding priority of square contour was weaker, which only appears in accuracy, not in reaction time. This study found that subjective contours whose physical areas keep constant can generate similar expanding priority as solid objects. The results suggested that perceived and real expanding had the same priority and the degree of physical area change had nothing to do with the intensity of expanding priority.
Study 2 explored whether the search asymmetry found in research 1 can be influenced by the type of inducers, the size of subjective contours or motion of inducers per se. In experiment 4, classic Kanizsa Pacman subjective contours were used as experimental stimuli. The results demonstrated stronger expanding priority than experiment 1，indicating that stronger inducers can lead to stronger expanding priority. In experiment 5, subjects were required to complete a static search task. Results showed that subjects were able to find out a bigger target in smaller distractors more accurately and quickly, compared to searching for a smaller target in bigger distractors. In experiment 6, we found that the search asymmetry disappeared when the subject contours were impaired by changing the orientations of four inducers, indicating that the motion of the inducers per se cannot lead to expanding priority, which ruled out the possibility of aggregating priority. It can be proved that the inducers were grouped correctly and perceived as a whole which can produce expanding priority.
In Study 1 we found the expanding priority of square contours was weaker than that of subjective squares and solid squares. We argue that the solidness of the expanding stimuli might play an important role. In Study 3, we try to manipulate the solidness of the experimental stimuli by using a background of dot texture. In experiment 7, we reduced the solidness of border stimuli, finding the expanding priority enhanced. In experiment 8 and 9, we found weaker expanding priority effects when solidness were decreased. Research 3 proved that thesolidness of stimuli could influence expanding priority. If the stimulus is more like a solid surface, the expanding priority is stronger. If the stimulus is more like a penetrable contour, the expanding priority is weaker.
In this thesis, we found that subjective contours whose physical areas keep constant could generate similar expanding priority effect as solid objects did. The solidness of the expanding stimuli plays an important role. More solid the expanding stimuli are, stronger expanding priority effect could be observed. Furthermore, it seemed that subjective contours were not only represented as merely contours but also represented as solid-like surfaces.