Perceived group size is determined by the centroids of the component elements

Abstract

To accomplish the deceptively simple task of perceiving the size of objects in the visual scene, the visual systemcombines information about the retinal size of theobject with several other cues, including perceiveddistance, relative size, and prior knowledge. When localcomponent elements are perceptually grouped to formobjects, the task is further complicated because agrouped object does not have a continuous contourfrom which retinal size can be estimated. Here, weinvestigate how the visual system solves this problemand makes it possible for observers to judge the size ofperceptually grouped objects. We systematically varythe shape and orientation of the component elements ina two-alternative forced-choice task and find that theperceived size of the array of component objects can bealmost perfectly predicted from the distance betweenthe centroids of the component elements and the centerof the array. This is true whether the global contourforms a circle or a square. When elements werepositioned such that the centroids along the globalcontour were at different distances from the center,perceived size was based on the average distance. Theseresults indicate that perceived size does not depend onthe size of individual elements, and that smoothcontours formed by the outer edges of the componentelements are not used to estimate size. The currentstudy adds to a growing literature highlighting theimportance of centroids in visual perception and mayhave implications for how size is estimated forensembles of different objects.