The Interaction of Motion and Form in the Perception of Global Structure: A Glass-Pattern Study

This dissertation, "The Interaction of Motion and Form in the Perception of Global Structure: a Glass-pattern Study" by Chun-fai, Charles, Or, 柯駿輝, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled "The interaction of motion and form in the perception of global structure: a Glass-pattern study" Submitted by Or Chun Fai, Charles B.Cog.Sc. H.K. for the degree of Master of Philosophy at The University of Hong Kong in December 2005 The processing of visual motion and form is commonly thought to occur in two separate pathways (dorsal and ventral streams) in the visual cortex. This view has been recently re-assessed on the evidence that the analysis of visual information in brain areas along these pathways is not limited to a particular visual attribute, but is specialised for a number of different attributes. For example, brain areas responsible for detecting motion (e.g., MT) also respond to form cues such as the orientation of stationary lines, and similarly those areas responsible for detecting orientation can be stimulated by motion. Given that brain areas are not exclusively limited to the analysis of a particular visual attribute, this thesis addresses whether interactions between motion and form occur when detecting the global structure of patterns undergoing motion. Glass patterns formed by oriented dot-pairs, or dipoles, were used as stimuli. In the first experiment, observers were required to discriminate either the perceived direction of motion (Experiment 1.1-1.2) or dipole orientation (Experiment 1.3) of a frontoparallel Glass pattern, as a function of the angular difference between the orientation and motion direction of dipoles. Perceived motion direction was examined with Glass patterns consisting of same- and opposite-polarity dot pairs. In the latter pattern, the perception of global structure is much reduced and thus form may exert no, or only a minimal, effect on global motion (Experiment 1.2). The results show that the apparent direction of motion was attracted towards the dipole orientation for small angular differences, and surprisingly this effect was comparable regardless of the polarity of dot pairs. However, under the same stimulus conditions, the apparent dipole orientation remained less affected by motion direction. These results indicate that motion and form interact, and that form has a stronger influence on motion perception. As shown in Experiment 1, despite the difficulty in perceiving the global structure of an opposite-polarity Glass pattern when it is stationary, the global structure is detectable when the pattern is set in motion. This finding suggests that the visual system is capable of extracting local orientation from opposite-polarity dipoles with greater efficiency due to the contribution of motion. In Experiment 2 to investigate the role of contrast in local grouping, the contrast of one dot, relative to the other, forming a dipole was varied to assess its impact on the detection of global structure in static and dynamic Glass patterns. Regardless of the contrast variation, the perception of global structure was more salient with dynamic than static Glass patterns. Salience was also improved when both dots in a dipole were lighter than the background, and the global Glass-pattern structure was still visible in spite of the change in polarity of one dot within a pair. These results demonstrate that while motion and form interact, motion processing mechanisms are more powerful in tolerating contrast differences in favour of the perception of glo