Roles of attention and form in visual motion processing: Psychophysical and brain imaging studies

INTRODUCTION It goes without saying that the human visual information processing system is very complex. Marr (1982) proposed that several principles should be taken into consideration to improve understanding of this complex system. These include the modularity principle and the feedforward principle. The modularity principle assumes that the visual system consists of several processes that are relatively independent of each other in early stages of visual processing (frontend processing). Therefore, each of these processes can be largely examined independently without considering the others. The feedforward principle assumes that the majority of visual functions can be understood without taking feedback effects into consideration. While research that follows these principles has greatly contributed to understanding some important aspects of visual information processing, other important aspects of information processing have been relatively neglected, especially, interactions between different processes and feedback from higher to lower levels (however, see Grossberg & Mingolla, 1985; Koch & Davis, 1994). Research on motion perception is not exceptional in this sense. Over the last decade, as in other aspects of visual processing, the majority of research on the processing of velocity of a moving object has mainly explored feedforward processing within the motion module (for a review, Hildreth & Koch, 1987; Nakayama, 1985; Snowden, 1992).