Steven Shofner¹, Paul Dassonville¹; ¹University of Oregon

Images consist of visual information at multiple scales, from fine details to gestalt. Reaction time (RT) studies of the perceptual mechanisms associated with these different levels typically find that information at the global scale is processed more quickly than information at the local scale (the global precedence effect), and that the impact of irrelevant global information on a local judgment is greater than the opposite (global interference effect). However, reaction times are a blunt measure of these effects across all stages of processing, from early sensory analysis to eventual motor response. The present study is an attempt to more precisely measure the time course of local and global processing, by employing a finger tracking task. Participants began each trial by initiating a tracing movement upward on a touch screen. Shortly after movement onset, a hierarchical C (a large C made of small c’s, both of which could be open to the right or left in a mirror-reversed fashion) was presented, with the participant required to quickly and accurately alter the trajectory of the ongoing movement toward one of two target locations on the left or right, as indicated by the orientation of the c’s at the attended level (local or global, in separate blocks). Trajectory analysis indicated that global-level orientation information began to be incorporated into the response ~300ms after stimulus onset, and local-level information in ~335ms, yielding a global precedence effect of ~35ms (smaller than the ~60ms global precedence measured in an RT task using the same stimuli). In contrast, the tracking task exhibited a larger global interference effect (~30ms) than in the RT task (~15ms). These findings provide a clear demonstration that a finger tracking task can be used to more precisely determine the time course at which global and local visual information is available to guide movements.