The Role of Optic Flow in Visual Control of Steering toward a Goal

The visual control strategies for steering toward a goal include aligning the instantaneous direction of travel (i.e., heading; Gibson, 1950) or future path (Wann & Swapp, 2000) specified by optic flow with the target, equating the time-to-closure of the target-heading angle with the time-to-passage of the target (tau-equalization, Fajen, 2001), and centering the target in the perceived straight ahead (Rushton et al., 1998). In this talk, I present experiments that systematically examined and differentiated the use of these different strategies to determine the role of optic flow in visual control of steering toward a goal. In Experiment 1, the displays were rendered such that the target egocentric direction was fixed and thus unavailable for steering. In Experiment 2, the target drifted naturally on the screen, and the heading specified by optic flow was displaced 10° away from the participant's straight ahead. When the target egocentric direction was fixed, participants steered to align their heading with the target. When the target drifted on the screen, participants steered to center the target in their perceived straight ahead, which was on average shifted 32% toward the displaced heading specified by optic flow. Centering the target also generated 44% (375 ms) faster responses than aligning heading with the target. We found no evidence to support the use of the path or the tau-equalization strategy. Our data demonstrate that when target egocentric direction is available for steering, heading specified by optic flow is no longer used for steering. Instead, it is used for the recalibration of the perceived straight ahead in egocentric space.