Bill C, Braddick O J, 1998, "Can information about visual motion direction be combined across different speeds?" Perception 27 ECVP Abstract Supplement
Can information about visual motion direction be combined across different speeds?
C Bill, O J Braddick
The encoding of visual motion must include information about both direction and speed. We have studied the interaction of these differently encoded dimensions, by testing whether information from different speeds can be integrated in a directional judgment.
Directional performance was measured from the coherence threshold (% signal dots in noise) required to discriminate two directions of global flow, differing by +/-10° either side of vertical, in a random-dot kinematogram. In single-speed conditions, coherence threshold was found to be a U-shaped function of the speed of the signal dots between 0.5 and 64 deg s-1. In mixed-speed experiments, signal dots moved in a single direction but at two different speeds chosen to have comparable thresholds when tested alone: 'slow' (0.5 or 1 deg s-1) and 'fast' (8 deg s-1). In one experiment, signal dots of one speed were added to a constant subthreshold number of dots of either the same or different speed. Thresholds were lower when the added dots and the non-varying dots had the same speed than when they were mixed, suggesting little or no integration of directional information across speeds. In a second experiment, the total number of signal dots in the mixed signal was equally divided between slow and fast dots. In this case, some observers showed as low a threshold for the mixed as for a single speed, which implies effective integration.
These results suggest that visual motion processing may be able to operate with alternative strategies, either separating information from different speeds or combining them.
[Supported by a BBSRC studentship and MRC project grant G9509331.]
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