The role of speed lines in subtle direction judgments

Stimuli moving in slightly different directions trace trajectories that differ slightly in orientation. These different 'speed lines', in principle, could generate responses in orientation mechanisms, and such responses could determine how well we judge subtle direction differences. Alternatively, the ability to judge subtle direction differences could be determined by direction mechanisms rather than by orientation mechanisms. To distinguish between these possibilities we exploited the fact that opposite directions of motion share an orientation: Across trials, participants judged a constant orientation difference between trajectories having either the same or opposite motion signs. The probabilities of the motion signs were also manipulated. When the probabilities were consistent with those typically used to assess fine direction discrimination, direction mechanisms set the limit on performance. In other conditions where orientation mechanisms could have set the limit on performance, responses were neither more precise nor faster than when performance was limited by direction mechanisms.