Evidence for Two Speed Signals: A Coarse Local Signal for Segregation and a Precise Global Signal for Discrimination

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Examined whether an initial coarse estimate of local speed can segregate the motions of different objects prior to a precise calculation of object speed. In Exp 1, each of 256 dots alternated between 2 speeds every 133 msec. When speed alternations were asynchronous across dots, Ss (the authors and a naive observer) saw 2 transparent surfaces moving at different speeds, and their ability to discriminate changes in the slow speed were unaffected by the presence of the fast speed. In Exp 2, dots with 133-msec lifetimes were used to generate the 2 speeds. Although individual dots permitted only crude speed discrimination, Ss perceived this stimulus as 2 surfaces moving at different speeds and they precisely judged the slower speed. In Exp 3, the dots again moved at 2 speeds, each generated by a range of spatial and temporal displacements. Ss saw 2 surfaces and precisely judged the speed of the slower surface.



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