Axis-of-motion affects direction discrimination, not speed discrimination

The motion of an object can be described by a single velocity vector, or eq uivalently, by direction and speed separately. Similarly, our ability to se e subtle differences in the motion of two objects could be constrained by e ither a velocity-based sensory response, or separate sensory responses to d irection and speed. To distinguish between these possibilities we investiga ted whether direction discrimination and speed discrimination were differen tially affected by changes in the axis-of-motion. Psychophysical data from 12 naive observers indicated that direction discrimination depended on axis -of-motion, but speed discrimination did not. The difference suggests that a velocity-based sensory response is not the limiting factor on the two tas ks. Instead, the results imply that the sensory response which constrains s peed discrimination is at least partially independent from the sensory resp onse which constrains direction discrimination. (C) 1999 Elsevier Science L td. All rights reserved.