Evidence for a mechanism sensitive to the speed of cyclopean form

We measured Weber fractions for discriminating the speed of cyclopean grati ngs and Weber fractions for discriminating the speed of luminance gratings. Of our 14 observers, five were unable to see the cyclopean grating suffici ently well to discriminate its speed. One observer experienced great diffic ulty in discriminating the speed of cyclopean gratings, even though her thr eshold for detecting cyclopean gratings was low, and even though she discri minated the speed of luminance gratings on the basis of the task-relevant v ariable. But several observers based their speed discriminations on trial-t o-trial variations of the task-relevant variable while ignoring associated trial-to-trial variations in all task-irrelevant variables (specifically: d isplacement; temporal frequency; spatial frequency; and presentation durati on). We conclude that the visual systems of these observers contain a speci alized neural mechanism for the speed of cyclopean gratings that supports a cute discriminations of speed (Weber fractions were as low as 0.05-0.07). ( C) 1998 Elsevier Science Ltd. All rights reserved.