Velocity discrimination in chromatic gratings and beats

It is commonly assumed that the ability to discriminate velocity in a stimulus directly reflects the properties of the underlying directionally-selective mechanism. The results presented here show that this assumption is not always correct. Speed discrimination tasks over a range of base velocities were carried out for luminance gratings, chromatic gratings and contrast (beat) gratings of equivalent periodicity and contrasts. At low contrasts (0.5 log units above detection threshold), speed discrimination in luminance gratings was at least twice as good (when expressed as a Weber fraction), than in either chromatic gratings or beats. This is similar to the situation seen for tasks of direction discrimination using these stimuli [e.g. Cropper and Derrington (1990) Perception, 19, A31]. When the stimulus contrasts were increased to 1.5 log units above detection threshold, the ability to discriminate speed in both chromatic and beat stimuli improved to a performance level comparable to that shown for luminance gratings at all contrasts. This effect is not seen for tasks of direction discrimination when the same increase in stimulus contrast has little effect on the lower threshold of motion (LTM) measured for beat patterns. These results indicate that the ability to discriminate velocity in a stimulus does not necessarily directly reflect the characteristics of the ability to discriminate the direction of motion of that stimulus.