Discrimination of direction of motion in human vision.

1. Differences as low as 0.5 degrees can be discriminated in the direction of motion of a single spot of light moving with optimum speed and seen in the fovea for < 250 ms. There is no improvement for a cloud of random dots or a short line. 2. For high velocities the thresholds approach those for the discrimination of orientation of a single line, when the length of the line is equal to the excursion of the dot and when the line is shown for the same duration. 3. The sensitivity for orientation of line of motion of a moving spot also shares two other attributes with that for the orientation of a single solid line of similar temporal and spatial extents: discrimination is seriously impaired when flanked by related close-by stimuli, and sensitivity is subject to simultaneous orientation contrast. 4. It is suggested that the orientation both of features and of lines of motion is processed by the same mechanism.