Abstract

When two drifting cosine gratings with different orientations are superimposed they can form a coherently moving 2-D pattern (plaid). The resultant speed of such a pattern increases as the angle (θ) between the component directions becomes broader, provided that the speed of the components does not change. It has been reported that speed discrimination for moving plaids is limited by noise in the component speed signals.¹ Thus, one would predict that speed discrimination for plaids would depend on component speed, but not on θ. We measured speed discrimination for both plaids and gratings using a 2AFC paradigm. We found that for plaids with slow moving components (drift rate =1.0 Hz), speed discrimination improves with increasing θ. For example, we found that speed discrimination Weber fractions (ΔV/V) for a grating drifting at 1.0 Hz were around 20%, but decreased to 12% for a plaid with 1.0-Hz components and θ = ±75°. In this case, the speed of the plaid was four times the speed of the components, however, speed discrimination was not as good as for a 4.0-Hz grating (ΔV/V = 6%). It is still possible to account for these results based on component speed uncertainty because, when component uncertainty is propagated through a transformation such as the intersection-of-constraints, the variance in the resultant does not increase as fast as its mean speed. Therefore, the Weber fraction for discriminating the speed of the resultant may decrease by a factor of √2.

© 1989 Optical Society of America