Contrast detection and direction discrimination of drifting gratings

Observers performed simple detection and left/right discrimination of drifting sinusoidal gratings. Ratio of detection to discrimination sensitivities was measured under variations in several experimental parameters. In the first experiment, it was found that combinations of spatial and temporal frequency which resulted in the same velocity produced similar detection-discrimination ratios. At an exposure duration of 800 msec, the relationship between the ratio and velocity described a power function with the intercept at 0.6 sec-1. Decreasing duration shifted the curve to higher velocities. I examined the effect of grating orientation in a second experiment. Visual sensitivity was poorer for oblique than for vertical gratings with detection and discrimination exhibiting similar size anisotropies. In a third experiment, observers viewed gratings presented to different retinal loci. Visual performance in both detection and discrimination fell with greater eccentricity. However, motion discrimination fell more steeply resulting in an increase in the ratio. The results demonstrate that form and motion analyzing mechanisms cannot be distinguished by their response to changes of spatial frequency, orientation or retinal locus.