Abstract

In a previous study (ARVO, 1989), we found that contrast thresholds for discriminating upward- from downward-moving isoluminant gratings (motion discrimination) are 4 to 16 times the thresholds measured for grating detection. We also found that minimum and maximum thresholds for motion discrimination fell on azimuths intermediate between those which define the isoluminant plane in Krauskopf-Lennie space. These results were obtained using 1.3-c/d, 13.75-Hz gratings produced by modulation of chromaticity around an 18-cd/m² white point and presented in a 2.3 × 2.3° field located 0.75° from fixation. We now report that, in the same conditions, the contrast required for discrimination of the orientation (horizontal vs vertical) of moving isoluminant gratings is at or near that required for grating detection. Furthermore, preliminary measurements at an azimuth for which motion discrimination is poorest indicate that the discrimination of a moving from a stationary grating occurs at a contrast well above that required for grating detection, but below that required for motion discrimination. Thus, the failures of up-vs-down motion discrimination at contrasts well above those required for grating detection are related specifically to the processing of motion information in isoluminant gratings and cannot be attributed to failures of the visual system to encode an adequate representation of the spatial structure of the stimuli.

© 1989 Optical Society of America