Effects of contrast and temporal frequency on orientation discrimination for luminance and isoluminant stimuli

We compared the mechanisms responsible for orientation discrimination of stimuli defined by luminance and red/green isoluminant contrast. A four-alternative forced-choice (4-AFC) paradigm was used to determine thresholds for discriminating 1 cpd sinewave gratings differing in orientation, contrast, or both. When measuring orientation thresholds as a function of stimulus contrast, we found a decrease in thresholds with increasing stimulus contrast. For three temporal frequencies (0, 1, and 8 Hz) the functions relating orientation thresholds to stimulus contrast had similar shapes for luminance and isoluminant gratings, indicating similar processing mechanisms. Thresholds for stationary or slowly moving gratings were consistently lower for isoluminant than for luminance gratings, when contrast was expressed on an absolute RMS-cone-contrast scale. When contrast was defined as multiples of detection thresholds, discrimination was slightly better for luminance gratings. Thresholds for fast moving gratings were similar, irrespective of the definition of contrast. In contrast to previous work, we found a marked "oblique-effect" for both luminance and isoluminant gratings, when measuring discrimination thresholds as a function of standard orientation. Finally, we measured discrimination thresholds for gratings that varied in contrast and orientation simultaneously. The shapes of the resulting two-dimensional threshold contours were similar for luminance and isoluminant gratings, indicating again that these stimuli undergo similar neuronal processing. Performance of the observers could be described by probability summation of the orientation and contrast cues, resulting in an elliptical shape of the two-dimensional threshold contours. In conclusion, our results show similar performance for luminance and isoluminant gratings in several orientation discrimination tasks. The similarity in shape of the different threshold functions presents strong evidence that similar mechanisms underlie orientation discrimination of luminance and isoluminant stimuli.