Luminance mechanisms mediate the motion of red-green isoluminant gratings: the role of "temporal chromatic aberration".

In this paper we use a dynamic noise-masking paradigm to explore the nature of the mechanisms mediating the motion perception of drifting isoluminant red-green gratings. We compare contrast thresholds for the detection and direction discrimination of drifting gratings (1.5 cpd), over a range of temporal frequencies (0.5-9 Hz) in the presence of variable luminance or chromatic noise. In the first experiment, we used dynamic luminance noise to show that direction thresholds for red-green grating motion are masked by luminance noise over the entire temporal range tested, whereas detection thresholds are unaffected. This result indicates that the motion of nominally isoluminant red-green gratings is mediated by luminance signals. We suggest that stimulus-based luminance artifacts are not responsible for this effect because there is no masking of the detection thresholds. Instead we propose that chromatic motion thresholds for red-green isoluminant gratings are mediated by dynamic luminance artifacts that have an internal, physiological origin. We have termed these "temporal chromatic aberration". In the second experiment, we used dynamic chromatic noise masking to test for a chromatic contribution to red-green grating motion. We were unable to find conclusive evidence for a contribution of chromatic mechanisms to the chromatic grating motion, although a contribution at very high chromatic contrasts cannot be ruled out. Our results add to a growing body of evidence indicating the presence of dynamic, internal luminance artifacts in the motion of chromatic stimuli and we show that these occur even at very low temporal rates. Our results are compatible with our previous work indicating the absence of a chromatic mechanism for first order (quasi-linear) apparent motion [Vision Res. 40 (2000) 1993]. We conclude that previous conclusions based on the motion of chromatic red-green gratings should be reassessed to determine the contribution of dynamic luminance artifacts.