Effect of contrast on detection of motion of chromatic and luminance targets: retina-relative and object-relative movement

It is now clear, contrary to previous conclusions, that motion may be conveyed by purely chromatic stimuli. The question considered here is how the mechanisms for assessing motion of luminance and chromatic stimuli differ. The dependence on contrast of amplitude thresholds for the perception of oscillatory motion was measured. The targets were Gabor patches modulated either chromatically along the L-M isoluminant axis or in luminance. When single targets were presented, the slope of the function relating log threshold amplitude to log contrast was approximately -0.50 for chromatic targets and 0.00 for luminance targets. When a reference target was present the slopes were approximately -0.50 for both types of target. The results imply that perception of motion of chromatic targets is based on the assessment of changes in relative position of target elements while motion of luminance targets may be signalled either by relative motion of target elements or by local motion of an image relative to the retina.