Abstract
The ongoing debate over the existence of a colour motion mechanism has appeared to lose its focus over the past few years. The purpose of the current study is to clarify the reasons for some differences in the published data, and to explain them in terms of a single stimulus structure in the context of the early processes of the system. Observers were required to discriminate the direction of motion of amplitude modulated (AM) gratings. The envelope and carrier of the grating moved independently, the carrier was masked by a static grating mask and, in an über-masking condition, the whole stimulus was masked with a dynamic noise mask. Each component was independently described in cardinal colour space and calibrated for subjective equiluminance using multiple methods. This sets up a stimulus structure whereby the motion of each of the first and second-order modulations were in conflict, the ‘energy’ and ‘feature’ predictions of motion direction further confounded within the stimulus, and, most importantly, explanations on the basis of artefactual signals become untenable. The spatial and temporal configuration of the test and mask were manipulated to replicate all previous data showing that evidence both for and against a chromatic motion mechanism could be generated within the same stimulus structure. When the structure and function of the retina, LGN and early cortex are considered in relation to the stimulus size, the mean luminance level of the stimulus and the mask-to-test frequency relationships, the reasons for the differences in the published data become clearer. Furthermore, strong evidence is found for a low-level motion mechanism independently sensitive to chromatic modulation; but this is only revealed under optimum conditions of size and location when all other critical factors (such as attention and rigidity) are controlled for.
The Dead Kennedys