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Abstract

Does an independent chromatic mechanism for the perception of the direction of motion exist? We analyzed detection of position and discrimination of direction of the shift of a chromatic isoluminant edge situated at 40 arcmin eccentricity to the left or right of the centre of gaze. For each observer 16 isoluminant and equally detectable colors were created as described previously (Teufel Wehrhahn 2000). Stimuli were well above threshold and respective contributions of cone mechanisms were measured showing that stimuli cover the range of available colors on the monitor used. Edges were 20 arcmin high and shift amplitudes, position (left or right) and direction were varied at random. Detection thresholds for the position of the shift, and discrimination thresholds for its direction were assessed using the method of constant stimuli. Subjects were required to report both decisions simultaneously. As a control, detection and discrimination were tested in separate experiments. The different experimental protocols were found not to affect the results. For all subjects detection thresholds were elevated for colors dominated by S-cone signals. To a lesser extent this was also found for discrimination thresholds. However, for all subjects and for all 16 colors tested discrimination thresholds were consistently lower than detection thresholds. This confirms that a directionally selective chromatic mechanism 'sui generis' exists. Furthermore, this chromatic mechanism is even more sensitive than the chromatic detection mechanism operative under identical conditions. This implies that a luminance signal is required for the detection of shift position.