Displacement thresholds for motion detection under conditions of chromatic adaptation

— The smallest, continuous target displacement which is sufficient to cause a sensation of movement was measured under conditions of chromatic adaptation. The experiments were carried out for test target and background field wavelengths which selectively isolate the activity of the colour mechanisms of the human eye. The results show that, when target detection is largely mediated by the red or green colour mechanism, a target displacement of 2–3′ in the foveal region is sufficient to cause a reliable sensation of movement. These displacement thresholds show only a small change as a function of target contrast or background retinal illuminance level. Similar results were obtained when the colour of the test target was the same as that of the background field (e.g. green, yellow or red), when no attempt was made to isolate a single colour mechanism. Test target and background wavelengths which isolate the activity of the blue colour mechanism yield displacement thresholds in the range 5–7′ and show a stronger dependence on target contrast and background retinal illuminance level. The experiments were repeated at several eccentricities and the results show that, for a test target and background field of the same colour and under conditions which isolate the green or red spectral response mechanism, the minimum target displacement required for motion detection appears to increase in a manner similar to the centre-width receptive-field size of broad-band, centre-surround transient neurons in the rhesus monkey retina [De Monasterio and Gouras (1975) J. Physiol., Lond.251, 167–195]. In contrast, displacement thresholds obtained under conditions which isolate the blue colour mechanism do not show significant variation for target presentation locations up to 25° of visual angle. These data relate well to other psychophysical results which reveal the spatial properties of motion-detecting mechanisms in human vision [Barbur and Ruddock (1980b) Biol. Cybernetics37, 93–98].