Motion coherence detection as a function of luminance level in human central vision

We studied the changes and invariances of foveal motion detection upon dark adaptation. It is well-documented that dark adaptation affects both spatial and temporal aspects of visual processing. The question we were interested in is how this alters motion coherence detection for moving random texture. To compare motion sensitivity at different adaptation levels, we adjusted the viewing distance for equal detectability of a stationary pattern. At these viewing distances we then measured velocity tuning curves for moving random pixel arrays (RPAs). Mean luminance levels ranged from 50 down to 0.005 cd m-2. Our main conclusion is that foveal velocity tuning is amazingly close to luminance-invariant, down to a level of 0.05 cd m-2. Because different viewing distances, and hence, retinal image sizes were used, we performed two control experiments to assess variations of these two parameters separately. We examined the effects of retinal inhomogeneities using discs of different size and annuli filled with RPAs. Our conclusion is that the central visual field, including the near periphery is still rather homogeneous for motion detection at 0.05 cd m-2, but the fovea becomes unresponsive at the lowest luminance level. Variations in viewing distance had marked effects on velocity tuning, both at the light adapted level and the 0.05 cd m-2 level. The size and type of these changes indicated the effectiveness of distance scaling, and show that deviations from perfect invariance of motion coherence detection were not due to inaccurate distance scaling.