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Purpose: The retina codes variations in luminance by adapting to and hence discounting, the mean luminance. During adaptation to a moving pattern, perceived speed decreases. Thus we know that the adapted visual system does not simply code the absolute speed of a stimulus. We hypothesize that adaptation to a moving stimulus serves to optimize coding of changes in speed at the expense of maintaining an accurate representation of absolute speed. In this case we would expect discrimination of speeds around the adapted level to be preserved or enhanced by motion adaptation. Methods and results: After adaptation to motion in the same direction as a subsequent test stimulus, seven of eight subjects showed a reduction of perceived speed in the adapted region and seven showed enhanced discrimination. Conclusions: We conclude that motion adaptation preserves or enhances differential speed sensitivity at the expense of an accurate representation of absolute speed in a manner analogous to retinal light adaptation. [ABSTRACT FROM AUTHOR]