Adaptation to temporal modulation can enhance differential speed sensitivity

During adaptation to a moving pattern, perceived speed decreases. Thus we k now that the adapted visual system does not simply code the absolute speed of a stimulus. We hypothesised that adaptation to a moving stimulus serves to optimise coding of changes in speed at the expense of maintaining an acc urate representation of absolute speed. In this case we would expect discri mination of speeds around the adapted level to be preserved or enhanced by motion adaptation. Speed discrimination thresholds were measured for sinuso idal gratings (1.25 cpd; 12.5 Hz; 40% contrast) with and without prior adap tation to moving, static, and flickering stimuli. After adaptation to motio n in the same direction as the test, seven of eight subjects showed a reduc tion of perceived speed in the adapted region, and seven showed enhanced di scrimination. Similar effects were found for adaptation to motion in the op posite direction to the test and to counter-phase flicker, suggesting that adaptation is driven by temporal modulation rather than by motion per se. W e conclude that motion adaptation preserves or enhances differential speed sensitivity at the expense of an accurate representation of absolute speed. (C) 1999 Elsevier Science Ltd. All rights reserved.