whether adaptation is a universal feature of cortical computation or whether it mostly occurs early in a sensory processing stream and is simply inherited as adapted signals pass to downstream cortical areas. With this motivation in mind, we studied adaptation in neurons in cortical area MT (or V5), an extrastriate visual area that contains a high proportion of neurons that are selective for the direction of motion of visual stimuli (Zeki, 1974; Maunsell and Van Essen, 1983a). Adaptation in MT is of interest for several reasons. First, psychophysical studies suggest that visual motion pro-cessing is strongly affected by adaptation (for a review, see Mather et al, 1998). For instance, the prolonged viewing of a moving stimulus causes subsequently viewed static or motion-balanced stimuli to appear to

Several fMRI studies have reported MT � response increases correlated with perception of the motion aftereffect (MAE). However, attention can strongly af-fect MT � responses, and subjects may naturally at-tend more to the MAE than control trials without MAE. We found that requiring subjects to attend to motion on both MAE and control trials produced equal levels of MT � response, suggesting that attention may have confounded the interpretation of previous experi-ments; in our data, attention accounts for the entire effect. After eliminating this confound, we observed that direction-selective motion adaptation produced a direction-selective imbalance in MT � responses (and earlier visual areas), and yielded a corresponding asymmetry in speed discrimination thresholds. These findings provide physiological evidence that popula-tion level response imbalances underlie the MAE, and quantify the relative proportions of direction-selective neurons across human visual areas.