Huk AC, Ress D, & Heeger DJ, Neuronal basis of the motion aftereffect reconsidered. Neuron, 32:161–172, 2001.
Abstract: Several recent fMRI studies have reported response increases in human MT+ correlated with perception of the motion aftereffect (MAE). However, MT+ responses can be strongly affected by attention, and subjects may naturally attend more strongly during the MAE than during controls without MAE. We found that requiring subjects to attend to the motion of the stimulus on both MAE and control trials produced equal levels of MT+ response, suggesting that attention may be a major confound in the interpretation of previous fMRI MAE experiments; in our data, attention appears to account for the entire effect. After eliminating this confound, we sought to measure direction-selective motion adaptation in human visual cortex. We observed that adaptation produced a direction-selective imbalance in MT+ responses (as well as earlier visual areas including V1), and yielded a corresponding psychophysical asymmetry in speed discrimination thresholds. These findings provide physiological evidence of a population-level response imbalance related to the MAE, and quantify the relative proportions of direction-selective neurons in human cortical visual areas.