Differential effect of attention to translation and expansion on motion aftereffects (MAE).

Recent fMRI findings have shown that selective attention to translating dots enhances V1 and MT complex activity whereas attention to expansion enhances MT complex activity rather than V1 (Watanabe et al., Proceedings of the National Academy of Sciences of the USA, (1998a) 95(19), 11489-11492). In order to clarify whether or not attention actually enhances the neural mechanism for the attended motion direction(s), we took advantage of the motion aftereffect (MAE), using superimposed groups of translating and expanding dots as the adaptation stimulus. During the adaptation stage, the subject was instructed to direct attention to one-way translation, expansion, or no particular motion or location, while gazing at the fixation point. The strength of the MAE in the attended monocular condition was greater than the sum of the unattended monocular MAE and the attended binocular MAE. In another experiment the monocular and binocular components showed linear additivity. These results suggest that attention enhances the monocular mechanism for the attended translational direction and that bottom-up monocular signals and top-down attentional signals simply add linearly. In contrast, no significant monocular contribution was found for attention to expansion. This is not only in accord with previous fMRI findings (Watanabe et al., 1998a), but also supports the thesis that attention to translation or expansion enhances the activation of the mechanism for the attended motion, rather than simply increasing arousal as a result of a heavier task load.