Surface based mechanisms of attentional facilitation and inhibition in motion perception

Purpose. The goal of this study is to investigate the mechanisms of surface-based attentional modulation of visual motion processing. We used the motion aftereffect (MAE) as a tool for quantifying the attentional facilitation and inhibition. Methods. Adapting stimuli consisted of two dot populations, moving transparently with the same speed in different directions: Dots in the "effector" population E, the MAE of which was measured, moved coherently in one direction (0°). Dots in the "distractor" population D moved in a direction orthogonal to that of E, and this direction reversed roughly every 3 s (+90° or -90° with respect to E's direction). Luminance-change "episodes," i.e., a proportion of dots becoming slightly brighter or dimmer, occurred randomly within each population. MAE was measured in three conditions: 1 Passive: observers had no attentional task. 2) Attend to effectors: Observers reported the direction of luminance change (brighter or dimmer within population E. 3) Attend to distractors: Observers reported the direction of luminance change within D. MAE strength was measured by the percentage of directionally biased dots, among randomly moving dots, required to null the MAE. Results. We found that the strength of MAE was increased when observers attended to the luminance change of the dots in population E, and decreased when they attended to that in the orthogonal component D, as compared to the passive viewing condition. Conclusions. Attention to one of the features, in our case the luminance of transparently moving surfaces, results in both enhancement of the motion signal associated with the attended surface, as well as inhibition of that associated with the unattended surface. These results strongly suggest that, in the case of bivectorial transparent motion, attention is directed to the moving surface as a whole rather than to its individual features.