Attention mechanisms for multi-location first- and second-order motion perception

We applied the external noise plus attention paradigm to study attention me chanisms involved in concurrent first-order and second-order motion percept ion at two spatial locations. Cued to attend to one of the locations, the o bserver was instructed to independently judge direction of motion of either first-order (Experiment 1) or second-order (Experiment 2) motion stimuli a t both locations in every trial. Across trials, systematically controlled a mounts of external noise were added to the motion displays. We measured mot ion threshold at three performance criteria in every attention x external n oise condition. We find that observers could, without any loss, simultaneou sly compute first-order motion direction at two widely separated spatial lo cations across a broad range of external noise conditions. However, conside rable loss occurred at the unattended location in processing second-order m otion direction at two separated spatial locations. We conclude that, under the conditions investigated in the current study, (1) in first-order motio n perception, the visual system could simultaneously process motion directi on at two widely separated locations without any capacity limitation; (2) i n second-order motion perception. attending to a spatial location enhances stimulus contrast at that location by a factor of about 1.37 (or equivalent ly, reduces the internal additive noise by a factor of about 0.73). (C) 200 0 Elsevier Science Ltd. All rights reserved.