ATTENTION-GENERATED APPARENT MOTION

MOTION perception mechanisms have recently been divided into three cat egories(1). First-order mechanisms(2-4) primarily extract motion from moving objects or features that differ from the background in luminanc e. Second-order mechanism(5,6) extract motion from moving properties, such as a moving area of flicker in which there is no difference in me an luminance between target and background. These first- and second-or der motion mechanisms are primarily monocular. The existence of purely binocular, interocular and various other unusual kinds of apparent mo tion(7-13) has promoted conjectures of a third-order mechanism(1,14,15 ), but there has been no clear suggestion as to the actual computation s that such a mechanism might perform. Here we demonstrate 'alternatin g feature' stimuli that produce apparent motion only when the observer selectively attends to one of the embedded features in the display. T he latent motion in the alternating feature stimuli is invisible to fi rst-or second-order motion mechanisms, and the direction of apparent m otion depends on the particular feature attended. These findings sugge st the mechanism of third-order motion: the locations of the most sign ificant features are registered in a salience map, and motion is compu ted directly from this map.