Attention-generated apparent motion

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Abstract
MOTION perception mechanisms have recently been divided into three categories1. First-order mechanisms2-4 primarily extract motion from moving objects or features that differ from the back-ground in luminance. Second-order mechanism5,6 extract motion from moving properties, such as a moving area of flicker in which there is no difference in mean luminance between target and back-ground. These first- and second-order motion mechanisms are primarily monocular. The existence of purely binocular, interocular and various other unusual kinds of apparent motion7-13 has promoted conjectures of a third-order mechanism1,14,15, but there has been no clear suggestion as to the actual computations that such a mechanism might perform. Here we demonstrate 'alternating feature' stimuli that produce apparent motion only when the observer selectively attends to one of the embedded features in the display. The latent motion in the alternating feature stimuli is invisible to first- or second-order motion mechanisms, and the direction of apparent motion depends on the particular feature attended. These findings suggest the mechanism of third-order motion: the locations of the most significant features are registered in a salience map, and motion is computed directly from this map.