PubMed · 4686438
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1973-01-06. Learning disabilities.. https://doi.org/10.5694/j.1326-5377.1973.tb119570.x
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Although attention plays a significant role in vision, its spatial deployment and spread in the third dimension is not well understood. In visual search experiments we show that we cannot easily focus attention across isodepth loci unless they are part of a well-formed surface with locally coplanar elements. Yet we can easily spread our attention selectively across well-formed surfaces that span an extreme range of stereoscopic depths. In cueing experiments, we show that this spread of attention is, in part, obligatory. Attentional selectivity is reduced when targets and distractors are coplanar with or rest on a common receding stereoscopic plane. We conclude that attention cannot be efficiently allocated to arbitrary depths and extents in space but is linked to and spreads automatically across perceived surfaces.
Positron emission tomography was used to measure changes in the regional cerebral blood flow of normal people while they searched visual displays for targets defined by color, by motion, or by a conjunction of color and motion. A region in the superior parietal cortex was activated only during the conjunction task, at a location that had previously been shown to be engaged by successive shifts of spatial attention. Correspondingly, the time needed to detect a conjunction target increased with the number of items in the display, which is consistent with the use of a mechanism that successively analyzes each item in the visual field.
Motion perception mechanisms have recently been divided into three categories. First-order mechanisms primarily extract motion from moving objects or features that differ from the background in luminance. Second-order mechanism extract motion from moving properties, such as a moving area of flicker in which there is no difference in mean luminance between target and background. These first- and second-order motion mechanisms are primarily monocular. The existence of purely binocular, interocular and various other unusual kinds of apparent motion has promoted conjectures of a third-order mechanism, 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.