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Hayley Buchholz, Mazyar Fallah; Selection of superimposed surfaces by density. Journal of Vision 2008;8(6):662. doi: 10.1167/8.6.662.
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Bottom-up attention is driven by stimulus features, which may be occurring at the level of the object or the feature itself. Most studies investigating stimulus-driven selection have used spatially separate stimuli and thus are confounded by spatial mechanisms. To investigate whether it is spatial or object-based, we superimposed two surfaces (random dot kinetograms, RDKs). We asked whether dot density, previously found to be involved in bottom-up attention, can also drive object-based selection in the absence of spatial mechanisms. Superimposing two surfaces of different densities places both densities at the same spatial location and thus, those densities are separated only by the surface on which they appear. We performed 2 experiments to investigate the effect of density on surface selection. In experiment 1, subjects fixated a central dot and an aperture with a single surface of dots moving left or right appeared in the periphery. After a random period of time, the fixation spot disappeared which was the cue for the subjects to saccade to the aperture. The density of the surface varied trial-by-trial from 0.24–30.6 dots/deg2. Saccading to the surface resulted in an automatic pursuit of that surface. We found that pursuit speed varied with surface density. In experiment 2, one surface again varied in density while a second surface was placed in the aperture, moving at the same speed in the opposite direction and of a constant density. We varied the opposing surface's density across sessions (range: 0.24–1.9 dots/deg2). As the relative difference in density between the two surfaces decreased, the gain of pursuit to the higher density surface decreased. At equal densities, no pursuit occurred. These findings are consistent with competitive circuitry between the two surfaces. Overall, these results suggest that density is a salient object feature that can drive automatic selection, regardless of location-based mechanisms.
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