Abstract
The finding that an item of type A pops out from an array of distractors of type B typically is taken to support the inference that human vision contains a neural mechanism that is activated by items of type A but not by items of type B. Such a mechanism might be expected to yield a neural image in which items of type A produce high activation and items of type B low (or zero) activation. Access to such a neural image might further be expected to enable accurate estimation of the centroid of an ensemble of items of type A intermixed with to-be-ignored items of type B. Here, it is shown that as the number of items in stimulus displays is increased, performance in estimating the centroids of horizontal (vertical) items amid vertical (horizontal) distractors degrades much more quickly and dramatically than does performance in estimating the centroids of white (black) items among black (white) distractors. Together with previous findings, these results suggest that, although human vision does possess bottom-up neural mechanisms sensitive to abrupt local changes in bar-orientation, and although human vision does possess and utilize top-down global attention filters capable of selecting multiple items of one brightness or of one color from among others, it cannot use a top-down global attention filter capable of selecting multiple bars of a given absolute orientation and filtering bars of the opposite orientation in a centroid task.
Highlights
The experiments reported here demonstrate that participants are dramatically worse at this centroid-estimation task than they are if target vs. distractor items are defined by brightness instead of by orientation, suggesting that human vision possesses attention filters selective for black vs. white (Drew, Chubb & Sperling, 2010), it does not possess corresponding attention filters selective for vertical vs. horizontal bar-orientation
Human vision has mechanisms that enable global selection of items based on contrast, color, shape, and other attributes, we find that human vision possesses no mechanism that enables selection of multiple items based on absolute bar-orientation
Participants were tested in a task requiring them to use topdown selective attention to estimate the centroids of target items with one feature value while ignoring distractor items with the opposite feature value
Summary
A target bar of a given orientation pops out from a field of distractor bars of some other fixed orientation provided the difference in orientation between target and distractor bars is large enough (Beck & Ambler, 1973; Foster & Ward, 1991a; Foster & Westland, 1995, 1998; Nothdurft, 1991, 1992, 1993, 1994; Sagi & Julesz, 1985; Treisman, 1985; Treisman & Souther, 1985; Wolfe et al, 1992) This frequently replicated result might be taken to suggest that human vision possesses a number (perhaps a continuum) of mechanisms selective for different orientations. The experiments reported here demonstrate that participants are dramatically worse at this centroid-estimation task than they are if target vs. distractor items are defined by brightness (black vs. white) instead of by orientation, suggesting that human vision possesses attention filters selective for black vs. white (Drew, Chubb & Sperling, 2010), it does not possess corresponding attention filters selective for vertical vs. horizontal bar-orientation
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