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Lynn Robertson, Thomas VanVleet; Feature binding and spatial awareness. Journal of Vision 2007;7(9):449. doi: https://doi.org/10.1167/7.9.449.
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Feature integration theory proposes that searching for a feature v. conjunction of two or more features is qualitatively different. Spatial attention is theoretically needed to bind features together, whereas basic features can be processed without spatial attention. Neuropsychological studies of patients with spatial deficits (unilateral visual neglect, Balint's syndrome) have supported this distinction. Features can be detected without intact spatial maps, while conjunctions are either missed entirely or conjoined inaccurately (produce illusory conjunctions). However, it is unclear whether features are conjoined implicitly below the level of awareness, making accurately bound conjunctions inaccessible to awareness, or remain separate and are conjoined at the time of awareness. The present study was designed to address this question. We tested 3 patients with unilateral visual neglect after right MCA stroke and presented feature or conjunction shape/color displays in a circular array around fixation. We used a psychophysical staircase procedure and dual threshold measures to first estimate stimulus presentation times (TPT) that produced approximately 25% detection and 75% detection for both features and conjunctions in the defective field. In this way search was equated at two levels of difficulty for each type of display. This was immediately followed by a priming procedure showing the same feature or conjunction primes at the two estimated TPTs. Participants were asked to discriminate a single target that appeared in the center of the screen where stimuli were clearly perceived. Significant priming effects in reaction time to the probe were present for both types of primes and for both TPTs. More importantly, performance was independent of explicit detection rates in the feature prime condition but mirrored detection rates in the conjunction prime condition. These findings support a fundamental role for spatial awareness in conjunction search but not feature search and are not consistent with implicit color/shape binding.
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