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Zhong-Lin Lu, Xiangrui Li, Bosco S. Tjan, Barbara A. Dosher, Wilson Chu; Mechanisms of covert attention: External noise exclusion and stimulus enhancement in early visual areas. Journal of Vision 2007;7(9):783. doi: 10.1167/7.9.783.
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© ARVO (1962-2015); The Authors (2016-present)
Using external noise analysis and a theoretical framework from the perceptual template model, we showed that covert spatial attention operates via two independent mechanisms (Dosher & Lu, 2000; Lu & Dosher, 2000): (1) excluding external noise/distractors around the target, and (2) enhancing the target stimulus. Behaviorally, external noise exclusion is only effective in high external noise conditions; stimulus enhancement only benefits performance in low external noise conditions. Here, we investigated the effect of covert attention in high external noise on the BOLD contrast response functions in retinotopically defined early visual brain areas. Using a rapid event-related design, BOLD responses to a brief (100 ms), spatially windowed (5–7° annulus) sinusoidal grating embedded in high external noise were obtained in V1, V2, V3, V3A, and V4v. A counter-balanced pseudo-random sequence tested four grating contrast conditions in each fMRI run: 0, 1x, 3x, and 10x of each subject's orientation identification threshold in noise. In separate runs, subjects reported either the orientation of the grating (45±5°) (“attended”) or the identity of a letter in the center of the display (“unattended”). In V1 and V2, BOLD responses without attention were relatively independent of signal contrast, reflecting responses to external noise. Covert attention reduced the BOLD responses for low signal contrasts and increased them for high contrasts, increasing the role of signal and decreasing the role of external noise. In higher cortical areas, attention did not alter the magnitude of the BOLD responses for low contrasts but increased them for high contrasts. Attention reduces the impact of external noise in early visual areas, resulting in increased signal to noise ratio and therefore better performance. Attention also enhances stimulus (“contrast gain”), which does not affect signal to noise ratio in high external noise — a finding unobservable psychophysically in high noise (Lu & Dosher, 1998).
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