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Published online on April 14, 2008, 10.1073/pnas.0801390105
PNAS | April 22, 2008 | vol. 105 | no. 16 | 6202-6207


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BIOLOGICAL SCIENCES / PSYCHOLOGY
Blood oxygenation level-dependent contrast response functions identify mechanisms of covert attention in early visual areas

Xiangrui Li{dagger}, Zhong-Lin Lu{dagger},{ddagger}, Bosco S. Tjan{dagger}, Barbara A. Dosher§, and Wilson Chu{dagger}

{dagger}Dana and David Dornsife Cognitive Neuroscience Imaging Center and Department of Psychology, University of Southern California, Los Angeles, CA 90089; and §Department of Cognitive Sciences, University of California, Irvine, CA 92697

Communicated by Richard M. Shiffrin, Indiana University, Bloomington, IN, February 11, 2008 (received for review November 23, 2007)

Covert attention can lead to improved performance in perceptual tasks. The neural and functional mechanisms of covert attention are still under investigation. Using both rapid event-related and mixed designs, we measured the blood oxygenation level-dependent functional MRI contrast response functions over the full range of contrast (0–100%) in the retinotopically defined early visual areas (V1, V2, V3, V3A, and V4) in humans. Covert attention increased both the baseline activities and contrast gains in the five cortical areas. The effect on baseline can be decomposed into a transient trial-by-trial component and a component across an entire attention block. On average, increase in contrast gain accounted for {approx}88.0%, 28.5%, 12.7%, 35.9%, and 25.2% of the trial-by-trial effects of attention in the five areas, respectively, and 22.2%, 12.8%, 7.4%, 19.7%, and 17.3% of the total effects of attention in those areas, consistent with single-unit findings in V4 and MT. The results provide strong evidence for a stimulus enhancement mechanism of attention as demonstrated in various behavioral studies.

contrast gain | increased baseline | response gain | stimulus enhancement

Author contributions: Z.-L.L., B.S.T., and B.A.D. designed research. X.L., Z.-L.L., and W.C. performed research; X.L., Z.-L.L., and B.S.T. analyzed data; and X.L., Z.-L.L., B.S.T., and B.A.D. wrote the paper.

The authors declare no conflict of interest.

This article contains supporting information online at www.pnas.org/cgi/content/full/0801390105/DCSupplemental.

{ddagger}To whom correspondence should be addressed. E-mail: zhonglin{at}usc.edu

© 2008 by The National Academy of Sciences of the USA


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