Most fMRI studies are based on the detection of a positive BOLD response (PBR). Here, we demonstrate and characterize a robust sustained negative BOLD response (NBR) in the human occipital cortex, triggered by stimulating part of the visual field. The NBR was spatially adjacent to but segregated from the PBR. It depended on the stimulus and thus on the pattern of neuronal activity. The time courses of the NBR and PBR were similar, and their amplitudes covaried both with increasing stimulus duration and increasing stimulus contrast. The NBR was associated with reductions in blood flow and with decreases in oxygen consumption. Our findings support the contribution to the NBR of (1) a significant component of reduction in neuronal activity and (2) possibly a component of hemodynamic changes independent of the local changes in neuronal activity.
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We thank B. Rosen and N. Logothetis for helpful discussions; S.C. Ngan, S. Sarkar, J.C. Zhuang, X.D. Zhang, and B. Aufferman for sharing their software and skills; H. Merkle, P. Andersen, J. Strupp, B. Hanna, and J. Zeltins for their excellent technical support; D. Leopold, A. Tolias, and G. Rainer for their comments on the manuscript; and D. Blaurock for English editing. This study was supported by National Institute of Health grants MH55346, EB000331, and RR08079, the W.M. Keck Foundation, and the MIND Institute. Instrumentation funds for the purchase of the 7 Tesla magnet were received from NSF grant DBI-9907842 and NIH grant S10 RR13957.