The Negative BOLD effect

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 Functional magnetic resonance imaging (fMRI) studies of early sensory cortex often measure stimulus-driven increases in the blood oxygenation level-dependent (BOLD). However, these positive responses are frequently accompanied by reductions in the BOLD signal in adjacent regions of cortex. This negative BOLD response (NBR) is thought to result from neuronal suppression but the precise mechanism of action remains unknown. By measuring BOLD signals in human primary visual cortex and varying contrast levels in the region affected by the NBR, we test computational models of neuronal modulation which could explain this phenomenon. We have shown that the NBR is best-modeled as a spatially-asymmetric, contrast gain modulation with a large zero-contrast response offset in the surround. These findings alter our understanding of the way neural activity in the visual system depends on stimulus structure and location.