Negative BOLD Response Ipsi-lateral to the Visual Stimulus: Origin Is Not Blood Stealing

Negative BOLD responses (NBRs) are pervasive in human fMRI experiments, but commonly ignored. The NBR in the human occipital cortex, triggered by stimulating part of the visual-field, is correlated with reductions in cerebral blood flow (CBF) and with decreases in oxygen consumption (Shmuel et al., 2002). The findings from this human study corroborate contributions to the NBR by 1) a significant component of reduction in neuronal activity, and possibly 2) a component of hemodynamic changes independent of the local changes in neuronal activity (e.g. passive reduction in CBF, also termed ’vascular blood steal’, due to increased flow to nearby, more demanding areas). Shmuel et al. (2002) indicated that vascular steal cannot account for NBR ipsi-lateral to a one hemi-field visual stimulus. In an accompanying abstract we show that the NBR in monkey V1 is associated with decreases in neuronal activity