Negative BOLD-fMRI signals in large cerebral veins.

Reductions in blood oxygenation level dependent (BOLD)-functional magnetic resonance imaging (fMRI) signals below baseline levels have been observed under several conditions as negative activation in task-activation studies or anticorrelation in resting-state experiments. Converging evidence suggests that negative BOLD signals (NBSs) can generally be explained by local reductions in neural activity. Here, we report on NBSs that accompany hemodynamic changes in regions devoid of neural tissue. The NBSs were investigated with high-resolution studies of the visual cortex (VC) at 7 T. Task-activation studies were performed to localize a task-positive area in the VC. During rest, robust negative correlation with the task-positive region was observed in focal regions near the ventricles and dispersed throughout the VC. Both positive and NBSs were dependent on behavioral condition. Comparison with high-resolution structural images showed that negatively correlated regions overlapped with larger pial and ependymal veins near sulcal and ventricular cerebrospinal fluid (CSF). Results from multiecho fMRI showed that NBSs were consistent with increases in local blood volume. These findings confirm theoretical predictions that tie neural activity to blood volume increases, which tend to counteract positive fMRI signal changes associated with increased blood oxygenation. This effect may be more salient in high-resolution studies, in which positive and NBS may be more often spatially distinct.