Dynamic metabolic changes in human visual cortex in regions with positive and negative blood oxygenation level-dependent response.

Dynamic metabolic changes were investigated by functional magnetic resonance spectroscopy (fMRS) during sustained stimulation of human primary visual cortex. Two established paradigms, consisting of either a full-field or a small-circle flickering checkerboard, were employed to generate wide-spread areas of positive or negative blood oxygenation level-dependent (BOLD) responses, respectively. Compared to baseline, the glutamate concentration increased by 5.3% ( = 0.007) during activation and decreased by -3.8% ( = 0.017) during deactivation. These changes were positively correlated with the amplitude of the BOLD response ( = 0.60,  = 0.002) and probably reflect changes of tricarboxylic acid cycle activity. During deactivation, the glucose concentration decreased by -7.9% ( = 0.025) presumably suggesting increased consumption or reduced glucose supply. Other findings included an increased concentration of glutathione (4.2%,  = 0.023) during deactivation and a negative correlation of glutathione and BOLD signal changes ( = -0.49,  = 0.012) as well as positive correlations of aspartate ( = 0.44,  = 0.035) and -acetylaspartylglutamate ( = 0.42,  = 0.035) baseline concentrations with the BOLD response. It remains to be shown in future work if the observed effects on glutamate and glucose levels deviate from the assumption of a direct link between glucose utilization and regulation of blood flow or support previous suggestions that the hemodynamic response is mainly driven by feedforward release of vasoactive messengers.