Attention strongly increases oxygen metabolic response to stimulus in primary visual cortex

Top-down attention enhances neural processing, but its effect on metabolic activity in primary visual cortex (V1) is unclear. Combined blood flow and oxygenation measurements provide the best tool for investigating modulations of oxidative metabolism. We measured the human V1 response to a peripheral low contrast stimulus using fMRI and found a larger fractional modulation of blood flow with attention compared to the blood oxygenation level dependent (BOLD) response, thus indicating a much larger modulation of oxygen metabolism than was previously thought. These findings point to different aspects of neural activity driving flow and metabolic changes to different degrees. We propose that V1 flow is driven strongly but not exclusively by the initial sensory-driven neural activity, which dominates the response in the unattended condition, while V1 oxygen metabolism is driven strongly by the overall neural activity, which is modulated by top-down signals related to attention.