The signal of functional MRI, which for decades has been considered a direct indicator of brain activity, may be misleading in 40% of cases. This conclusion was reached by scientists from the Technical University of Munich. The study was published in Nature Neuroscience (NatNeuro).

Functional magnetic resonance imaging (fMRI) is based on the assumption that cerebral blood flow is linked to neuronal activity. Active neurons consume more oxygen, which increases blood flow and, consequently, the recorded signal.

More than 40 healthy volunteers took part in the experiment. During scanning, they performed standard cognitive tasks—mental arithmetic and the recall of personal memories. Alongside conventional fMRI, the researchers used quantitative MRI, which allows for direct measurement of oxygen consumption by brain tissue.

It was this comparison that revealed the discrepancies. In a number of regions, especially those involved in computations, neurons did indeed consume more oxygen, but blood flow did not increase. Instead, the brain began to extract oxygen more efficiently from the blood already being delivered.

According to the authors, this means that changes in the fMRI signal may reflect not neuronal activity itself, but features of the vascular system and the brain’s energy efficiency. This is particularly important for clinical research: in depression, Alzheimer’s disease, and other neurological disorders, fMRI signals are often interpreted as indicators of decreased or increased brain activity, whereas in reality they may be related to age-related or vascular changes.