Background

FMRI is one of the most recently developed forms of neuroimaging but the idea underpinning the technique - inferring brain activity by measuring changes in blood flow - is not new. The following account of an experiment performed by the Italian scientist Angelo Mosso can be found in William James’ The Principles of Psychology, published in 1890:

"The subject to be observed lay on a delicately balanced table which could tip downwards either at the head or the foot if the weight of either end were increased. The moment emotional or intellectual activity began in the subject, down went the balance at the head-end, in consequence of the redistribution of blood in his system…"

The reported success of this early experiment can only have been wishful thinking on the investigators behalf. But the suggestion that blood flow is coupled to neural activity was insightful. In 1890 the prevailing view was that since the brain is encased by the skull, local increases in blood flow and volume would be impossible. It was thought instead that any changes in blood flow were caused by systemic changes in blood pressure or cardiac output.

Toward the end of the 19th century, Charles S. Roy and Charles S. Sherrington provided the first evidence supporting a coupling between energy metabolism and blood flow in the brain. In their experiments, a monitoring device was placed on the brain surface of anesthetized dogs, which measured fluctuations in blood volume. They showed that blood volume (and presumably flow) does change locally in the brain. However it was still unclear whether the brain itself was responsible for mediating these changes.

It was not until 1948 in a seminal experiment measuring oxygen metabolism and blood flow in the brain that Seymour Kety and Carl Schmidt confirmed that blood flow in the brain is regionally regulated by the brain itself. They demonstrated that when neurons use more oxygen, chemical signals cause nearby blood vessels to dilate. The increase in vascular volume leads to a local increase in blood flow. At the time of these publications Kety and Schmidt were considered vascular physiologists more than brain scientists. Nevertheless the ability to measure CBF, a proven correlate of brain metabolism, opened up the remarkable possibility of studying brain function in humans.

The development of FMRI in the 1990s, generally credited to Seiji Ogawa and Ken Kwong, is the latest in long line of innovations, including positron emission tomography (PET) and near infrared spectroscopy (NIRS), which use blood flow and oxygen metabolism to infer brain activity. As a brain imaging technique FMRI has several significant advantages:

1. It is non-invasive and doesn’t involve radiation, making it safe for the subject.
2. It has excellent spatial and good temporal resolution.
3. It is easy for the experimenter to use.

The attractions of FMRI have made it a popular tool for imaging normal brain function – especially for psychologists. Over the last decade it has provided new insight to the investigation of how memories are formed, language, pain, learning and emotion to name but a few areas of research. FMRI is also being applied in clinical and commercial settings.

Image credits:

• Photo of Angelo Mosso - Courtesy of the National Library of Medicine, USA.

• Sensory regions of the brain - Courtesy of Peter Hobden, FMRIB.