With the further development of MRI, functional MRI or fMRI was invented. Like PET, the principle of fMRI is based on the oxygen supply to the brain. This oxygenation takes place by bringing more oxygen-rich blood to a particular brain area. In oxygenated blood, oxygen is bound to haemoglobin. When a certain area is active, oxygen-rich blood is brought to this area within a few seconds. This natural response of the body is called the haemodynamic response, and can be measured using fMRI.
Oxygenated and deoxygenated blood have different magnetic properties, with oxygenated blood being diamagnetic and deoxygenated blood paramagnetic. Oxygen-rich blood gives a high fMRI signal, and deoxygenated blood gives a low fMRI signal. When a lot of oxygen-rich blood flows to a certain brain area, the fMRI signal suddenly becomes stronger. This leads to a typical signal from the brain, the BOLD signal. BOLD stands for Blood Oxygenation Level Dependent, and basically represents the ratio of oxygenated to deoxygenated blood.
There are several characteristics by which the BOLD signal can be recognised.
The signal always starts with a small, initial, dip, which means that there is a reduction in oxygenated blood. This is caused by the fact that the active brain area immediately needs oxygen. However, it takes a few seconds for the extra oxygen-rich blood to reach this brain area.
Approximately three seconds after the start of the activity, the extra blood supply can be seen in the signal, there is extra oxygen-rich blood coming to this brain area. This extra amount is explained by both an increasing blood flow, and an increasing volume of blood flow. So there is more blood coming one after the other, but also more blood at the same time, so the elastic blood vessels expand slightly.
After a while, the brain area has received enough oxygen to perform the task properly, and the oxygen supply will decrease again to the baseline level. The blood vessels, however, need a little more time to return to their original state. Therefore, a lot of deoxygenated blood remains in these vessels for a while. This causes the BOLD signal to drop below the baseline.
Only when the blood vessels have returned to their original state will the BOLD signal be equal to that at rest.
Two types of tasks are distinguished that can be used in fMRI.
In a blocked design, several trials of the same kind are clustered in blocks. First, for example, there is a block in which only numbers are presented as stimuli. This can be followed by a block with only words as stimuli. During the analysis of this design, the average activity during the blocks is compared with each other.
In an event-related design, trials of different conditions are no longer clustered, but offered in a random order. After this, the individual trials of one condition are put together and an average is calculated. This mean is compared to the mean of the other condition. The analysis of event-related designs is basically the same as for ERP. However, one has to take into account that fMRI shows activity much slower than EEG. This can be done by allowing more time between stimuli, so that the signal can return to baseline level. The ideal time interval for this is 12 seconds.
On cognitive tasks, an event-related design is preferred. To begin with, subjects cannot develop a strategy to solve the problem, as they do not know what type of stimulus will occur. In a blocked design, this is possible, because the subjects know what is coming (after all, it is all the same). In addition, in an event-related design, you can easily link the activity to the stimuli offered. This means that the basic activity, or the activity between two trials, is not included in the average. Another advantage of event-related design is that you can separate correct reactions from incorrect reactions. You can therefore calculate the difference between them.
Author: Myrthe Princen (translated by Thomas von Rein)
Images: Marcel Loeffen
