Magnetic resonance imaging MRI and fMRI

What happens in magnetic resonance imaging (MRI) is that radio waves are used to excite atoms in the brain. This produces magnetic changes which are detected by an 11-ton magnet surrounding the patient. These changes are then interpreted by a computer and turned into a very precise three-dimensional picture. MRI scans (Figure 1.9) can be used to detect very small brain tumours. MRI scans can be obtained from numerous different angles. However, they only tell us about the structure of the brain rather than about its functions.

The MRI technology has been applied to the measurement of brain activity to provide functional MRI (fMRI). Neural activity in the brain produces increased blood flow in the active areas, and there is oxygen and glucose within the blood. According to Raichle (1994a, p. 41), "the amount of oxygen carried by haemoglobin (the molecule that transports oxygen.) affects the magnetic properties of the haemoglobin. MRI can detect the functionally induced changes in blood oxygenation in the human brain." The approach based on fMRI provides three-dimensional images of the brain with areas of high activity clearly indicated. It is more useful than PET, because it provides more precise spatial information, and shows changes over shorter periods of time. However, it shares with PET a reliance on the subtraction technique in which brain activity during a control task or situation is subtracted from brain activity during the experimental task.

A study showing the usefulness of fMRI was reported by Tootell et al. (1995b). It involves the so-called waterfall illusion, in which lengthy viewing of a stimulus moving in one direction (e.g., a waterfall) is followed immediately by the illusion that stationary objects are moving in the opposite direction. There were two key findings. First, the gradual reduction in the size of the waterfall illusion over the first 60 seconds of observing the stationary stimulus was closely paralleled by the reduction in the area of activation observed in the fMRI. Second, most of the brain activity produced by the waterfall illusion was in V5, which is an area of the visual cortex known to be much involved in motion perception (see Chapter 2). Thus, the basic brain processes underlying the waterfall illusion are similar to those underlying normal motion perception.

Evaluation

Raichle (1994a, p. 350) argued that fMRI has several advantages over other techniques:

The technique has no known biological risk except for the occasional subject who suffers claustrophobia in the scanner (the entire body must be inserted into a relatively narrow tube). MRI provides both anatomical and functional information, which permits an accurate anatomical identification of the regions of activation in each subject. The spatial resolution is quite good, approaching the 1-2 millimetre range.

One limitation with fMRI is that it provides only an indirect measure of neural activity. As Anderson et al. (1996, p. 423) pointed out, "With fMRI, neural activity is reflected by changes in the relative concentrations of oxygenated and deoxygenated haemoglobin in the vicinity of the activity." Another limitation is that it has poor temporal resolution of the order of several seconds, so we cannot track the time course of cognitive processes. A final limitation is that it relies on the subtraction technique, and this may not accurately assess brain activity directly involved in the experimental task.

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