fMRI measures brain activity indirectly, namely by measuring change in oxygen content; this is called the Blood Oxygenation Level Dependent (BOLD) contrast mechanism.
However, fMRI has certain limitations. For instance, the fMRI signal reflects changes in oxygen content with high but insufficient spatial-time resolution. Hemodynamics in response to neuronal activity is revealed on a spatial-temporal scale far longer than the neuronal activity itself. Here, so-called temporal "blurring" of the fMRI signal is caused by both inertance and residual effects. In spite of such obstacles, changes in neural activity associated with individual trials or components of a trial in a task can be observed. Moreover, it is possible to capture brain activity associated with a single momentary cognitive act of mentally rotating a stimulus, without recourse to averaging over events (Buckner & Logan 2001, p. 31). Special fMRI experimental designs such as event-related fMRI designs are required in these cases.
There are numerous difficulties in separating the processing roles of specific brain areas. Usually such separation is provided either by well matched task comparisons or through convergence across multiple studies. However, brain activity changes can be relative changes between pairs of tasks, gradual or even nonlinear changes across a series of tasks, or correlations between different tasks. How can tasks and trials within a task be constructed to separate brain cognitive operations This is a key problem of fMRI experimental design. There are several approaches for its solving.
The basic approach is to have subjects engage in a target behavioural task for a period of time and then contrast that task period with periods where subjects perform a reference task. Here, the subject might perform a target task, and the measurement obtained during the performance of that task would be contrasted with a measurement obtained when the subject performed a matched reference task.
How to substantiate this approach It is obvious that brain activity will change between the two task states and therefore will correlate selectively with the manipulated task demands. When using fMRI, images are taken of the brain repeatedly and in sequence. Brain areas of activation are identified by examining which specific regions change signal intensity as the task state changes from the reference condition to the target task. Then, statistical procedures ranging from direct comparisons between task states to more sophisticated estimations of correlations among task states can be employed to identify those regions whose activity change is unlikely to occur by chance.
Unfortunately, tasks designed by such approach may cause differences in the processing strategies adopted by subjects during task performance by means of the blocking of trials, which may result in differential patterns of neural activity that do not have to do with the item-specific processes elicited by the individual trials. This issue can appear in delicate forms in cognitive paradigms where subject strategies may be