The main limitation with fMRI experimental design planning arises from the fact that the signal changes being measured are very small. Thus, fMRI can be used only for determining relative signal intensity changes within a single image session . There are two main methods of planning an fMRI experimental design: block design and event-related design.
A typical block design consists of two alternating situations: the „activation condition" and the „resting condition", which often have the same duration. Each condition is considered as a mean static value across the whole period. The principles of BOLD data processing are based on the subtraction of the rest from the active periods; thus rest behaves as a baseline with respect to activation. The resulting difference between the two signals is the specific activation effect we are dealing with. It follows that the rest condition is very important, as much as the active condition, and is more important the smaller the eloquent area to be revealed. Finally, the rest condition is not always the absence of anything, but may also consist of another task, which may differ in the different paradigms.
An event-related design considers the time course of the event, and not a time-integrated averaging procedure. Each trial is considered separately as being time-locked to the beginning of the stimulus and signal changes are explored in relation to the onset of the event generated by the trial. Event-related signal averaging requires the repetition of trials and the alignment of the data to a reference point, such that repeated time-locked epochs of data can be recorded and subsequently averaged together. In this way, event-related design can explore the temporal shape of the event and discover if there is a temporal evolution, e.g. if a transient signal change is followed by a sustained signal change (in a block paradigm the transient and sustained signal changes would not be resolved, because activity is mediated over each condition) .
With respect to event related design, block design, besides being simpler, has a higher power in signal detection, i.e. has a higher sensitivity, due to the intrinsic characteristics of the method (to find significant differences between means). So a block design seems a more convenient way to perform presurgical fMRI paradigms .
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