Functional MRI Experimental Designs

The particular characteristics of fMRI (i.e., its noninvasiveness and its spatiotemporal resolution) allow for its use in a variety of experimental designs. Two commonly employed schemas are block design and event-related design.

In a block design experiment, the experimental conditions alternate in blocks of some tens of seconds, each consisting of multiple repetitions of the same condition. In the subsequent statistical analysis, the scans acquired during the same block are pooled together to give an average effect. In an event-related design, each single experimental event/stimulus is modeled separately, which usually allows more latitude both in terms of the analysis (one can, for instance, separate the scan corresponding to a correct behavioral response from the scans corresponding to the subject's errors) and of the experiment itself (for instance, one can alternate between events of different types to avoid effects related to habituation). The analysis of such studies relies on the assumption of linearity in the BOLD response: if events of the same type are presented not too closely one to the other (the "linearity" seems to break down with an interstimulus interval of 1-2 seconds), then the resulting BOLD signal is simply the sum of the BOLD responses for the individual events.

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