Subtractions, conjunctions, and interactions in experimental design of activation studies

This paper contrasts different experimental designs for revealing the neural correlates of phonological retrieval (i.e., naming). Cognitive subtraction designs require a minimum of one task pair and the comparison between tasks reveals the differing functional task components. Conjunction analysis requires a minimum of two task pairs, each differing by the same component, and this component is revealed as the difference which is common to both task pairs. Two different limitations of cognitive subtraction are highlighted: 1) the difficulty of finding baseline tasks that activate all but the process of interest, and 2) activation differences (between the two tasks of a pair) include the interaction term, i.e., the effect that the added component in the activation task has on preexisting components. The problem of baseline selection can be overcome by conjunction analysis, for which there may be many processing differences for each task pair, providing that the only common difference between pairs is the component of interest. The problem of interactions can be overcome when the experimental design is factorial. This permits the effect that an added component has on the expression of preexisting components (i.e., the interaction term) to be evaluated explicitly. We demonstrate that when the design is factorial, conjunction analysis reveals commonalities in activation, while the interactions reveal task-specific effects.