Dear SPM experts,
In each trial of my experiment, subjects saw the first item (it1) and, after seconds, saw the second item (it2). Subjects had to decide if the second one was identical to the first one. After each trial, they were required to give a difficulty rating. Because we were interested in the decision process, i.e., brain activity from the start of the second item to the moment when subjects made a decision, this period is designated as the duration of a decision ev in each subject's first-level model in spm8. Moreover, we hypothesize that there exists a brain region that calculates the morphological difference between both items (i.e., it1-it2). Thus, trial-by-trial it1-it2 value is entered as parametric modulation of this ev. In group analysis, we collected each subject's 1st-level contrast of this parametric modulation for a one-sample t-test. In this group-level model, we also entered each subject's average difficulty scores across all trials as a covariate. Finally we found a brain region whose activity was correlated with difficulty scores.
So (1) or (2) is the interpretation for this region? Or both are incorrect?
(1) This brain region is associated with calculating morphological difference during decision and reflects between-subject difficulty during the decision process. (2) This brain region reflects between-subject difficulty during calculating morphological difference in the decision process.
Thanks.
Mike
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