Research in neuroscience often assumes universal neural mechanisms, but increasing evidence points towards sizeable individual differences in brain activations. What remains unclear is the extent of the idiosyncrasy and whether different types of analyses are associated with different levels of idiosyncrasy. Here we compute the within-subject reliability and subject-to-group similarity of activations for task, reaction time (RT), and confidence at both the single trial and block levels. We find that maps of RT and confidence effects at the block level feature substantially lower subject-to-group similarity values compared to RT and confidence effects at the trials level, and that this effect occurs despite high within-subject reliabilities for both analyses. Critically, we develop a computational model that can quantify the relative strength of group- and subject-level factors in observed brain activations. The model suggests that subject-level factors carry similar weight as group-level factors for analyses at the trial level, but are over six times more important for RT and confidence analyses at the block level. These results show that different analyses are associated with different levels of idiosyncrasy and specifically suggest that behavioral changes that occur at the block level are markedly more idiosyncratic compared to other mental processes.HighlightsBrain activation among people differ.The extent of these differences at trial- and block-level analyses is unknown.Results indicate that block-level analyses are more idiosyncratic.These findings suggest the need to examine individual-subject results.
