Reassessing the Functional Significance of Blood Oxygen Level Dependent Signal Variability

BOLD signal variability (SD_BOLD) has emerged as a unique measure of the adaptive properties of neural systems that facilitate fast, stable responding, based on claims that SD_BOLD is independent of mean BOLD signal (mean_BOLD) and is a powerful predictor of behavioral performance. We challenge these two claims. First, the apparent independence of SD_BOLD and mean_BOLD may reflect the presence of deactivations; we hypothesize that although SD_BOLD may not be related to raw mean_BOLD, it will be linearly related to “absolute” mean_BOLD. Second, the observed relationship between SD_BOLD and performance may be an artifact of using fixed-length trials longer than RTs. Such designs provide opportunities to toggle between on- and off-task states, and fast responders likely engage in more frequent state-switching, thereby artificially elevating SD_BOLD. We hypothesize that SD_BOLD will be higher and more strongly related to performance when using such fixed-length trials relative to self-paced trials that terminate upon a response. We test these two hypotheses in an fMRI study using blocks of fixed-length or self-paced trials. Results confirmed both hypotheses: (1) SD_BOLD was robustly related with absolute mean_BOLD, and (2) toggling between on- and off-task states during fixed-length trials reliably contributed to SD_BOLD. Together, these findings suggest that a reappraisal of the functional significance of SD_BOLD as a unique marker of cognitive performance is warranted.