Neural correlates of individual variation in two-back working memory and the relationship with fluid intelligence

Working memory has been examined extensively using the N-back task. However, less is known about the neural bases underlying individual variation in the accuracy rate (AR) and reaction time (RT) as metrics of N-back performance. Whereas AR indexes the overall performance, RT may more specifically reflect the efficiency in updating target identify. Further, studies have associated fluid intelligence (Gf) with working memory, but the cerebral correlates shared between Gf and N-back performance remain unclear. We addressed these issues using the Human Connectome Project dataset. We quantified the differences in AR (critical success index or CSI) and RT between 2- and 0-backs (CSI_2-0 and RT_2-0) and identified the neural correlates of individual variation in CSI_2-0, RT_2-0, and Gf, as indexed by the number of correct items scored in the Raven's Standard Progressive Matrices (RSPM) test. The results showed that CSI_2-0 and RT_2-0 were negatively correlated, suggesting that a prolonged response time did not facilitate accuracy. At voxel p < 0.05, FWE-corrected, the pre-supplementary motor area (preSMA), bilateral frontoparietal cortex (biFPC) and right anterior insula (rAI) showed activities in negative correlation with CSI_2-0 and positive correlation with RT_2-0. In contrast, a cluster in the dorsal anterior cingulate cortex (dACC) bordering the SMA showed activities in positive correlation with CSI_2-0 and negative correlation with RT_2-0. Further, path analyses showed a significant fit of the model dACC → RT_2-0 → CSI_2-0, suggesting a critical role of target switching in determining performance accuracy. Individual variations in RT_2-0 and Gf were positively correlated, although the effect size was small (f² = 0.0246). RT_2-0 and Gf shared activities both in positive correlation with the preSMA, biFPC, rAI, and dorsal precuneus. These results together suggest inter-related neural substrates of individual variation in N-back performance and highlight a complex relationship in the neural processes supporting 2-back and RSPM performance.