Effects of verbal working memory load on corticocortical connectivity modeled by path analysis of functional magnetic resonance imaging data

We investigated the hypothesis that there are load-related changes in the integrated function of frontoparietal working memory networks. Functional magnetic resonance imaging time-series data from 10 healthy volunteers performing a graded n-back verbal working memory task were modeled using path analysis. Seven generically activated regions were included in the model: left/right middle frontal gyri (L/R MFG), left/right inferior frontal gyri (L/R IFG), left/right posterior parietal cortex (L/R PPC), and supplementary motor area (SMA). The model provided a good fit to the 1-back (χ² = 7.04, df = 8, P = 0.53) and 2-back conditions (χ² = 9.35, df = 8, P = 0.31) but not for the 3-back condition (χ² = 20.60, df = 8, P = 0.008). Model parameter estimates were compared overall among conditions: there was a significant difference overall between 1-back and 2-back conditions (χ²_diff = 74.77, df = 20, P <0.001) and also between 2-back and 3-back conditions (χ²_diff = 96.28, df = 20, P <0.001). Path coefficients between LIFG and LPPC were significantly different from zero in both 1-back and 2-back conditions; in the 2-back condition, additional paths from LIFG to LPPC via SMA and to RMFG from LMFG and LPPC were also nonzero. This study demonstrated a significant change in functional integration of a neurocognitive network for working memory as a correlate of increased load. Enhanced inferior frontoparietal and prefrontoprefrontal connectivity was observed as a correlate of increasing memory load, which may reflect greater demand for maintenance and executive processes, respectively.