Load- and practice-dependent increases in cerebro-cerebellar activation in verbal working memory: an fMRI study

Load-dependent and practice-related changes in neocortical and cerebellar structures involved in verbal working memory (VWM) were investigated using functional MRI (fMRI) and a two alternative forced choice Sternberg paradigm. Using working memory loads ranging from 2 to 6 letters, regions exhibiting linear and quadratic trends in load-dependent activations were identified. Behaviorally, reaction time measurements revealed significant linear increases with increasing memory load, and significant decreases with increased task practice. Brain activations indicated a preponderance of linear load-dependent responses in both superior (lobule VI/Crus I) and inferior (lobule VIIB/VIIIA) cerebellar hemispheres, as well as in areas of neocortex including left precentral (BA 6), inferior frontal (BA 47), parahippocampal (BA 35), inferior parietal (BA 40), cingulate (BA 32), and right inferior and middle frontal (BA 46/47) regions. Fewer voxels exhibited quadratic without linear trends with the most prominent of these activations located in left inferior parietal (BA 40), precuneus, and parahippocampal regions. Analysis of load × session interactions revealed that right inferior cerebellar and left inferior parietal activations increased with practice, as did the correlations between activation in each region with reaction time, suggesting that changes in this cerebro-cerebellar network underlie practice-related increases in efficiency of VWM performance. These results replicate and extend our previous findings of fMRI activation in the cerebellum during VWM, and demonstrate predominately linear increases in cerebro-cerebellar activation with increasing memory load as well as changes in network function with increased task proficiency.