Specific versus nonspecific brain activity in a parametric n-back task

In this study functional magnetic resonance imaging (fMRI) was used to exam ine cerebral activity patterns in relation to increasing mental load of a w orking memory task. Aim of the experiment was to distinguish nonspecific ta sk-related processes from specific workload processes analytically. Twelve healthy volunteers engaged in a spatial n-back task with four levels. FMRI data were acquired with the SD-PRESTO pulse sequence. Analysis entailed a t wo-step multiple regression algorithm, which was specifically designed to m easure and separate load-sensitive and load-insensitive activity simultaneo usly, while preserving the original high spatial resolution of the fMRI sig nal. Load-sensitive and load-insensitive activity was found in both dorsola teral-prefrontal and parietal cortex, predominantly bilaterally, and in the anterior cingulate. As expected, the left primary sensorimotor cortex show ed predominantly load-insensitive activity. Load-sensitive activity reflect s specific working memory functions, such as temporary retention and manipu lation of information, while load-insensitive activity reflects supportive functions, such as visual orientation, perception, encoding, and response s election and execution. Good performance was correlated with a large area o f load-sensitive activity in anterior cingulate, and with a small area of l oad-insensitive activity in the right parietal cortex. The findings indicat e that nonspecific and specific working memory processes colocalize and are represented in multiple frontal and parietal regions. Implication of this analytical strategy for application in research on psychiatric disorders is discussed. (C) 2000 Academic Press.