Functional MR imaging of the prefrontal cortex: specific activation in a working memory task

Functional magnetic resonance imaging was used to identify cortical regions activated by a working memory task involving letter detection. Twenty four normal subjects were scanned with a conventional 1.5-T magnet while performing one of two tasks: In the activation task, subjects responded by pressing a button whenever any presented letter was the same as the second last in the sequence. In the control condition, subjects had to respond to a single predefined letter without memory update requirements. The activation task and the control condition were identical with regard to perceptual input and motor output. They were different only regarding the task demand. Movement artifacts were minimized in a two way strategy and eight subjects were excluded from further analysis. Functional MR data from the remaining 16 subjects were analyzed on the basis of anatomical regions-of-interest which were manually defined in each subject. The engagement of working memory produced significant activation in the dorsolateral prefrontal cortex (Brodmann's areas 9, 10, 46, and 47) in both hemispheres. Results demonstrate the applicability of the paradigm within a clinical MRI setup and corroborate previous findings of non-lateralized dorsolateral prefrontal activation during continuous context updating and active maintenance.