Dynamic pattern of brain activation during sequencing of word strings evaluated by fMRI

An impaired ability to recite highly automated word strings (e.g., the name s of the months of the year) in reverse order concomitant with preserved pr oduction of the conventional sequence has been considered a salient sign of frontal lobe dysfunction. Using functional magnetic resonance imaging (fMR I), the spatial and temporal pattern of brain activation during covert perf ormance of these tasks was evaluated in healthy subjects. As compared to th e response obtained during forward recitation, re-sequencing of the word st ring yielded additional activation of the bilateral middle and inferior fro ntal gyri, the posterior parietal cortex and the left anterior cingulate gy rus. The prefrontal responses are in accordance with the clinical findings referred to. However, the set of activated areas, as a whole, presumably re flects contribution of the various components of the working memory system to the sequencing of word strings. During successive periods of task admini stration, subjects showed a linear increase of production speed. Analysis o f corresponding dynamic changes of regional hemodynamic responses revealed a significant increase at the level of the left inferior parietal cortex an d a decrease within the mesial aspect of the left superior frontal gyrus. P resumably, the former finding reflects increasing demands on the phonologic al short-term memory store, due to faster updating of its content under inc reased word production rate. Decreasing activation within the superior fron tal gyrus might indicate contribution of this area to the initiation of the cognitive processes subserving the sequencing of verbal items. These findi ngs demonstrate the capability of fMRI as a powerful tool for the analysis of dynamic brain activation. (C) 1999 Elsevier Science B.V. All rights rese rved.