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 names of the months of the year) in reverse order concomitant with preserved production of the conventional sequence has been considered a salient sign of frontal lobe dysfunction. Using functional magnetic resonance imaging (fMRI), the spatial and temporal pattern of brain activation during covert performance of these tasks was evaluated in healthy subjects. As compared to the response obtained during forward recitation, re-sequencing of the word string yielded additional activation of the bilateral middle and inferior frontal gyri, the posterior parietal cortex and the left anterior cingulate gyrus. The prefrontal responses are in accordance with the clinical findings referred to. However, the set of activated areas, as a whole, presumably reflects contribution of the various components of the working memory system to the sequencing of word strings. During successive periods of task administration, subjects showed a linear increase of production speed. Analysis of corresponding dynamic changes of regional hemodynamic responses revealed a significant increase at the level of the left inferior parietal cortex and a decrease within the mesial aspect of the left superior frontal gyrus. Presumably, the former finding reflects increasing demands on the phonological short-term memory store, due to faster updating of its content under increased word production rate. Decreasing activation within the superior frontal gyrus might indicate contribution of this area to the initiation of the cognitive processes subserving the sequencing of verbal items. These findings demonstrate the capability of fMRI as a powerful tool for the analysis of dynamic brain activation.