Distinct areas in parietal cortex involved in long-term and short-term action planning: a PET investigation

The sequential organization of events within a script, can be considered as a means to explore the cognitive mechanisms involved in action planning. Scripts are composed of goal-oriented sequences of events that typically occur in a specific and systematic order. The purpose of this study was to investigate the effect of temporal variation between script events (short-term versus long-term) on neural activity in normal volunteers. Subjects were required to judge whether the chronological order of three events was correct or not, while regional cerebral blood flow was measured using positron emission tomography. The modality of script presentation (graphic versus lexical) was manipulated. In the left hemisphere, the supramarginal gyrus, the middle frontal gyrus, the inferior temporal gyrus, and the middle occipital gyrus were more activated in the short-term script conditions, while the angular gyrus, the medial superior frontal gyrus, the precuneus bilaterally, the anterior cingulate gyrus and the inferior and middle temporal gyrus on the left were more involved during long-term scripts processing. The occipito-temporal junction (MT/V5) was significantly more activated in the graphic conditions as compared to the lexical ones, and in the short-term scripts conditions as compared to the long-term ones although all stimuli were static. Our results support the notion that two distinct frontal-parietal networks are engaged in short-term and long-term script processing. In addition, our study demonstrates two kinds of dissociations in the parietal lobe. A macro-dissociation between the anterior and the posterior portion of the inferior parietal lobe was found which accounts for processing short-term and long-term scripts respectively, as well as a micro-dissociation in each of these regions which is associated with distinct processing depending on the presentation modalities. These results may help to better understand apraxia at the neurophysiological level.