VISUOMOTOR TRANSFORMATIONS FOR REACHING TO MEMORIZED TARGETS - A PET STUDY

Positron emission tomography (PET) was used to identify cortical and s ubcortical regions involved in the control of reaching to visual targe ts, Regional cerebral blood flow (rCBF) was measured in eight healthy subjects using (H2O)-O-15 PET during the performance of three differen t tasks, All tasks required central fixation while a 400-ms target was hashed every 5 s at a random location around a virtual circle centere d on the fixation target, Additional instructions differed according t o the task: (i) visual detection of the target without overt responses ; (ii) immediate pointing to the most recent target in the sequence, a nd (iii) pointing to the previous target in the sequence. By design, t he two motor tasks differed in the cognitive processing required, In e ach trial of immediate pointing, the spatial location of only the most recent target needed to be processed, In each trial of pointing to th e previous, instead, while the most recent target was stored in memory for the movement of the next trial, the previous target had to be ret rieved from memory to direct the current movement, Limb trajectories w ere comparable between the two motor tasks in terms of most spatiotemp oral parameters examined. Significant rCBF increases were identified u sing analysis of covariance and t statistics. Compared with visual det ection there was activation of primary sensorimotor cortex, ventrolate ral precentral gyrus, inferior frontal gyrus in the opercular region, supramarginal gyrus, and middle occipital gyrus, all these sites in th e hemisphere (left) contralateral to the moving limb, and cerebellar v ermis, during both immediate pointing and pointing to the previous. Du ring immediate pointing there was additional activation of left inferi or parietal lobule close to the intraparietal sulcus, and when compare d with pointing to the previous, dorsolateral prefrontal cortex bilate rally. During pointing to the previous, instead, there was additional activation of supplementary motor cortex, anterior and midcingulate, a nd inferior occipital gyrus in the left hemisphere; superior parietal lobule, supramarginal gyrus, and posterior hippocampus in the right he misphere; lingual gyri acid cerebellar hemispheres bilaterally; anteri or thalamus; and pulvinar. The activation of two partially distinct ce rebral networks in these two motor tasks reflects the different nature of signal processing involved. In particular, the specific activation of intraparietal sulcus and prefrontal cortex in immediate pointing a ppears characteristic of a network for visuospatial working memory. By contrast, the corticolimbic network engaged in pointing to the previo us could mediate spatial attention and the sequence of encoding, recod ing, and decoding of spatial memories required by a dual task with two competing targets. (C) 1997 Academic Press.