PLANNING AND SPATIAL WORKING-MEMORY - A POSITRON EMISSION TOMOGRAPHY STUDY IN HUMANS

Previous work with both neurosurgical and neurodegenerative patient gr oups has suggested that high level planning is mediated by neural circ uitry which includes both the prefrontal cortex and the striatum. In t his study, the functional anatomy of cognitive planning was investigat ed further, using positron emission tomography (PET). Regional cerebra l blood flow (rCBF) was measured in 12 normal volunteers while perform ing easy and difficult versions of (i) a modified Tower of London plan ning task; (ii) a mnemonic Variant of this task that required short-te rm retention and reproduction of problem solutions; and (iii) a contro l condition that involved identical visual stimuli and motor responses . Significant increases in rCBF were observed in the left hemisphere, in both the mid-dorsolateral frontal cortex and in the head of the cau date nucleus, when the difficult planning task was compared with the c ontrol condition. Moreover, subtraction of a simple planning condition from the more difficult one revealed focal increases in rCBF in the c audate nucleus and the thalamus only. During both mnemonic variants of the planning task, changes were also observed in the mid-dorsolateral frontal cortex and in more ventral frontopolar regions, bilaterally. When compared directly, the planning and memory conditions differed in terms of these ventral activation foci, but not in the pattern of act ivation observed in the mid-dorsolateral frontal cortex. These finding s further implicate frontostriatal circuitry in high-level planning an d provide evidence for functionally distinct contributions from ventra l and dorsolateral frontal regions to spatial working memory.