A POSITRON EMISSION TOMOGRAPHY STUDY OF VISUAL AND MENTAL SPATIAL EXPLORATION

We measured normalized regional cerebral blood flow (NrCBF) using positron emission tomography (PET) and oxygen-15-labeled water in eight young right-handed healthy volunteers selected as high-imagers, during 2 runs of 3 different conditions: 1, rest in total darkness 2; visual exploration of a map 3; mental exploration of the same map in total darkness. NrCBF images were aligned with individual magnetic resonance images (MRI), and NrCBF variations between pairs of measurements (N = 15) were computed in regions of interest having anatomical boundaries that were defined using a three-dimensional (3-D) reconstruction of each subject MRI. During visual exploration, we found bilateral activations of primary visual areas, superior and inferior occipital gyri, fusiform and lingual gyri, cuneus and precuneus, bilateral superior parietal, and angular gyri. The right lateral premotor area was also activated during this task while superior temporal gyri and Broca's area were deactivated. By contrast, mental exploration activated the right superior occipital cortex, the supplementary motor area, and the cerebellar vermis. No activation was observed in the primary visual area. These results argue for a specific participation of the superior occipital cortex in the generation and maintenance of visual mental images.