Mapping the visual recognition memory network with PET in the behaving baboon

By means of a novel F-18-fluoro-deoxyglucose PET method designed for cognitive activation imaging in the baboon, the large-scale neural network involved in visual recognition memory in the nonhuman primate was mapped for the first time. In this method, the tracer is injected in the awake, unanesthetized, and unrestrained baboon performing the memory task, and brain imaging is performed later under light anesthesia. Brain maps obtained during a computerized trial-unique delayed matching-to-sample task (lists of meaningless geometrical patterns and delay > 9 seconds) were statistically compared pixel-by-pixel to maps obtained during a specially designed visuomotor control task. When displayed onto the baboon's own anatomic magnetic resonance images, foci of significant activation were distributed along the ventral occipitotemporal pathway, the inferomedial temporal lobe (especially the perirhinal cortex and posterior hippocampal region); and the orbitofrontal cortex, consistent with lesion, single-unit, and autoradiographic studies in monkeys, as well as with activation studies in healthy humans. Additional activated regions included the nucleus basalis of Meynert, the globus pallidus and the putamen. The results also document an unexpected left-sided advantage, suggesting hemispheric functional specialization for recognition of figural material in nonhuman primates.