PET correlates of normal and impaired memory functions
Heiss, W.D.; Pawlik, G.; Holthoff, V.; Kessler, J.; Szelies, B.
Cerebrovascular and Brain Metabolism Reviews 4(1): 1-27
1992
ISSN/ISBN: 1040-8827 PMID: 1562450 Document Number: 391881
PET was repeatedly used to investigate metabolic and/or blood flow abnormalities in patients with various amnestic syndromes. In cases with uni- or bilateral lesions of mesiotemporal structures, caused by surgery, herpes simplex encephalitis, or permanent ischemic, anoxic, or toxic damage, disturbances of metabolism and blood flow typically extended far beyound the morphological defects detected by computed tomography or magnetic resonance. In acute transient global amnesia, CBF (cerebral blood flow) and metabolism were decreased bilaterally in the mesiotemporal lobes, where hypometabolism persisted for some time, while higher values were observed in thalamus and some cortical areas. Diencephalic lesions causing Korsakoff's syndrome were associated with decreased rCMRGl in the hippocampal formation, upper brainstem, cingulate, and thalamus. Discrete thalamus infarcts caused amnesia and metabolic depression in the morphologically intact ipsilateral thalamus and in various projection areas of the infarcted nuclei. In ischemic forebrain lesions, amnestic deficits could be related to involvement of the anterior cingulate and of basal cholinergic nuclei. A large number of pathologies are diffusely spread out in the brain and affect partially or predominantly structures in memory processing. This holds true especially in the various dementias where memory disturbances are a consistent and often leading feature. Notably, Alzheimer's disease can be distinguished from other dementias by its characteristic pattern of metabolic dysfunction, with the most prominent changes occurring in parietotemporal and frontal association cortex whose residual metabolism is related to the severity of the disease. Therefore, activation studies using paradigms involving memory functions enhance that typical pattern. Only in the activated state is metabolism of mesiotemporal structures significantly correlated with the performance in memory tests. Other dementias also affect some of the distributed memory networks, with Huntington's disease suggesting a role of the striatum in memory processing. In posttraumatic encephalopathies, brain damage usually is multifocal, typically involving most severely inferotemporal and basal frontal areas. In selective amnesias, focal metabolic lesions ofter are found in regions subserving the correspondence between unimodal association cortex and the limbic system. Accordingly, prosopagnosia could be related to functional disturbances in the right occipitotemporal junction and parietal lobe. Subjects complaining of senescent forgetfulness or age-associated memory impairment exhibited rCMRGl values in the range of age-matched controls without memory deficits. The described PET results in normals during memory tasks and in patients with memory disorders of various etiologies emphasize both the important role of the limbic system and the wide regional representation of this fundamental and complex brain function.