Increasing perceptual difficulty is associated with increased activation of frontal cortex. Activation of rCBF during stimulus encoding was seen in young subjects in left frontal cortex and right hippocampus but not in old subjects, suggesting age-related impairment of learning processes. A study of visual selective attention showed that attention within a sensory modality is associated with increased activity in those brain areas that process the relevant stimulus features and decreased activity in brain areas that process input from other sensory modalities. A review of the literature revealed that some sparing of cognitive function early in Alzheimer disease is probably due to the efforts of the brain to compensate for loss of circuits, either through synaptic plasticity or reorganization of functional networks. White matter hyperintensities (on the MRI) are predictive of metabolic brain alterations and reductions in cognitive function, even in very healthy individuals. Patients with dementia of the Alzheimer type (DAT) who also have white matter changes on MRI, showed relatively lower metabolism in subcortical gray matter and calcarine cortex and smaller reductions in neocortical association regions, compared to DAT patients without white matter changes, suggesting that leukoencephalopathy alters the pattern of metabolic abnormality associated with DAT.