Hippocampal atrophy and disconnection in incipient and mild Alzheimer's disease.

Quantitative imaging techniques allow the in vivo investigation of age and disease related changes in the brain and their relation to cognitive function. In this chapter we review imaging evidence indicating that the entorhinal cortex and hippocampus show atrophy very early in Alzheimer's disease (AD) and in individuals who are at risk of developing AD compared to age appropriate controls. Furthermore, the extent and rate of atrophy of the entorhinal cortex, a brain region pathologically involved very early in the disease process, can predict who among the elderly will develop AD. Techniques that assess the integrity of white matter further demonstrate that alterations in the parahippocampal white matter in the region that includes the perforant path could partially disconnect the dentate gyrus and other hippocampal subfields from incoming sensory information. Such partial disconnection and degradation in transmission of sensory information in people at risk of AD and in patients with very mild AD could contribute to the memory dysfunction associated with the early stages of the disease.