AAMI is seen in humans, monkeys, and rodents and is not accompanied by significant neuron loss, but is more likely caused by synaptic changes. The corticocortical circuits that are vulnerable in AD are also affected in normal aging in that spines on the neurons providing these circuits are lost, but the circuits are still largely intact (Fig. 2, Duan et al., 2003). In addition, there are age-related changes in the perforant path projection such as a loss of presynaptic markers in rodents, and a loss of postsynaptic glutamate receptors, particularly N-methyl-d-aspartate (NMDA) receptors, in monkeys (Morrison and Hof, 2002). The electrophysiological properties of hippocampal neurons reflective of NMDA receptor function are also compromised in aged animals (see Section 5 below, aging brain and plasticity).