The effects of aging on layer 1 of primary visual cortex in the rhesus monkey

The effect of age on layer 1 in primary visual cortex was determined in 19 rhesus monkeys of various ages. Twelve of the monkeys had been behaviorally tested. With age layer 1 becomes thinner and the glial limiting membrane b ecomes thicker. In the neuropil of layer 1 many of the dendrites in old mon keys appear to be degenerating and, as a consequence, electron micrographs from old monkeys display fewer dendritic and spine profiles per unit area t han in young monkeys. As determined using both the disector and size-freque ncy methods, there is also a concomitant decrease in the numerical density of synapses with age. Although there is a significant correlation between t he thinning of layer 1 in area 17 and age, there is no significant correlat ion between either the thinning of layer 1 or its loss of synapses and any of the behavioral measures of memory function obtained from the 12 behavior ally tested monkeys. Similar morphological changes with age occur in layer 1 of prefrontal cortex of these same monkeys, but in area 46 both the thinn ing of layer 1 and the loss of synapses shaw a significant correlation with behavioral measures of memory function. These differences between layer 1 in these two cortical areas presumably relate to the fact that prefrontal c ortex has a greater role in subserving cognition than does primary visual c ortex.