It has been suggested that age-related cognitive decline is due in part not to neuronal death but to synaptic alterations. Evidence in support of this idea from animal work has also suggested that this cognitive deficit is due to functional and biochemical factors such as changes in enzymatic activity, chemical messengers, or gene expression in cortical circuits. Thinning of the cortex
Advances in MRI technology have provided the ability to see the brain structure in great detail in an easy, non-invasive manner in vivo. Bartzokis et al., has noted that there is a decrease in grey matter volume between adulthood and old age, whereas white matter volume was found to increase from age 19–40, and decline after this age. Studies using Voxel-based morphometry have identified areas such as the insula and superior parietal gyri as being especially vulnerable to age-related losses in grey matter of older adults. Sowell et al., reported that the first 6 decades of an individuals life were correlated with the most rapid decreases in grey matter density, and this occurred over dorsal, frontal, and parietal lobes on both interhemispheric and lateral brain surfaces. It is also worth noting that areas such as the cingulate gyrus, and occipital cortex surrounding the calcarine sulcus appear exempt from this decrease in grey matter density over time. Age effects on grey matter density in the posterior temporal cortex appear more predominantly in the left versus right hemisphere, and were confined to posterior language cortices.
Changes in attention
Many older adults notice a decline in their attentional abilities. Attention is a broad construct that refers to "the cognitive ability that allows us to deal with the inherent processing limitations of the human brain by selecting information for further processing" (p. 334). Since the human brain has limited resources, people use their attention to zone in on specific stimuli and block out others. If older adults have fewer attentional resources than younger adults, we would expect that when two tasks must be carried out at the same time, older adults performance will decline more than that of younger adults. However, a large review of studies on cognition and aging suggest that this hypothesis has not been wholly supported. While some studies have found that older adults have a more difficult time encoding and retrieving information when their attention is divided, other studies have not found meaningful differences from younger adults. Similarly, one might expect older adults to do poorly on tasks of sustained attention, which measure the ability to attend to and respond to stimuli for an extended period of time. However, studies suggest that sustained attention shows no decline with age. Results suggest that sustained attention increases in early adulthood and then remains relatively stable, at least through the seventh decade of life. More research is needed on how normal aging impacts attention after age eighty. It is worth noting that there are factors other than true attentional abilities that might relate to difficulty paying attention.