cognitive decline in the elderly. Age-related neuronal morphology There is converging evidence from cognitive neuroscientists around the world that age-induced cognitive deficits may not be due to neuronal loss or cell death, but rather may be the result of small region-specific changes to the morphology of neurons. Studies by Duan et al., have shown that dendritic arbors and dendritic spines of cortical pyramidal neurons decrease in size and/or number in specific regions and layers of human and non-human primate cortex as a result of age (Duan et al., 2003; morph). A 46% decrease in spine number and spine density has been reported in humans older than 50 compared with younger individuals. An electron microscopy study in monkeys reported a 50% loss in spines on the apical dendritic tufts of pyramidal cells in prefrontal cortex of old animals (27–32 years old) compared with young ones (6–9 years old). Neurofibrillary tangles Age-related neuro-pathologies such as Alzheimer's disease, Parkinson's disease, diabetes, hypertension and arteriosclerosis make it difficult to distinguish the normal patterns of aging. One of the important differences between normal aging and pathological aging is the location of neurofibrillary tangles. Neurofibrillary tangles are composed of paired helical filaments (PHF). In normal, non-demented aging, the number of tangles in each affected cell body is relatively low and restricted to the olfactory nucleus, parahippocampal gyrus, amygdala and entorhinal cortex. As the non-demented individual ages, there is a general increase in the density of tangles, but no significant difference in where tangles are found. The other main neurodegenerative contributor commonly found in the brain of patients with AD is amyloid plaques. However, unlike tangles, plaques have not been found to be a consistent feature of normal aging. Role of oxidative stress Cognitive impairment has been attributed to oxidative stress, inflammatory reactions and changes in the cerebral microvasculature. The exact impact of each of these mechanisms in affecting cognitive aging is unknown. Oxidative stress is the most controllable risk factor and is the best understood. The online Merriam-Webster Medical Dictionary defines oxidative stress as, "physiological stress on the body that is caused by the cumulative damage done by free radicals inadequately neutralized by antioxidants and that is to be associated with aging." Hence oxidative stress is the damage done to the cells by free radicals that have been released from the oxidation process. Compared to other tissues in the body, the brain is deemed unusually sensitive