levels of consumption. Nervous system Chronic heavy alcohol consumption impairs brain development, causes alcohol dementia, brain shrinkage, physical dependence, alcoholic polyneuropathy (also known as 'alcohol leg'), increases neuropsychiatric and cognitive disorders and causes distortion of the brain chemistry. At present, due to poor study design and methodology, the literature is inconclusive on whether moderate alcohol consumption increases the risk of dementia or decreases it. Evidence for a protective effect of low to moderate alcohol consumption on age-related cognitive decline and dementia has been suggested by some research; however, other research has not found a protective effect of low to moderate alcohol consumption. Some evidence suggests that low to moderate alcohol consumption may speed up brain volume loss. Chronic consumption of alcohol may result in increased plasma levels of the toxic amino acid homocysteine; which may explain alcohol withdrawal seizures, alcohol-induced brain atrophy and alcohol-related cognitive disturbances. Alcohol's impact on the nervous system can also include disruptions of memory and learning (see Effects of alcohol on memory), such as resulting in a blackout phenomenon. Strokes Epidemiological studies of middle-aged populations generally find the relationship between alcohol intake and the risk of stroke to be either U- or J-shaped. There may be very different effects of alcohol based on the type of stroke studied. The predominant form of stroke in Western cultures is ischemic, whereas non-western cultures have more hemorrhagic stroke. In contrast to the beneficial effect of alcohol on ischemic stroke, consumption of more than two drinks per day increases the risk of hemorrhagic stroke. The National Stroke Association estimates this higher amount of alcohol increases stroke risk by 50%. "For stroke, the observed relationship between alcohol consumption and risk in a given population depends on the proportion of strokes that are hemorrhagic. Light-to-moderate alcohol intake is associated with a lower risk of ischemic stroke which is likely to be, in part, causal. Hemorrhagic stroke, on the other hand, displays a log-linear relationship with alcohol intake." Brain Alcohol misuse is associated with widespread and significant brain lesions. Alcohol related brain damage is not only due to the direct toxic effects of alcohol; alcohol withdrawal, nutritional deficiency, electrolyte disturbances, and liver damage are also believed to contribute to alcohol-related brain damage. Cognition and dementia Excessive alcohol intake is associated with impaired prospective memory. This impaired cognitive ability leads to increased failure to carry out an intended task at a later date, for