Throughout the 19th and 20th century, much progress was made by breaking the brain down into various components, with individual neurons viewed as the fundamental unit for human brain activity. The number of these neurons is roughly a hundred billion; the number of contacts between neurons is a hundred trillion. The average neuron receives thousands of distinct inputs, with each neuron connecting to many other neurons; however the exact physical "circuitry" of these individual components is unknown. Taxpayer funded research in neuroscience has been crucial to advancing our understanding into the workings of the brain. During FY 2013, the National Institutes of Health (NIH) spent over $5.6 billion in supporting neuroscience-related research. The economic implications for this research are significant. The monetary cost of dementia, including Alzheimer's disease, in the United States ranges from $157 billion to $215 billion annually. Dementia is more costly to the nation than either heart disease or cancer.