Brain Efficiency

At one time, scientists believed that people with cognitive reserve had simply been born with bigger brains; that they had more neurons, so if some nerve cells died as a result of Alzheimer's disease or other disease processes, others were waiting to take over. This theory has been disproved —any connection between brain size and cognition is quite subtle.

Imaging studies have provided compelling evidence that brain efficiency, not size, underlies cognitive capacity. Brain scans show that a person learning a new task engages a great deal of the brain or has greater activity in a particular region. As the task becomes familiar, less and less of the brain is needed or used, or the particular brain region becomes less activated. Think about how much concentration it takes to learn a foreign language, and how much easier it becomes over time to speak in that language as the brain becomes more efficient at the task. Dr. Bennett told us, "A good brain is an efficient brain. Cognitive reserve probably has to do with efficiency and the way the brain operates."

A brain with cognitive reserve may have a rich system of "alternative routes" for neural connections. Dr. Bennett compared this to the side streets off a city freeway. "If there is an accident on the freeway, you can get off and use the side streets. You might need to meander around a bit, but you could get where you were going. The side streets weren't built for through traffic, but you can use them for this different purpose. On the other hand, if you're on an isolated two-lane highway and there's an accident, you may have no alternate route." Dr. Bennett speculates that a brain with cognitive reserve may have many more pathways and connections, or "alternate routes," that can rewire themselves when challenged by disease. These may prevent a deficit in memory even though there is pathology in the brain.

A more efficient brain may also help a person perform better on cognitive tests. Processing speed, or how quickly one scans and comprehends information, tends to decline with aging. But according to Dr. Bennett, having a conversation, reading a book or newspaper, listening to the radio, and watching TV all involve processing information, and people who spend more time processing greater varieties of information are found to be better able to maintain youthful scores on tests of processing speed. If a person has better processing resources, it might protect her from expressing Alzheimer's pathology as memory loss.

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Excerpted from THE ALZHEIMER'S PROJECT: MOMENTUM IN SCIENCE, published by Public Affairs, www.publicaffairsbooks.com.

Alzheimer's Disease (AD)

A progressive degenerative disease of the brain that causes impairment of memory and other cognitive abilities.

Amyloid Precursor Protein (APP)

The larger protein from which beta-amyloid is formed.

ApoE Gene

A gene that codes for a protein that carries cholesterol to and within cells; different forms of the ApoE gene are associated with differing risks for late-onset Alzheimer's disease. This gene may be referred to as a risk factor gene or a "susceptibility gene" because one form of the gene, called APOE4, is associated with the risk of developing late onset AD.

Beta-Amyloid

Derived from the amyloid precursor protein and found in plaques, the insoluble deposits outside neurons. May also be called A-beta.

Beta-Amyloid Plaque

A largely insoluble deposit found in the space between nerve cells in the brain. The plaques in Alzheimer's disease are made of beta-amyloid and other molecules, surrounded by non-nerve cells (glia) and damaged axons and dendrites from nearby neurons.

Cognitive Reserve

The brain's ability to operate effectively even when some damage to cells or brain cell communications has occurred.

Dementia

A broad term referring to a decline in cognitive function that interferes with daily life and activities. Alzheimer's disease is one form of dementia.

Functional MRI (fMRI)

An adaptation of an MRI (see magnetic resonance imaging) technique that measures brain activity during a mental task, such as one involving memory, language, or attention.

Hippocampal Formation

A structure in the brain that plays a major role in learning and memory and is involved in converting short-term to long-term memory. Also called the hippocampus.

Inflammation

The process by which the body responds to cellular injury by attempting to eliminate foreign matter and damaged tissue.

Insulin Resistance

A condition in which the pancreas makes enough insulin, but the cells do not respond properly to it; characterizes and precedes type 2 diabetes.

Magnetic Resonance Imaging (MRI)

A diagnostic and research technique that uses magnetic fields to generate a computer image of internal structures in the body.

Mild Cognitive Impairment (MCI)

A condition in which a person has cognitive problems greater than those expected for his or her age. Amnestic MCI includes memory problems, but not the personality or other cognitive problems that characterize AD.

Neurodegenerative Disease

A disease characterized by a progressive decline in the structure and function of brain tissue. These diseases include AD, Parkinson's disease, frontotemporal lobar degeneration, and dementia with Lewy bodies. They are usually more common in older people.

Oligomers

Clusters of a small number of beta-amyloid peptides.

Oxidative Damage

Damage that can occur to cells when they are exposed to too many free radicals.

Pittsburgh Compound B (PiB)

The radioactive tracer compound used during a PET (see Positron Emission Tomography) scan of the brain to show beta-amyloid deposits.

Pittsburgh Compound B (PiB)

The radioactive tracer compound used during a PET (see Positron Emission Tomography) scan of the brain to show beta-amyloid deposits.

Synapse

The tiny gap between nerve cells across which neurotransmitters and nerve signals pass.

Tau

A protein that helps to maintain the structure of microtubules in normal nerve cells. Abnormal tau is a principal component of the paired helical filaments in neurofibrillary tangles.

Tangles

A protein that helps to maintain the structure of microtubules in normal nerve cells. Abnormal tau is a principal component of the paired helical filaments in neurofibrillary tangles.

Memory

Normal Aging

Genetic Risk Factor

Dominant and Recessive Genes

Genes and Proteins

Protein-Misfolding Disease

Cholesterol

Biomarkers

Disease-Modifying Drug

Transgenic Mice

An animal that has had a gene (such as the human APP gene) inserted into its chromosomes for the purpose of research. Mice carrying a mutated human APP gene often develop plaques in their brains as they age.

Pathology

Microglia

Insulin & Insulin Resistance

Susceptibility Gene

A variant in a cell's DNA that does not cause a disease by itself but may increase the chance that a person will develop a disease.

Susceptibility Genes

A variant in a cell's DNA that does not cause a disease by itself but may increase the chance that a person will develop a disease.

Genome-Wide Association Study

Vascular Disease

Genetics

Genetics

Normal Aging