Explain this: if Alzheimer’s disease  is caused by the accumulation of sticky amyloid plaques and neurofibrillary tangles that kill neurons (as the leading theories of the disease hold), then how can holding a job that poses minimal cognitive demands and watching a lot of TV raise your risk of developing the disease? Nothing in neuroscience suggests that either activity speeds the formation of plaques or tangles—or, conversely, that mentally-demanding jobs and spending your leisure time in more intellectually-engaging pursuits blocks their formation.

New research is leading to an inescapable conclusion. Being extra brainy when you enter old age has no effect on the basic pathological processes that attack the brain. Sorry. But here’s what it does do: when that pathology kicks in (or even when less insidious, normal brain-aging starts to impair memory and neuronal processing speed), a brain that has a “cognitive reserve” can take more hits before showing the effects than a brain with less cognitive reserve.

Think of it this way: if a cyber-thief siphons $100 a week from your bank account, you’ll have enough to pay the bills longer if you start with a large balance than if you start with a small one. Cognitive reserve is your brain’s bank balance.

This is very different from the commonly-held belief that mental activity slows the rate of cognitive decline, as Timothy Salthouse of the University of Virginia, a veteran of studies on aging and cognition, pointed out in a 2006 analysis. This “mental exercise” hypothesis rests on studies finding that higher mental activity, such as reading and doing crossword puzzles, is associated with better cognitive function in old age. That does not mean, however, that older adults who stay more mentally active are slowing their rate of cognitive decline. Instead, it is becoming increasingly clear, they enter old age (i.e. their 30s, which is the decade when cognitive function starts heading south, accelerating in your 50s) with a cognitive reserve, a sort of mental cushion. That starts them off far enough above the threshold where impaired mental function affects the ability to think and remember that a normal decline leaves them pretty much okay for a longer period. That cyber-thief’s $100-per-week leaves the well-endowed with enough to pay the bills for many years.

But here’s the awful part. In the most mentally-engaged elderly, such as professors who enter their 60s or 70s with smarts to spare, the rate of mental decline is typically just as steep as in dullards. True, a brain functioning far above the average will still be pretty good even with the equal rate of decline—but these people notice their diminishing capacities nonetheless. Sadly, therate of mental decline is not affected by cognitive exercise, Salthouse wrote back in 2006, arguing that there is “little scientific evidence that engagement in mentally stimulating activities alters the rate of mental aging.” The belief that it does is “more of an optimistic hope than an empirical reality.”

The latest study, too, supports the cognitive-reserve idea. People with greater thinking, learning and memory abilities are able to delay symptoms of Alzheimer’s disease—but not necessarily the underlying pathology: their brains still suffer neuron-killing plaques and tangles, but they function adequately longer than people with a smaller cognitive cushion. As Catherine M. Roe and colleagues at the Washington University School of Medicine write in Archives of Neurology in a paper being posted this afternoon, “a greater pathological burden is required to show an effect on cognition among persons with more education.”

They studied 37 people with Alzheimer’s and 161 healthy individuals, noting their education history and giving them cognitive tests. They were then tested for the presence of the beta-amyloid plaques considered a marker of Alzheimer’s. Even in people with lots of beta-amyloid plaques, the more years of education they had had the better they did on the test. “The results support the hypothesis that cognitive reserve influences the association between Alzheimer disease pathological burden and cognition,” the authors write. Translation: the smarter you are, the more beta-amyloid plaques your brain can tolerate before showing symptoms of Alzheimer’s.