Edited by Kate Findley and proofread by Angela Shoemaker, The Great Courses Daily
Why does your brain slow down as you get older? Professor Vishton explains the mechanisms behind this process and how you can replace dead neurons.
Charting Cognitive Decline
While basic cognitive faculties decline slowly, they nevertheless decline starting after the age of 20. One of the primary reasons for this decline is that our brain shrinks. Neurons die off faster than they are replaced, leaving us with a smaller brain.
The human body is made up of trillions of cells, and those cells die all the time. In some parts of the body, they die very quickly—every cell on the inner lining of your digestive tract dies within a few days.
Fortunately, those cells are replaced just as quickly as they die off. The digestive tract works just fine with these fresh, new, inexperienced cells. Cells in other parts of the body die off very slowly.
The cells in your brain are organized in an intricate fashion. The memories you possess, the skills you have, and the language you know are all built into the pattern of connections between those billions of cells in your brain.
Signals are conducted through that network according to those connections to produce all of the behaviors you will ever exhibit. When new cells are added into this system, the process of integrating them into those existing circuits takes a lot of work. Those new cells have to learn—in a sense—to contribute to the existing computational system.
The neurons die very slowly, but they are created even more slowly. As neurons die off, the brain loses some of its computational ability.
Why Do Neurons Die Off?
Now, the human brain is remarkably robust. When we encode a memory, it’s not saved in a single place like it would be in the memory of a computer chip. All of our memories are encoded across thousands—in some cases millions—of neuronal cells.
If a few of those cells die, the memory will still be accessible. If a few neurons involved in performing some skill you know die off or are destroyed by injury, you’ll still be able to perform that skill.
However, if enough neurons die off at a constant rate, eventually the system starts to be compromised. Those basic abilities measured by cognitive testing decline.
Many things can cause a neuron to die, including physical injury. There is a large body of evidence now demonstrating that frequent trauma to the head—even trauma that doesn’t result in a trip to the hospital—results in cognitive decline even more than that associated with typical aging.
Sports like boxing and American football come with this risk of premature shrinkage of the brain and the cognitive deficits associated with aging. Disease can kill neurons as well.
There are particular viruses and bacteria that specifically attack brain tissue. Circulatory issues can kill brain tissue. If an artery is blocked, preventing blood from flowing to a region of the brain, the neurons in that region will quickly begin dying off.
Keeping Your Brain Active
All of these things can kill off neurons, but as we age, it’s not how most of them go. The majority of neuronal death that occurs is due to simple inactivity.
Neurons exhibit bursts of electrochemical activity called action potentials. These cause signals to be sent to neurons nearby. If one neuron receives enough signals from the excitatory neurons around it, then that neuron may be excited enough to exhibit its own action potential.
Action potentials are the codes that our brain uses to perform computations that support our perception, cognition, and action control. However, those action potentials are also a critical part of the neuronal cells’ physiology.
It’s part of how they pull nutrients and oxygen from the bloodstream and how they expel waste products. If a neuron stops exhibiting action potentials long enough, it will starve.
To reduce age-related brain shrinkage, Professor Vishton recommends that you keep your brain active. Physical activity—even moderate exercise like walking—inspires neural activation and even the creation of new neurons in certain areas of the brain such as the hippocampus.
One of the best ways to inspire broad-based neural activity, however, is to try new things. As we become more experienced at some activity—playing a particular sport, for example—our brain becomes faster and more accurate in producing the proper actions at the right moments in time.
As we become very expert at that activity, several lines of research have shown that we don’t employ more of our brain to support the activity. On the contrary, we learn to use less and less of our brain, developing specialized circuits that rapidly, automatically mediate the task performance.
Peter M. Vishton is Associate Professor of Psychology at William & Mary. He earned his PhD in Psychology and Cognitive Science from Cornell University. Before joining the faculty of William & Mary, he taught at Northwestern University and served as the program director for developmental and learning sciences at the National Science Foundation.