Can an old head injury suddenly cause detrimental effects much later in life?

—Anonymous, via e-mail

Douglas Smith, professor of neurosurgery and director of the Center for Brain Injury and Repair at the University of Pennsylvania, answers:

ALTHOUGH A BRAIN INJURY from a car accident or a collision during a football game often seems to cause a sudden change to cognitive ability years later, this change does not just appear out of the blue—the damage has been building up slowly, unnoticed, over time.

Postinjury, the progressive brain deterioration that may occur likely reaches a tipping point, after which the loss of function “suddenly” becomes obvious. Depending on the type and severity of the traumatic brain injury (TBI), it can accelerate memory loss or increase a person’s chance of succumbing to Alzheimer’s disease.

TBI commonly damages nerve fibers in the brain called axons. These thin, tubelike structures transmit electrical and chemical signals that are vital for carrying information among different regions of the brain. For unknown reasons, these fragile structures not only disconnect shortly after injury but can continue to disconnect even for decades later in some patients. Once disconnected, the blunt end of an axon seals itself off, swells with fluids, enzymes and proteins and eventually bursts. When axons burst open, they often distribute amyloid proteins through the neighboring brain tissue. These sticky proteins are a hallmark of Alzheimer’s, and in fact many TBI patients exhibit signs of dementia later in life that mimic the deterioration observed in Alzheimer’s patients.

In addition, with axons disappearing or not functioning well after TBI, a person’s ability to process new information may slow down. Surviving axons may compensate for the damage by increasing electrical signaling and thus restoring the normal speed of information processing in the brain. This temporary fix, however, can cause these axons to become even more sensitive to damage if a second concussion occurs.

Most people with TBI will have progressive axonal damage, but it is difficult to predict who will suffer from cognitive changes years later. TBIs have a devastating effect on society, with more than 1.5 million cases documented in the U.S. every year. Currently no therapies exist for either short- or long-term damage, which means for now the best treatment is protection and prevention.

What is the memory capacity of the human brain? Is there a physical limit to the amount of information it can store?

—J. Hawes, Huntington Beach, Calif.

Paul Reber, professor of psychology at Northwestern University, replies:

"MR. OSBORNE, may I be excused? My brain is full,” a student with a particularly tiny head asks his classroom teacher in a classic Far Side comic by Gary Larson. The deadpan answer to this question would be, “No, your brain is almost certainly not full.” Although there must be a physical limit to how many memories we can store, it is extremely large. We don’t have to worry about running out of space in our lifetime.

The human brain consists of about one billion neurons. Each neuron forms about 1,000 connections to other neurons, amounting to more than a trillion connections. If each neuron could only help store a single memory, running out of space would be a problem. You might have only a few gigabytes of storage space, similar to the space in an iPod or a USB flash drive. Yet neurons combine so that each one helps with many memories at a time, exponentially increasing the brain’s memory storage capacity to something closer to around 2.5 petabytes (or a million gigabytes). For comparison, if your brain worked like a digital video recorder in a television, 2.5 petabytes would be enough to hold three million hours of TV shows. You would have to leave the TV running continuously for more than 300 years to use up all that storage.

The brain’s exact storage capacity for memories is difficult to calculate. First, we do not know how to measure the size of a memory. Second, certain memories involve more details and thus take up more space; other memories are forgotten and thus free up space. Additionally, some information is just not worth remembering in the first place.

This is good news because our brain can keep up as we seek new experiences over our lifetime. If the human life span were significantly extended, could we fill our brains? I’m not sure. Ask me again in 100 years.