Henry Molaison had become the world’s most famous psychological patient but had no knowledge of that important fact. He’d been told of his fame many times, but to no avail. Mr. Molaison had no way to remember that or anything else.
Called HM to preserve his anonymity, Henry had developed severe epilepsy as a young man and, seeking relief, had the inner parts of both temporal lobes removed, including the hippocampus. Relief he obtained, but at a terrible price. From that day on, HM could not form a lasting memory. He could hold a conversation and performed well on an intelligence test, but he couldn’t remember anything about his day-to-day life.
We’ll talk further about HM’s contribution to science but it should be pointed out that, in his own way, HM sacrificed greatly but contributed to a revolution in science that aims to accurately describe what it is to be human.
Let’s first get a sense of the geography of the brain and a few of the roles that various regions play. We previously learned that neurons form connections with each other at their synapses and that the magic of information processing in living flesh has to do with their massive inter-connectedness—up to one thousand trillion connections in the human brain. By themselves, neurons don’t do much. The magic of a nervous system lies in the connections and most of the connections form integrated units called circuits.
The process of circuit building begins very early during fetal development in the womb. Neurons under the genetic control of DNA spread out from discrete starting locations on pre-built scaffolds made of glial cells. Once a neuron is in its final location, it seeks out connections by sending out its single axon to make synaptic contact with other neurons, usually at their dendrites. The neural pattern that forms is the basic structure of the working brain—an assembly of circuits ready to experience the world and, in turn, be modified by that experience. It’s ready to learn.
A Good Pruning
Signals from the outside world now have a startling effect on the number of neurons in the brain. One would be inclined to postulate that with growth and experience, an infant should develop increasing numbers of synaptic connection. That intuition would be wrong. As we saw in the previous discussion of neurons, whenever a strong electric potential runs through the axon of a neuron, the connection to the receiving neuron is strengthened. This is the basis of learning, sometimes called Hebbian plasticity after the researcher who showed that neurons that fire together wire together. As an infant experiences the world, some synaptic connections become stronger than others. These stronger neuronal connections are allowed to remain but the weaker ones undergo a programmed cell death and are pruned from the brain.
The current evidence suggests that the pruning process in humans occurs in several waves. One happens during the first few years after birth when trillions of connections are eliminated and a second large event takes place at adolescence. The process is remarkably Darwinian and has come to be called neural selection—only the strongest neurons and synapses are selected for survival.
For some psychologists, neural pruning puts a time limit on learning, but most argue that learning is a lifelong process because Hebbian plasticity is never turned off. Neurons that fire together, continue to wire together for a lifetime in normally functioning brains. Perhaps some aspects of a person (personality, for example) are somewhat locked down, but the ability to learn information, alter one’s perceptions, and grow as individuals can operate well into old age.