Let's begin by talking about how the brain develops physically.
The brain goes through different phases in development. Before birth, a process called neurogenesis takes place, during which cells are generated. The cells then migrate to the places in the brain where they belong. Then the brain starts to wire itself. By the time a child is about 3 years old, 80 percent of the brain growth is finished.
Would you say that the early years are the most important time for brain growth?
Brain growth from conception to 3 is very important, but the words "most important" may be a bit difficult to buy into. Adulthood is the summation of all our development. Although many important changes go on from ages 0 to 3 and from 3 to 6, learning and brain changes take place over the life span.
In what ways does the brain wire itself?
Brain cells are different from other cells. They have a cell body just as a regular cell does, but they also have projections called axons and dendrites. These projections seem to send out connections to other cells. And the connections can be very long. Single cells can be as long as three feet. For instance, nerve cells in your spine can go from the bottom of your spine down to your toes. Cells make connections with other cells at synapses. A typical mature brain cell gets wires from about a thousand other cells and connects to about a thousand other cells. The wiring is very complicated.
How much of this process is genetically driven and how much is experiential?
That's the million-dollar question, and the answer is not as clean as we'd like it to be. The sending out of the wires seems to be under genetic control. There needn't be any activity in the brain to start the process. Even if you put those cells in a dish, they'll send out axons. And inside the brain, they find their way along pathways reasonably well. For example, when the eye is sending its nerve cells into the brain, it sends out some inappropriate connections, but many axons connect to the parts of the brain that will be related to vision. Then an activity-dependent stage occurs when the neurons begin sending electrochemical activity down the wires. The eyes aren't seeing yet at this stage. Finally, there's a stage during which you need patterned activity to make the brain connections. The eye has to be sending appropriate activity to the brain to allow the final precise wiring to be done. And that is an environment-driven, or experience-expectant, stage.
The visual system knows that it's going to have two eyes and that they are going to focus on objects, and it wires itself appropriately. We think that experience-expectant prewiring goes on in all sorts of brain parts. It is as if the brain is ready for certain kinds of information at different times.
Perhaps that is what is meant by the term "developmentally appropriate." Until a child's brain has reached certain stages of physical development, certain behavioral expectations may not be appropriate. Does this go back to the nature of the connections the nerve cells make?
Yes. Once the neurons make connections, the brain surrounds and insulates the nerve cells with myelin, which allows the conduction to go much faster. Different parts of the brain myelin-ate at different times in life. So, different capabilities will become efficient at different ages. Although children may experience an onset of capabilities at about the same age, there is not a fixed time—6 months, 18 months, 3 years—when a child will be capable of a specific behavior.
Incidentally, myelin is fat. So extreme diets, including extremely low-fat diets, should be avoided.
Let's go back to synapses for a moment. Research tells us that humans have the most rapid growth of synapses from birth to age 3, and then the number of synapses begins to decline. How important is it to preserve synapses?
It is probably correct that we peak in the number of synaptic connections around age 3. Some have taken this to mean that our knowledge starts degrading from that point. Yet, intuitively we know that is not true. I wouldn't ask a 3-year-old to run the MR (magnetic resonance) scanner for me because he's got more synapses than I do. Knowledge accumulates through our entire lives. And this takes place in spite of the fact that we probably have fewer synapses when we're older.
If synapses connect neurons and thoughts are transmitted over these connections, it would seem logical that more synapses would be better than fewer. How does science explain this?
It is a bit of a paradox. To a certain extent, more synapses are better than fewer synapses. But in development, we overgrow synapses, and then the inappropriate ones are pruned away. Many of the synapses we have at 3 are not carrying useful information for the long haul. And one of the ways we appear to learn, to refine our brains, is not by maintaining synapses or by growing new ones, but by retracting and getting rid of the inappropriate ones and selecting the appropriate ones. The brain sends out a whole bunch of possibilities, and we select the ones that allow us to do things efficiently.
There is some evidence, however, that very impoverished environments cause overretraction of the synapses. If you put rats in a very impoverished environment—don't let them exercise, don't have other rats around—the retraction may go too far. If we enrich the environment—give them exercise wheels and access to other rats—the rat brains will have more synapses. So it's a balance between retraction (selection of the right ones over wrong ones) and production of more synapses.
Einstein didn't necessarily have more synapses than your average college freshman. If you could go in and count, you wouldn't be able to predict somebody's intelligence from the number of synapses.
We often hear about critical periods for learning certain things, suggesting that if children are not taught certain things when they are open to learning them, the opportunity for most efficient learning will be lost.
Critical periods are sensitive periods in the development of animals and people. For example, between the ages of 3 and 12, children are capable of developing an incredible vocabulary—50,000 to 100,000 words. They are learning 50 words a day. Sensitive periods are times when we're particularly good at something.
There seems to be a period in which we develop stereoscopic vision, when the brain integrates information from the two eyes. But a critical period is not a window that's slammed shut at a certain time. Although it's much better to correct vision problems that will allow eye integration to take place early, you still can encourage the development of stereoscopic vision as late as 12 years of age, perhaps later. It just takes much stronger manipulation of the environment, much more training. It is not that the brain becomes hardwired and then cannot take advantage of the remaining plasticity in the brain.
What are scientists discovering about intentional and incidental learning?
The distinction between intentional and incidental learning is an important one. Most of what we remember from our everyday life we have learned incidentally. We remember most clearly experiences that are salient to us—the day we married, the day we got engaged, the day our child was born. Most of what we most vividly recall we did not intentionally try to remember.
Hundreds of studies in the cognitive psychological literature show that, in most cases, incidental learning is as good as—and in some cases better than—intentional learning. If you compare a situation in which people are asked to remember a list of words with a situation in which people are asked to tell you what the words mean to them and how much they like those words, the latter group will remember the list of words just as well even though they haven't been intentionally trying to remember them.
In a classroom, indirectly focusing children's attention on the material you want them to learn may be just as good as telling them, "This is something you really want to learn."
The examples of incidental learning seem to be emotionally charged. What does that imply about the role of emotion in learning?
You can use emotion to direct attention, and that attention will lead to better learning. If people are uninvolved and unmotivated in an explicit learning situation, they probably won't learn. If you can get them emotionally involved, they'll probably learn well. If you get people in a very strong emotional state, however, they could go beyond the level that's effective. But to learn, you have to be involved, to have some emotion. In fact, even if you perceive a situation as slightly stressful, your learning will most likely be better than if you are in an absolutely neutral state.
What do we know about the types of memory, and how can we apply that knowledge in the classroom?
In the last 20 years, neuroscience and cognitive science have clearly shown us that the brain has multiple memory systems. Two of the systems are explicit (or declarative) memory and procedural (or implicit) memory. The explicit memory system involves remembering something consciously, for example, what you had for breakfast yesterday. Procedural memories come from iterative learning or skill development. Different brain parts seem to do those two jobs.
Teachers should realize that, in real life, we're probably using explicit and implicit systems of learning all the time. If I wanted to reinforce students' explicit or declarative memory of 8 x 7 = 56, I'd teach about how multiplication is defined as multiple additions. To reinforce procedural memory, I'd teach 8 x 7 = 56 on flash cards over and over again so memory is quick and fluent. Both are useful pathways for getting information into the brain, and teachers should use both.
How did scientists discover these two separate memory systems?
Probably the most famous patient in all of neuroscience is a man named HM. HM had epilepsy that couldn't be controlled by medication. His life was ruined by seizures. So, in the 1950s, neurosurgeons removed the medial temporal lobe on both sides of his brain (including the hippocampus) in hopes of relieving the seizures. The surgery controlled HM's epilepsy but left him profoundly amnesiac. From that day forward, he could learn nothing that he could consciously recall later. Each day, he had to be introduced to the people who had worked with him for years. He didn't remember ever having seen them before. He could not form new long-term declarative memories. He could not remember what he was doing 10 minutes before.
But despite this, HM learned many complicated new skills. For example, he learned to take apart and put together little wooden Chinese puzzle boxes. The first time he did it, he was terrible at it, just as anybody would be. Over time, he learned to take a box apart and put it together very quickly. The interesting thing was that if you took the box away from him and waited a few minutes to give it back to him, he didn't remember ever having seen it before. He implicitly learned how to take the box apart and put it together, but he explicitly could not recall learning the skill.
This led scientists to think that there are multiple systems for learning in the brain. The explicit learning—the conscious kind—uses the medial temporal lobes. The implicit learning is done by other parts of the brain that in HM's case had been left intact.
Educators are eagerly looking to brain research to inform the decisions they make about their daily work with students. In addition to doing research in neurology, you are also a professor. What insights can you share with us about what brain research might mean for classroom practice?
Neuroscience has only the broadest outline of principles to offer educators at this time. And in a lot of cases, the principles suggest strategies that educators already know. But teachers should take advantage of incidental situations. Most of the learning that we carry forward in our everyday life we remember because we were interested in the learning or attended to it. In schools, the learning environment can be artificial. We have to take advantage of the principles we know about how brains learn.
What would you say to parents concerned about enhancing opportunities for brain development in their children?
The most important thing they should know about how the brain develops is that barring extreme deprivation, sensory deficits, nutritional problems, or brain injury, children will develop normally—normal language, normal vision, normal social interactions. If you forget to teach your 2-year-old the word dog, that child's capabilities will not be diminished at 18. For the most part, the brain will develop the way the brain wants to develop. If you feed your child a reasonable diet, make sure that your child can see and hear well, interact with your child, and love your child, the odds are really good that you'll develop a normal, healthy, well-developed, well-integrated child.
