Welcome to Your Child's Brain: How the Mind Grows From Conception to College (33 page)

Different forms of memory call upon different brain regions. In adults, the principal distinction in memory type is the difference between
declarative memory
and
nondeclarative memory
. Declarative memory is the recall of a fact or an episode from the past. Formation of declarative memories requires the hippocampus, a horn-shaped structure found on both sides of the brain (see
figure
) that is also necessary for spatial navigation. Another key region for declarative memory is the neocortex, especially its medial temporal regions. Declarative memory continues to improve throughout childhood.

It is possible to see glimmerings of hippocampus-dependent memory before age two. For example, your preschooler might be able to describe an episode that happened to her before she could speak. Even so, memory is difficult to probe in the very young because children cannot declare much until they have a good command of language. For this reason, compared with nonverbal measures, verbal measures of memory underestimate memory competence in children up to age six.

A better way to examine memory during this period is to observe spontaneous actions (see
chapter 1
) or to give children a task that does not require speaking. For example, in the laboratory, children of eighteen to twenty-four months can learn to navigate with the help of distinctively shaped objects as landmarks to remind them where to go. In real life, your child may complain if you take an unusual route to school or day care. These behaviors are evidence of memory for the locations in question.

Infant remembrance can be boosted with timely reminders.

The other major category of memory is nondeclarative, encompassing a wide range of nonverbal memories: learned associations, skills, and habits. One type of nondeclarative memory that is readily studied in infants is the formation of associations: for example, anticipating that Mom is about to leave the house when she puts on her coat. Associative learning is present from a very early age, though infants also forget associations relatively quickly (see
Did you know? Babies forget faster
). Associative learning can involve the amygdala for emotional responses or the cerebellum for other forms of sensory learning.

Another type of nondeclarative memory is
procedural learning
, the acquisition of habits and skills, such as learning to tie your shoes. Shortly after the first birthday, children can perform procedures in a sequence, such as making a rattle by putting a ball inside a container, putting the lid on, and shaking it. This procedural learning requires the striatum, a structure in the basal ganglia that is necessary for movement and initiating actions. Procedural skills are learned more robustly than declarative memories requiring the hippocampus. This is why you never forget how to ride a bike.

To learn the new, an infant must get used to the old. As described in
chapter 1
, infants are good at detecting new information in their environments. A necessary part of this capacity is the ability to identify objects as familiar. Infants less than one year old will look with interest at a new object revealed when a cloth is pulled away. With successive repetitions, they slowly lose interest and eventually stop orienting altogether, a process called
habituation
. After a waiting period of minutes, the baby recovers from habituation and looks with renewed interest. Some of this memory is longer lasting; if you start over
again, it doesn’t take as long for the infant to habituate as it did the first time. Eventually the infant ignores the object entirely.

Habituation does not initially require long-term changes in the brain. It is present in nearly all animals, including sea slugs, fruit flies, and even single-celled organisms. Habituation depends on short-term changes, such as the accumulation or depletion of intracellular chemical signals, which then inhibit neurons from firing or prevent synaptic terminals from releasing neurotransmitter.

Similar chemical signals are also triggered by new experiences. These signals can cause long-term changes in the structure and composition of neurons and synapses—the nuts and bolts of learning. Most of these changes are unlikely to include the generation of new neurons. Nearly all neurons that the brain will ever contain are already present soon after birth (see
chapter 2
). After that, new neurons are produced only in the olfactory bulb, part of the hippocampus, and at a trickle in the neocortex. Also, because neurons have formed most of their axons and dendrites within the first few years of life, the possible locations where a neuron can form new synapses are also somewhat constrained.

Given these commitments, a major site for learning to occur in older children and adults is within existing neurons and at the synapses between them. Existing synapses can grow stronger or weaker, as neurotransmitter receptors are added or removed, or as the neurotransmitter chemical becomes more or less likely to be released at the connection when the presynaptic neuron fires. In response to external events or internal processes, information in the brain flows along paths of neurons that fire with characteristic patterns and sequences.

The process by which individual synapses become stronger is called
long-term potentiation
. It requires particular patterns of neuronal firing as well as signals such as dopamine, acetylcholine, and other long-distance neurotransmitters. When these conditions are met, a group of neurons firing together in sequence can trigger biochemical processes that strengthen the connections between all the neurons. As we mentioned in
chapter 5
, cells that fire together, wire together.

Just as important in learning is the weakening of synapses. Synapse strength is reduced in a process called
long-term depression
. This process occurs when two connected neurons fire independently of one another, which can happen when the postsynaptic neuron is being driven by other neurons. Or, in a slogan often used to teach neuroscience students, “Out of sync, lose your link.” In addition, as we saw in
chapters 5
and
9
, childhood is a period of ongoing synapse elimination, in which many synapses disappear entirely as a part of normal development—the refinement of your child’s brain circuits. Synapse elimination is driven not only by activity but also by the availability of trophic factors, proteins that are necessary for the growth and maintenance of dendrites and axons.

PRACTICAL TIP: THE BEST STUDY HABITS

Decades of research have identified study techniques that can vastly improve learning, but most teachers don’t practice them, and few parents know about them. Fortunately, these strategies aren’t complicated, and your child can use them at home. Students often wait until the last minute, then make up for lost time in a marathon study session. This approach flies in the face of one of the most reliable results in research on learning: the power of spaced study. The brain retains many kinds of information longer if there is time to process the learned information between training sessions. Two study sessions with time between them can result in twice as much learning as a single study session of the same total length. Spaced training works with students of all ages and ability levels, across a variety of topics and teaching procedures. In general, the longer the gap between study sessions (up to a year in some cases), the longer people will remember the material.

One possible reason why this approach works is that breaks allow time for newly acquired information to be consolidated (see
p. 187
). Memories are not written just once but reinforced either during recall or even offline, for instance, during sleep. Both declarative and procedural memory appear to be consolidated during sleep, so it’s important to make sure your child gets enough rest.

Because memories are reconsolidated when they are recalled, tests actually improve learning (and slow down subsequent forgetting) by compelling the student to actively recall the course material. Passive reading is much less effective for learning. Multiple-choice tests do not improve learning, while short-answer questions do. You can take advantage of this fact by quizzing your child at home during study time to improve her performance at school and by teaching your child to test herself as a study strategy.

A third way to improve your child’s learning is to mix it up. Children who see ten similar examples in a row learn considerably less than children who see ten different examples. This strategy works across domains, affecting the way we learn sports, art history, math, or any other subject. Varying the timing and location of study sessions also improves recall, probably because learning is contextual, so learning in multiple contexts gives your child’s brain a deeper connection to the material.

At first, your child may find these approaches discouraging because they often result in more errors during studying—but they will produce better test performance with no more effort than traditional study techniques. Such good results should change his tune quickly.

The formation and breakage of synapses is another major site of information storage. In most of the brain, neurons form synapses with only a small fraction of their neighbors, so the growth and elimination of connections can establish entirely new pathways for information. This process is happening on a very large scale during childhood, when synapses are produced in large numbers, and then eliminated according to experience (see
chapters 5
and
9
). In addition, neurons can change how they respond to synaptic input, for instance, by changing their electrical and chemical response properties. All of these processes require new proteins and cellular structures to be made and broken down.

Tests improve learning by compelling the student to actively recall the course material.

You may know that Paris is the capital of France, but you are far less likely to recall where you were when you learned this fact—unless you learned it recently. In contrast, your child might be able to recall where he heard it. The conversion of short-term to long-term memory seems to involve physical transfer from one brain region to another. Initial storage of a fact requires the hippocampus and other brain structures nearby, in medial temporal parts of the neocortex. The hippocampus sends connections to the neocortex, and with time, factual information is reprocessed to join our storehouse of general knowledge. The relatively late development of synaptic connections in the hippocampus may be the reason that children have poor declarative memory in their early years. For other types of memory, the stored information may be transferred between brain regions in an analogous manner.

DID YOU KNOW? BABIES FORGET FASTER