It wasn’t until the mid-20th century that scientists discovered experimental evidence of a brain’s ability to dynamically strengthen, weaken and reconfigure its neural networks in response to stimuli. This phenomenon, called neuroplasticity (or brain plasticity or synaptic plasticity), has quickly become a primary focus of research on memory and learning. With increasingly sophisticated imaging and experimental methods, researchers are able to correlate changes in behavior with changes in synapse strength, signifying a direct connection between learning and the physical state of the brain.
In his recent book, Memory Makes the Brain, Christian Hansel, Professor of Neurobiology at the University of Chicago, explains not only how this life-long construction of our brains works, but suggests that it is at once broader and more fundamental than we may think. Hansel argues that the machinery a brain uses to change in observable ways is the same machinery it uses to change in much subtler ways and is, in turn, ultimately responsible for our individuality.
What is a memory?
When we recall a memory, we feel a sensation, visualize images, are filled with emotions. But this is just how we experience the memory. Like seeing an image on a computer screen, we may interpret it as something we recognize, but what it really is is something different.
Every one or zero behind every pixel on a screen is controlled by a physical object, a tiny capacitor, and whether or not you recognize a combination of pixels as an image depends on the physical state of each object and how they are connected.
It is similar for our memories. Each memory—each sensation we interpret as a memory—must be represented by a physical network of neurons (nerve cells) and synapses (neuron-neuron connections) in our brain. These networks are called neural circuits, and connections between these networks allow for association, the phenomenon of having one memory or thought lead us to another.
However, unlike networks of capacitors in computers, our brains’ neural networks change over time in response to our experiences. Simply speaking, neuroscientists define learning as a change in behavior upon having one or more experiences. A subject “remembers” an experience if it shapes their future behavior, and this is accompanied by observable morphological changes to the brain. The only way of measuring the making of memories, then, has been to observe these changes.
Engraving the brain
In his book, Hansel suggests that the working definition of memory is limiting, because it depends on our ability to measure pronounced behavioral or physical changes. In reality, he argues, our brain is constantly changing, influenced by each interaction, however slight. Like a canyon, our neural networks are being slowly carved out by our experiences, and at each moment our brains are a little different than before. Each subsequent experience is then filtered through a slightly different brain, and so on.
“We experience something every second,” said Hansel, “and so the brain is never the same.” Moreover, because each person’s set of interactions is distinct, so is each brain.
Does this subtle evolution of our brains count as making memories or learning? Hansel says yes. “Whether it’s a visual experience or an auditory experience, it’s not always immediately visible in the behavior,” he said, “but it’s a memory of the brain. It will influence everything you do, but it’s not always measurable.”
Hansel was inspired to broaden our concepts of memory and learning by the work of early-20th-century zoologist Dr. Richard Semon. “Without having any idea about the physiology or biochemistry at the time, Semon came up with this concept of ‘an engram.’ I see it as an engraving—a physical trace that remains in the brain after an experience,” explained Hansel.
Although the formation of this kind of memory is not as easy to observe as in the case of a rat learning to find its way in a maze, the mechanisms underlying both cases are similar. “We have a relatively simple toolbox that enables us to do really complex things,” Hansel said. For example, as opposed to creating or destroying synapses, a synapse can also become more or less efficient, by gaining or losing more signal receptors or releasing more or less transmitters. Something like this might be implicated in the subtle changes that form us, as opposed to more pronounced morphological changes.
Applying the engram
The idea of the brain as a dynamic matrix, unique to each person and to itself from moment to moment, is certainly intriguing, but is it useful? Hansel, of course, argues that it is. By focusing our understanding of learning on observations of simple behaviors in mouse models, for example, “we are reducing ourselves,” he said, “and we are reducing our capacity to think about these problems.”
As Hansel describes in his book, University of Chicago neurologist Dr. Peter Huttenlocher showed that from the time we are born until about two years old, we gain an enormous number of neuronal connections. At that point, certain connections begin to strengthen, while many are systematically “pruned” away. One place that irregularities in this pruning process are often implicated is in the brains of those with autistim. Hansel suggests that autism research is an area that could benefit greatly from thinking about memory more broadly: “There really is no memory deficit in autistic people in the usual sense of ‘remembering things,’ but if we understand memory as engraving information, changing the brain and making use of it, then it all starts to make much more sense.”
At the University of Chicago, Hansel’s research lab focuses on the cerebellum, a small region of the brain at the back of the head, below the cerebrum (the gray, wrinkled structure you might think of when you use your brain to imagine a brain). The cerebellum was long thought to be involved in motor control and not much else. However, there is now evidence that the cerebellum plays parts in emotion-control, reasoning, and learning as well. Hansel explains in his book that we see language deficits in people with cerebellum diseases that are similar to those in people with autism.
“These language deficits have to do with finding the right words, but also with the placement of words in order,” he said. He suggests that this struggle to find and “compose” words is related to the struggle to “compose” the right set of movements in the right order to complete a task. Especially given the broad range of abilities and deficits across the autism spectrum, these deficits and their physiological relationship may not have traditionally been categorized as having to do with memory. But as Hansel argues, with a broader view, we may be able to apply the concepts of learning and memory in new ways.
Through his book, Hansel is able to unify past and current research, including his own, in order to present a way of thinking about thinking that is deeper, more nuanced and could potentially lead to a truer understanding of what it means to learn, grow and become who we are.
By Amanda Parker, PhD