What is memory anyway? Can it be thought of as a pattern of neuronal activity, tracked down to individual cells making up the brain? Or is it fundamentally inscrutable and forever doomed to remain the topic of philosophical treatises?
Recent scientific advances have made it possible to directly observe, and even to manipulate memories in genetically-modified mice, allowing for the first time to directly address these questions. Yes, mice are very different from us humans, yet also remarkably similar on the level of the basic building blocks of the brain: neurons and their connections, called synapses.
A typical neuron consists of a cell body and its processes, called axons and dendrites. An axon forms the output of a neuron and connects with other cells in the brain to pass on information. Dendrites, on the other hand, can be thought of as the neuron’s antenna – they gather inputs arriving from other neurons’ axonal projections. The union of a dendrite and an axon is a synapse, the fundamental unit of information processing in the brain.
Memories are thought to arise from the formation and modification of synapses. This has been observed under the microscope in living, breathing mice. Moreover, interfering with synaptogenesis (formation of synapses) has disastrous consequences for memory and is thought to be the underlying cause of dementias such as Alzheimer’s Disease.
Now, none of this is exactly news so what is all the fuss with these genetically-modified mice, you ask? Well, these special new mice allow for observing all neurons involved in the formation of a memory, in real time, making it possible to create a blueprint for a specific memory and to later re-activate it. What’s really exciting though is that certain memories can also be erased or even created, just like in Christopher Nolan’s movie ‘Inception’ (but fortunately without the need to transplant the experimenter’s consciousness into the mind of a mouse!). These new experimental tools may, for example, help to one day devise therapies for people suffering from posttraumatic stress disorder (erasure of memory). For now, however, research in mice can offer many new insights into how normal memory is formed.
This has been the focus of my own research. I used genetically-modified mice to track the formation of memory in the retrosplenial cortex. The retrosplenial cortex is a very trendy brain region, implicated in just about every brain function (see a great review by my supervisor ‘What does the retrosplenial cortex do?’ downloadable from ResearchGate) yet we still do not know exactly how it works. There are clues of involvement of the retrosplenial cortex in aspects of episodic memory, that is remembering events. In my experiment, I modelled specifically the spatial aspect of episodic memory during learning of the position of strawberry milk rewards in a maze (mice are crazy for strawberry milk!). I found that as mice became better at finding the rewards, so did their retrosplenial neurons at activating according to a pattern, rather than at random. This pattern may be thought of as the biological representation of memory. Remarkably, even weeks from original training, mice that still found it easy to locate strawberry milk showed a very similar pattern of neuronal activity as they did before while their not-so-clever pals, which struggled to find the rewards, showed a less similar pattern.
Going back to the original questions from the beginning of this article, it seems like certain memories are indeed stored in specific patterns of neuronal activity. Moreover, the more stable the neuronal representation, the more stable the memory itself.
About the Author
Michał Milczarek: I am currently finishing my PhD in Integrative Neuroscience at Cardiff University. The title of my thesis is: ‘A multimodal investigation of retrosplenial function’. My interests are learning and memory and how it is stored in the brain. I obtained my bachelor’s degree in Neuroscience from the University of Glasgow in 2013. During my undergraduate degree, I also spent one year at Novartis in Horsham where I worked toward developing a new method of testing the activity of drugs on ion-channel targets. Outside of science, I like…what’s outside of science? Joking. I enjoy crime novels and learning new languages and frequently attend theatre productions.