Memory Makes the Brain

If you're interested in memory, you'll find a lot in Memory Makes the Brain: The Biological Machinery That Uses Experiences To Shape Individual Brains (World Scientific, 2021), from cellular processes to unique and interesting perspectives on autism.

• Detailed descriptions of cellular processes involved in forming a memory.
• Connecting those cellular processes to everyday experiences - like the memorable image of a butterfly seen during a hike decades ago.
• Comparisons of plasticity in different brain areas, like cortex, hippocampus, cerebellum.
• Comparisons of plasticity and learning in different phases of the human life.
• Important milestones in the history of neuroscience. Like Wiesel and Hubel's work identifying the critical period for plasticity, or Huttenlocher's discovery of synaptic pruning.
• Up-to-date science and open questions about autism, a wide range of phenomena that seems to be connected both to synaptic pruning and to the funtion of the cerebellum.
• An outlook on non-synaptic plasticity.

Professor Christian Hansel starts both the book and the conversation with establishing a very broad definition of memory. Traditionally, a lot of research focused on the "observable outcome" of learning: acquiring new skills, changing behavior. Instead, he defines memory as any event that changes the brain.

The first 2 chapters introduce major discoveries from the 1960s and 1970s. David Hubel and Thorsten Wiesel examined the visual system of kittens. They recognized that it's much more adaptive in a "critical period", ca. 4-8 weeks after birth. Peter Huttenlocher discovered another fascinating phenomena during childhood: synaptic pruning. In the years 2-12, a lot of synapses (connections between neurons) disappear.

Chapter 3 describes the molecular machinery behind all these observations. We'll get to know the terms LTP (long-term potentiation) and LTD (long-term depression), which mean the long-term strengthening and weakening of synaptic connections respectively. We find a detailed description of these processes, incl. the enzymes, neurotransmitters and receptors contributing to them.

Chapter 4 continues examining these processes across the human life span. The (perhaps surprising) conclusion: The machinery for synaptic plasticity is quite similar in a small child an in an adult. What's different is the magnitude of these processes.

Autism is a central topic both in the book and in the Hansel Lab's work. Chapter 5 starts with describing the difficulties of a definition. ASD (autism spectrum disorder) is an umbrella term encompassing several symptoms and intensities. In the last years, it has turned out that low degree of synaptic pruning plays a significant role in multiple forms of autism. We'll see in the next years how the research about synaptic pruning and learning can be used for clinical purposes. Understanding the biological mechanism can also make it easier to relate to some peculiar-sounding symptoms. If we consider that an individual's brain contains an unusually high number of synapses, their reports about unusually intense perceptions suddenly become more understandable.

A thought-provoking chapter is number 6. It describes various experiments with mouse models to study autism-like symptoms. The chapter might also make you reflect about the merits and shortcomings of animal models generally.

The cerebellum was long considered to be the brain area for finetuning movements. Now, it's clear that its repertoire is much bigger. In the interview, Christian describes a hypothesis that the cerebellum has a capability of timing. This in turn makes it a crucial factor in several behaviors beyond sophisticated movements, including speech.

We know a lot about synaptic plasticity and its significance in learning. Chapter 8 tackles the big, open question, what else? Which other processes contribute to memory formation? It presents some hypotheses about intrinsic plasticity.

Just like the introduction, the closing chapter describes a personal memory. It also revisits the concept of circuit learning and puts the whole topic into a bigger context: How do all these memories make the brain and the individual unique?