Written and submitted by Tomás Ryan and Lydia Marks.
This guest blog post is by Tomás Ryan, Assistant Professor and Principal Investigator at Trinity College Dublin. Tomás spoke on the TEDMED stage in 2016, and you can watch his talk here.
How is it that we can remember a single experience for our entire lifetime and yet simultaneously forget things that are centrally important to us? To answer this question, we need to understand the fundamental mechanisms that enable memory storage and retrieval. However, until recently, research in the field of memory has mainly been conducted through the lens of amnesia. While this approach provides us with valuable clues about deficits in memory, it doesn’t always inform us about the physical nature of a specific memory.
To understand the fundamental basis of memory, we need to investigate how individual memories are stored in the brain. The best way to do this is to trace a specific memory starting from its inception. The process of learning a new memory involves a material change in our brains, which we refer to as a specific memory engram. In my TEDMED 2016 Talk, I explained my research showing that the data of memory persists beyond the functional experience of amnesia, thus separating the physical entity of a memory from the process of remembering.1,2,3,4 These findings, which have since been replicated by Christine Denny’s lab at Columbia University, show that in cases of amnesia our brains may lose access to our memory engrams, but this does not mean that the engram itself is lost or damaged.5 Rather, the mechanism for recall has been hindered, but upon reactivation of a specific memory engram we see recovery of the behaviour associated with that memory.
Given that we are able to retrieve memories even after they have been obscured by amnesia, we know that a memory must be stable enough to withstand the ever-changing environment inside and outside of our bodies. To give us a clue about the nature of a single memory, we can turn to another stable and long-lasting mechanism that drives our behavior.
Instincts: the innate, hardwired rules that inform our thoughts, beliefs, and behaviors. These are the factory settings of our brain. They are pieces of genetically encoded information that are vital to our survival as individuals and as a population. Like instincts, memories are also imperative to our success because they allow us to learn about cause and effect from previous experiences and to make predictions about the world around us. While both memories and instincts last for a lifetime, they are also malleable. Memories change through the process of recall, while instincts adapt as we acquire new information about the world around us.
But the connection between memories and instincts goes deeper than just having similar properties. We know that memories build off of our instincts and we use both of these tools to help react to any situation. Given this relationship, we can assume that there must be some continuity between the way our brains encode both memories and instincts. We can think of it as a shared language, allowing us to process instincts and memories as similar pieces of information.
What if we were to look at memory engram research through the lens of evolutionary biology? We find a scenario where a particular learned engram becomes so valuable to a population that it is passed down through generations by culture or by environmental necessity. When an individual happens to inherit a genetic mutation (randomly formed in the sperm or egg producing cells of a parent) that allows this specific memory engram to become an innate instinct—or ingram—that person becomes better suited than their peers who have to learn this lesson through experience. After many generations, this ingram becomes more prevalent in the population by means of natural selection—out-competing the individually-acquired engrams. With this working hypothesis still in its infancy, there are many possibilities for investigation that will hopefully result in a better understanding of the fundamental basis of memory. If instincts are indeed formed by ‘copying’ memories, then it may eventually be possible for our future evolution to be driven not just by factors important for basic survival and reproduction rate, but also by our culture and education.
1. Ryan, T., Roy, D., Pignatelli, M., Arons, A. and Tonegawa, S. (2015). Engram cells retain memory under retrograde amnesia. Science, 348(6238), pp.1007-1013.
2. Tonegawa, S., Pignatelli, M., Roy, D. and Ryan, T. (2015). Memory engram storage and retrieval. Current Opinion in Neurobiology, 35, pp.101-109.
3. Ryan, T. and Tonegawa, S. (2016). Rehebbilitating Memory. Neuropsychopharmacology, 41(1), pp.370-371.
4. Roy, D., Arons, A., Mitchell, T., Pignatelli, M., Ryan, T. and Tonegawa, S. (2016). Memory retrieval by activating engram cells in mouse models of early Alzheimer’s disease. Nature, 531(7595), pp.508-512.
5. Perusini, J., Cajigas, S., Cohensedgh, O., Lim, S., Pavlova, I., Donaldson, Z. and Denny, C. (2017). Optogenetic stimulation of dentate gyrus engrams restores memory in Alzheimer’s disease mice. Hippocampus.