When Memory’s Trace Became Visible
In Memory of Susumu Tonegawa (1939–2026)
A giant has passed away. The Japanese molecular biologist and neuroscientist Susumu Tonegawa has died at the age of 86. He was one of those rare researchers who not only made a major breakthrough in one field, but later left that field and helped transform another.
Tonegawa received the 1987 Nobel Prize in Physiology or Medicine for showing how a limited number of genes can give rise to the immense diversity of antibodies required by the immune system. His discovery transformed immunology. Yet only a few years later, he turned to neuroscience. He wanted to understand how we learn, and how memories are formed and preserved.
It is this part of his research that speaks most directly to my own work on violence, sexual abuse, shame, and traumatic memories.
Since the early 1980s, I have been concerned with what people remember after violations. In child welfare, research, therapy, and legal proceedings, what a person is able to tell about what happened can become decisive. At the same time, we know that memory is not a photograph of the past. Two people may have been present at the same event and yet remember it differently. Memories can be incomplete, embodied, and difficult to put into words. They may return in fragments, or remain inaccessible even while the experience continues to shape a person’s life.
In an earlier essay entitled “The Ethics of Memory,” I asked why it is so important to remember, and whether it might sometimes be better to forget. Drawing on Paul Ricoeur, I described memory as an action. We use memory, and precisely for that reason it can also be misused. The ethical problem arises in the tension between use and misuse, between the obligation to preserve the traces of the past and the need not to become trapped by them.
Tonegawa’s basic research takes us into the biological foundations of this tension.
Using genetically engineered mouse models and optogenetics, his research group was able to label specific neurons that were active when a memory was formed. These neurons are known as engram cells. An engram can be understood as the physical trace that an experience leaves in the brain’s networks. By later activating the same cells with light, the researchers could evoke the associated memory.
The experiments revealed something that had previously been difficult to imagine. The researchers were able to activate memories in mice that were otherwise unable to retrieve them. They could also create a false association between a particular place and a frightening experience. The mouse then behaved as though something dangerous had happened in a place where, in fact, it had not. Other experiments showed that the emotional valence associated with a memory could be influenced. Tonegawa’s laboratory also helped identify the interaction between groups of neurons in the amygdala associated with positive and negative experiences.
This does not mean that human memories can simply be switched on and off, or that Tonegawa’s experiments with mice can be transferred directly to therapy, witness testimony, or human life stories. The distance between a controlled experiment with mice and a human being trying to speak about a violation is considerable. But the research shows that a memory is not merely stored as a finished narrative in a particular location in the brain. It comes into being through activity within networks of cells, it is bound to emotion, and it is also shaped when it is retrieved.
Here neuroscience meets the ethics of memory.
When a memory can change through recall, the person who listens must proceed with care. In therapy, leading questions can contribute to shaping what is later experienced as a memory. In court, the demand for a complete, chronological, and unchanging account may conflict with the very way traumatic memories exist. Remembering in fragments or changing certain details does not necessarily mean that a violation has been invented. Yet the vulnerability of memory does not mean that every recollection can simply be regarded as historical truth. Tonegawa’s research does not give us an easy method for determining what actually happened. It teaches us why humility, patience, and critical care are necessary.
Ricoeur wrote of memory as work. Freud wrote of remembering, repeating, and working through. In his own field, Tonegawa showed that there are concrete cells and connections that carry the trace of memory, but also that this trace is living and open to influence. His basic research does not abolish the distinction between the brain and lived experience. Rather, it makes the connection between them both more mysterious and more real.
Memory enables us to preserve who we have been, to bear witness to injustice, and to keep alive the histories of those who have suffered. At the same time, memory is not an immovable object. It is woven together with emotion, relationships, and the situation in which we attempt to remember. Care for memories must therefore also be care for the person who carries them.
In his Nature obituary for Tonegawa, Adrian Hayday describes a scientist who asked great questions and explored how new technologies might open them up. Tonegawa did this twice: first with the diversity of antibodies, and then with the mystery of memory. He leaves behind not only answers, but a new possibility for asking old questions with greater precision.
What is a memory? How truthful is it? Why can it sometimes not be retrieved? How does it become connected to fear, joy, and shame? And what do we owe the person who is trying to live with what is remembered—or with what cannot be remembered?
Susumu Tonegawa’s research did not resolve the ethical questions of memory. But it revealed something of the living and changeable foundation upon which these questions rest. It is a scientific life’s work worthy of profound respect.
References and Further Reading
Hayday, A. (2026). Obituary: Susumu Tonegawa, Japan’s first Nobel prizewinner in physiology or medicine. Nature, 656, 31. https://doi.org/10.1038/d41586-026-02340-8
Josselyn, S. A., & Tonegawa, S. (2020). Memory engrams: Recalling the past and imagining the future. Science, 367(6473), eaaw4325. https://doi.org/10.1126/science.aaw4325
Ramirez, S., Liu, X., Lin, P.-A., Suh, J., Pignatelli, M., Redondo, R. L., Ryan, T. J., & Tonegawa, S. (2013). Creating a false memory in the hippocampus. Science, 341(6144), 387–391. https://doi.org/10.1126/science.1239073
Ryan, T. J., Roy, D. S., Pignatelli, M., Arons, A., & Tonegawa, S. (2015). Engram cells retain memory under retrograde amnesia. Science, 348(6238), 1007–1013. https://doi.org/10.1126/science.aaa5542
Pettersen, K. T. (2026, April 16). The ethics of memory: A reflection from practice. Praktisk Filosofi – Practical Philosophy. https://kaaretorgnypettersen.blogspot.com/2026/04/the-ethics-of-memory.html