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The ability to form memory is an essential trait that allows learning and the accumulation of knowledge. But what is a memory? There has been a long history of searching for the neuronal substrate that forms memory in the brain, and the emerging view is that ensembles of engram cells explain how memories are formed and retrieved. In a Review, Josselyn and Tonegawa discuss the evidence for engram cells as a substrate of memory, particularly in rodents; what we have learned so far about the features of memory, including memory formation, retrieval over time, and loss; and future directions to understand how memory becomes knowledge.

The idea that memory is stored as enduring changes in the brain dates back at least to the time of Plato and Aristotle (circa 350 BCE), but its scientific articulation emerged in the 20th century when Richard Semon introduced the term “engram” to describe the neural substrate for storing and recalling memories. Essentially, Semon proposed that an experience activates a population of neurons that undergo persistent chemical and/or physical changes to become an engram. Subsequent reactivation of the engram by cues available at the time of the experience induces memory retrieval. After Karl Lashley failed to find the engram in a rat brain, studies attempting to localize an engram were largely abandoned. Spurred by Donald O. Hebb’s theory that augmented synaptic strength and neuronal connectivity are critical for memory formation, many researchers showed that enhanced synaptic strength was correlated with memory. Nonetheless, the causal relationship between these enduring changes in synaptic connectivity with a specific, behaviorally identifiable memory at the level of the cell ensemble (an engram) awaited further advances in experimental technologies.

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RELEVANT QUESTIONS

The question of where and how memories are encoded and retrieved has been challenging scientists and philosophers for centuries. In 1904, Richard Semon brought up the concept of memory engrams [1 ]. He defined an engram as a physical change in the brain introduced by a particular experience. After encoding, specific cues can reactivate this engram leading to memory retrieval (ecophry). Despite highly systematic attempts to find the engram, the lack of certain technologies made it very difficult for Lashley and others to find evidence of long-lasting changes in the brain that represent particular experiences (memory trace) during the 20th century [2 ,3 ]. However, the progress in methods of labeling and manipulating specific cell groups helped find strong support for the existence of neuronal populations that contain the engram, so-called memory engram cells [4 ]. Josselyn and Tonegawa [5 ] describe three major engram evidence strategies: First, finding the activity of the same neurons during encoding and retrieval; second, manipulating this respective network of cells and investigating if this can cause retrieval (gain of function) or suppress (loss of function) respective behavioral output; third, implanting a memory by mimicking the encoding and retrieval process.

NOTES

• https://www.bbc.com/future/article/20160125-the-blessing-and-curse-of-the-people-who-never-forget
• This method has allowed Tonegawa to show that existing theories about memory formation and storage are wrong, or at least incomplete. This past summer, along with Roy and other colleagues, he reported that — contrary to neuroscience dogma — the neural circuit in the brain structure called the hippocampus that makes a particular memory is not the same circuit that recalls the memory later. Instead, retrieving a memory requires what the scientists call a “detour circuit” in the hippocampus’s subiculum, located just off the main memory-formation circuit.
• To explain why the brain would form and recall memories using different circuits, Roy framed it in part as a matter of expediency. “We think these parallel circuits help us quickly update memories,” he said. If the same hippocampal circuit were used for both storage and retrieval, encoding a new memory would take hundreds of milliseconds. But if one circuit adds new information while the detour circuit simultaneously calls up similar memories, it’s possible to apply past knowledge to your current situation much more quickly. “Now you can update on the order of tens of milliseconds,” Roy said.
• The researchers called these cortical cells “silent engrams” because they contained the memory but did not respond to a natural recall cue. Over the next couple of weeks, however, these cells seemingly matured and became integral for recalling the memory.

Memory engrams: Recalling the past and imagining the future

• “Your memory is only a story of a story of a story. In fact, none of your memories ever happened. The mind only stores what it thinks happened … We only store a highly modified and distorted version of reality—not reality.”
• “Whenever we recall a memory, we reconstruct it. When we remember or recollect, we are putting together a story of what we think happened, not witnessing or replaying what happened. So, none of our memories ever really happened in the first place.”
• “Our memories have been demonstrated to change and shift dramatically over time. The truth is that memories are really just more thoughts.”
• “Memory forms one of the most fundamental aspects of the creation of the fictitious self. Memory allows us to create a chain of events in our mind that are central to establishing the idea that ‘all of this happened to me.’ We can then replay these memories over and over and apply them to any situation. This serves us well in many ways—our existence would be unrecognizable without memory—but it also comes with some problematic consequences.”
• “The idea of a self depends on these types of memories, called autobiographical memories. They are about your story. When someone asks, ‘Who are you?’ the first thing we do is rely on these special types of thoughts. We call up all kinds of categories that make up ‘us.’ I am my name, my gender, my job, my age, my personality. These are all thoughts of what we think happened in the past and they define what we think of as a stable sense of identity of self.”