How Our Brain Brings Memories Back and Forth Through Time
How does our brain play back memories for us? Is it like a movie in our mind?
Posted Aug 26, 2019
Most people are interested in memory, especially if they are reminiscing about the past or if they know someone suffering from Alzheimer’s disease. But most have not really thought about how you would go about making a memory in the brain.
I used to be the same way. Even as an undergraduate, the formation of memories did not interest me that much. But as a graduate student, I had the good fortune to go to a lecture by one of the most famous memory researchers then alive—Dick Thompson. He was the first person to find a memory trace in the mammalian brain. (I still think he should have gotten the Nobel Prize for this, but that is another story.)
His lecture was so exciting. He asked the audience to really think about memory: How in the world does the brain make a memory? How do we learn? How can a piece of tissue made up of protein and fat take what we experience in everyday life and keep a record of it to relive over and over again? How in the world could this even work?
On top of those questions is the fact that memories are made nearly instantaneously. Think about that for a second. Think about what I just told you. Do you remember the scientist’s name? You probably do—it’s Dick Thompson—because I just mentioned him. And you remember it now, already, even though you read it mere moments ago. That’s how fast memories are made. Many of those memories stay with us in our brain forever—some for the rest of our life, and even the ones we would prefer to forget.
I ended up working for Dick Thompson and learned many things about memory from him. Along the way, I learned why it is so unbelievably hard to study memory: because like us, our memories are always changing. Sure, memories are made in one moment in time—but then when we relive those memories by bringing them up into conscious awareness, and by doing so, we change them.
Therefore, memories are not only a part of the past; they are also attached to the present, at least when we bring them up to reflect on them. Moreover, they are not always that accurate or accessible. One person may remember something very well, whereas someone else with a similar experience may not be able to remember even the most basic information. I have a girlfriend who seems to remember everything—even waiters we both had in a restaurant over a decade ago—whereas I have no memory of the event whatsoever.
This memory disparity is especially true for traumatic experiences. Many people have very vivid memories of traumatic experiences, but others do not. Among those for who the memories are vivid, some people are able to shut them out entirely whereas others cannot stop thinking about it. As such, there is no one rule about trauma and memory.
When people think about memories, they usually assume that they are physically encoded in the brain, much like a videotape. But it turns out that the brain may not make and retrieve memories in this way. In a recent study, Linde-Domingoet et al. (2019) participants learned to associate word cues with everyday objects and then later were asked to recall the object in response to the word cue. They then measured their reaction time—which is simply a behavioral measure of how long it took for the person to bring the memory back into conscious awareness. This process is measured in tens to hundreds of milliseconds. The scientists also used EEG to measure how long it took for the brain to respond—again, in tens to hundreds of milliseconds.
What they found out was quite interesting. When visual memories were recalled, they actually came back into conscious awareness in the reverse order from which they were initially recorded.
Let’s think about how this would work. When you experience a traumatic life event—or any event, for that matter—your brain first responds to the experience itself, encoding what you see or hear or taste or smell. Then, it responds to what you feel in your body as you process that information. Then, it puts the entire experience in a context that you can understand—most often using words, but not always. At this point, the memory about what happened becomes associated with other memories of similar experiences or people, as well as all the thoughts and feelings that are occurring at the time.
Now, pretend that a lot of time goes by. What would it take for your brain to recreate that memory? Imagine you were asked to reflect on the most stressful day of your life. Your brain would search around for the memories of your life and you would select the specific event that you have decided is the most stressful. You might use words to describe it to yourself or simply see it in your mind: “Yes, this was definitely the most stressful day of my life.” Then it would all start to come back to you, and you would be able to access what you saw, heard, smelled, and even how you felt.
The data presented by Linde-Domingo et al. (2019) suggest that the brain prioritizes more abstract and in this case, semantic information before the perceptual details associated with the experience arise. As noted by the authors, “memories, once triggered by a reminder, unfold in a systematic and hierarchical way, and that the mnemonic processing hierarchy is reversed with respect to the major visual processing hierarchy.”
Now, of course, this scenario may not apply to all kinds of memories or organisms or even sensory systems, and the authors acknowledge this. I just like these data because they make us think more logically about how the brain creates and then recreates memories, especially meaningful ones with lots of context and history and perhaps trauma, and to resist the temptation to see them like most of do, as a simple replay of a movie in our mind.
Linde-Domingo, Treder, Kerren, Wimber (2019). Evidence that neural information flow is reversed between object perception and object reconstruction from memory. Nature Communications, volume 10, 179.