Read the full transcript of neuroscientist Aleena Garner’s talk titled “How Memories Shape Your Reality”, exploring how our brains integrate past experiences with present sensations to create our perception of reality, at TEDxNewEngland, September 12, 2025.
Listen to the audio version here:
The Power of Perception
ALEENA GARNER: I am going to show you a drawing of a man’s face. He has dark hair, a wide set jaw, and a slightly crooked nose. How would you describe his facial expression? Now I’m going to show you a drawing of a woman sitting on the ground. Her gaze is cast downwards. What do you think she’s thinking? Can you see both the man’s face and the woman? But the image isn’t changing. The sensory input entering your eyes is not changing. Only your thoughts are changing.
You may think that your thoughts are just yours, but sometimes they’re influenced by other people, as in this scenario, when I directed your thoughts by speaking to you. What you see affects what you perceive, but also what you’re thinking affects what you perceive.
So what is it that determines how we experience the world? Is it our senses? That is, what we see, hear, feel, smell, and taste? Or is it our thoughts? What scientists refer to as the brain’s internal model of the world. Turns out, it’s actually both what you’re sensing and what you’re thinking.
How Experiences Shape Our Neural Circuits
Every experience you have with the world enters your brain through sensation, through your sensory organs, like your eyes and your ears. But your sensations integrate with what’s already in your mind, what you’re already thinking, what you already believe. All of your new experiences are encoded in your brain, interleaved with past experiences.
But how do the neural circuits that make up our brains interleave sensation with thought, with the brain’s internal model of the world?
And once we know that, how can we take advantage of the way neural circuits work to improve our experiences? To answer these questions, we turn to neuroscience.
I’m Dr. Aleena Garner, and I’m a neuroscientist. I’m also a professor at Harvard Medical School, and my lab studies how neural circuits integrate sensation with the brain’s internal model of the world to create perception, to give rise to how we experience the external world.
Revolutionary Research on Memory Traces
We cannot watch neural circuits function in a human, but we can watch them in a mouse. In a mouse, we can simply replace part of the skull, that’s bone, with glass. This allows us to actually watch the activity of thousands of individual neurons as mice learn and experience the world.
Many years ago, we invented a way to tag a specific population of neurons that’s activated when a mouse has a unique experience. More specifically, we tagged the neurons that were activated when a mouse explored a little mouse-sized house. Now, this house was comfy and cozy. It had warm colors on the walls. We call the active neurons a memory trace. Each memory trace records one of our experiences in mice and in humans. For the mouse, the memory trace represented the house it had explored.
Then we developed a way to synthetically reactivate the memory trace, the neural circuit responsible for the memory. But we did so while we gave the mouse a new experience. While the mouse explored a mouse version of a slightly aversive doctor’s office, this room had cold colors and textures on the floor and walls, even some poking, similar to what you might experience during a slightly aversive visit to the doctor.
So what happened when we reactivated the old memory of the comfy house? As the mouse learned about this new aversive space, the mouse formed a hybrid memory in which both experiences were fused into a single memory trace. When we reactivated the old memory, as the mice learned something new, the new learning, the new sensations, physically integrated into the previous memory trace.
The Science Behind PTSD and Resilience
Our experiment showed that when you’re learning something new, all of the memories that are active in your brain while you are learning become part of what you are learning. This means that what you experience depends not only on what’s happening to you now, but also on what happened to you in the past.
Our experiments also opened the door to understanding something very subtle, but extremely important about sensing and the world and learning. If a negative memory is active in your brain while you learn something new, even if the new thing is awesome, you might not experience it as awesome. It may just seem mediocre or even kind of bad. This is what happens after trauma and during post-traumatic stress disorder, PTSD.
On the other hand, if a positive memory is active in your brain while you learn something new, even if the new thing is pretty aversive, you may actually experience it as not so bad or even pretty good. This is resilience.
So how we perceive and experience the world results not only from our present sensations, but also from our memories, from our experiences in the past. But what about the future? Can your possible future experiences change the way you perceive the world now? Yes, they can. Because experiences create expectations. What you expect to happen, what you predict, changes the way you perceive the world now.
But how do the neural circuits that make up our brains interleave sensation with something that hasn’t even happened yet, but that you expect to happen?
Training Mice in Virtual Reality
To answer this question, we start by training a mouse to play a video game in virtual reality. The mouse stands on a spherical treadmill and it uses its locomotion to control a video game being displayed on a mouse-sized iMac screen.
In our experiment, we let mice explore a virtual environment. And in that environment, we expose them to sequentially paired auditory and visual stimuli. And this allows mice to learn that certain sounds correspond with certain visual objects. Like when you hear a siren and then see the ambulance, or when you hear thunder and then see the lightning. Except in our experiment, the sound was a 10.5 kilohertz tone and the visual object was a triangle. The game was an audiovisual associative learning experience.
Measuring Brain Activity During Learning
At the same time, we measured neural activity in a part of the brain called the visual cortex, the part of the brain that allows us to see. If you show a mouse a triangle, its visual cortex will become active, just like in humans. And that makes sense, we can see triangles.
But we asked, how does the neural activity in the visual cortex change as mice learn to use the tone to predict the triangle? And how is this activity different from when mice don’t expect to see the triangle, when it suddenly appears without that predictive tone?
Our results showed that the visual stimulus, the triangle, is processed differently in the visual cortex when mice expected to see it than when it came as a surprise. The audiovisual associative learning resulted in a suppression of neural activity to the expected visual stimulus, that triangle that followed the tone, and a corresponding amplification of the unpredictable sensory input, that triangle that was presented in silence as a surprise. Expectation changed sensation.
Building Internal Models
We think that the suppression of neural activity in the visual cortex is due to the mice building an internal model of the virtual world based on their experience. And as long as the mice did not detect any errors, their internal models remain the same.
But because they were in virtual reality, we, the experimenters, had the ability to systematically create errors. And we did. We changed the game. So that sometimes sounds occurred with the wrong visual objects. Like when you hear a siren and then see a bicycle instead of an ambulance. Or when you hear thunder, and then see a train instead of lightning. Except in our experiment, it was the 10.5 kilohertz tone and a circle instead of a triangle.
Now, many mice detected these errors and their brains updated their internal models. And these mice learned. They learned a new relationship, that the tone was also predictive of circles, not just triangles. But some mice did not update their internal models. These mice failed to learn. They remained stuck, expecting the triangle every time they heard the tone, even though the world around them was changing to include triangles and circles.
Human Internal Models
As humans, we also have internal models. And we learn by updating our internal models as we experience the world around us. And that world is always changing. So if we don’t update our internal models, but instead use the same lens, the lens with which we’re comfortable to see the world, we can’t learn and experience the changes in that world.
Maybe you’re not aware of the internal model in your brain that you have been using to operate. But consider MS-DOS, Disk Operating System, a primary computer operating system in the 1980s. It had no graphics, command line only. Now consider Windows 11. It has colors and widgets and video conferencing. Depending on what operating system you are using, your experience is very different. If your brain is using an operating system that you have not updated, you are limiting your experiences.
So I hope I’ve guided your thinking so that you see that every experience is an interaction between our sensations and our thoughts. And our experiences create expectations. This means that the past, the present, and the future determine what we perceive. And it also means that if you change your thoughts and your expectations, you have the ability to change how you experience the world.
So what can we do with this information for ourselves? Well, choose your memories. We often reward ourselves after the fact. Maybe we try rewarding ourselves before the fact. Trigger that nice memory before you do something challenging or unpleasant. Be mindful of your sensations so that you can distinguish them from your thoughts and your expectations. Meditate and be deliberate with your expectations. We’re often just not aware of our expectations. But are you expecting to see a man’s face or a woman sitting on the ground?
And what can we do with this information for others? Your experiences create your version of the world, which means your perception of reality is unique to you. And every person has a different version of reality. We can’t have someone else’s experiences, of course, but we can be aware that they’re different from ours.
Transforming Education and Communication
And when we acknowledge this, we have the ability to reform and improve many important aspects of our lives. Like, for example, learning in our public and private education systems, all the way from kindergarten to teaching teachers and professors. Because we can appreciate that everyone learns and understands things through their own unique lenses. Everybody learns differently, but everybody can learn.
And anyone is capable of understanding a message that you want to convey. But because we all have our own internal models, our own versions of reality, we constantly need to adapt how we transmit information and communicate. And this seems overwhelming. But if we remain stagnant, just using that one internal model that’s comfortable and kind of gets us by, nothing really matters outside of ourselves.
Your Superpower
Everything we know about how the brain works doesn’t matter if we don’t apply it. Knowing that you can control the memories that are active in your mind when you do something and when you learn something, and knowing that what you expect to happen affects the way you perceive what really happens, is a superpower. This is your superpower.
And now that you know you have the superpower within you, what are you going to do with it? If you don’t use this superpower, you will only get more of the same. When you change the way you look at things, the things you look at change. You are always using an internal model to operate and experience the world. What internal model would you like to use? The choice is yours. Thank you.
