How your brain distinguishes memories from perceptions
memory and perception seem to be completely different experiences, and neuroscientists were sure that the brain also produced them differently. But in the 1990s, neuroimaging studies showed that parts of the brain that were thought to be active only during sensory perception are also active during memory recall.
“This began to raise the question of whether the representation of memory is really different from the representation of perception at all,” he said. Sam Ling, associate professor of neurology and director of the Visual Neuroscience Laboratory at Boston University. Could our memory of a beautiful forest clearing, for example, be simply a recreation of the neural activity that previously allowed us to see it?
“The argument has gone from debating whether there is any involvement of the sensory cortex at all, to saying, ‘Oh wait a minute, is there any difference?’” he said. Christopher Baker, Researcher at the National Institute of Mental Health, Head of Learning and Plasticity. “The pendulum has swung from side to side, but it has swung too far.”
Even if there is a very strong neurological similarity between memories and experiences, we know that they cannot be exactly the same. “People don’t get confused between the two,” he said. Serra Favilaresearch fellow at Columbia University and lead author of a recent Connection with nature studying. Her team’s work has identified at least one of the ways in which memory and image perception are differently assembled at the neurological level.
Blurred spots
When we look at the world, visual information about it comes through the photoreceptors of the retina to the visual cortex, where it is processed sequentially in different groups of neurons. Each group adds new levels of complexity to the image: simple points of light evolve into lines and edges, then contours, then shapes, and then complete the scenes that embody what we see.
In the new study, the researchers focused on a feature of vision processing that is very important for early groups of neurons: where objects are located in space. The pixels and outlines that make up an image need to be in the right places, otherwise the brain will create shuffled, unrecognizable distortions of what we see.
The researchers taught participants to memorize the location of four different patterns against a backdrop that looked like a dart board. Each pattern was located in a certain place on the board and was associated with a color in the center of the board. Each participant was tested to make sure they remembered this information correctly: for example, if they saw a green dot, they realized that the star shape was at the far left. Then, as the participants perceived and remembered the location of the patterns, the researchers recorded their brain activity.
The brain scans allowed the researchers to map out how neurons recorded where something was, as well as how they later remembered it. Each neuron visits one space or “receptive field” in your visual space, such as the lower left corner. The neuron “will only fire when you put something in that little spot,” Favila said. Neurons tuned to a specific point in space tend to cluster together, making it easy to detect their activity on brain scans.
Previous studies of visual perception have established that neurons at earlier, lower levels of processing have small receptive fields, while neurons at later, higher levels have larger ones. This makes sense because higher level neurons pick up signals from many lower level neurons, gathering information from a wider area of the visual field. But a larger receptive field also means less spatial fidelity, creating an effect similar to mapping a large ink blot over North America representing New Jersey. In essence, visual processing during perception is that small clear dots turn into larger, blurry, but more meaningful spots.