How your brain distinguishes memories from observations
Memory and perception seem to be completely different experiences, and neuroscientists have always believed that the brain produced them differently as well. But in the 1990s, neuroimaging studies revealed that parts of the brain that were thought to be active only during sensory perception are also active during memory recall.
“It started to raise the question of whether a memory representation actually differs from a perceptual representation,” he said Sam Ling, associate professor of neuroscience and director of the Visual Neuroscience Lab at Boston University. For example, could our memory of a nice clearing in the woods just be a recreation of the neural activity that allowed us to see it before?
“The argument has moved from this debate about whether there’s even any involvement of sensory cortices to saying ‘Oh, wait a minute, is there any difference?'” she said. Christopher Baker, a researcher at the National Institute of Mental Health who leads the learning and plasticity unit. “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 them,” he said Serra Favilaa postdoctoral scholar at Columbia University and the lead author of a recent one Nature communication study. Her team’s work has identified at least one of the ways memories and perceptions of images are put together differently at the neurological level.
When we look at the world, visual information about it flows 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 turn into lines and edges, then contours, then shapes, then complete scenes that embody what we see.
In the new study, the researchers focused on a feature of image processing that is very important in the early groups of neurons: where things are in space. The pixels and contours that make up an image have to be in the right places, otherwise the brain creates a confused, unrecognizable distortion of what we see.
The researchers trained participants to remember the positions of four different patterns on a background that resembled a dartboard. Each pattern was placed in a very specific location on the board and associated with a color in the center of the board. Each participant was tested to make sure they remembered this information correctly – that if they saw a green dot, for example, they knew the star shape was on the far left. When the participants observed and remembered the locations of the patterns, the researchers recorded their brain activity.
The brain scans allowed the researchers to map out how neurons registered where something was, and how they later remembered it. Each neuron participates in one space, or “receptive field,” in the vastness of your field of vision, such as the lower left corner. A neuron “is only going to fire if you put something on that little spot,” Favila said. Neurons tuned to a particular spot in space tend to cluster together, making their activity easy to detect in brain scans.
Previous studies of visual perception have shown that neurons in the early, lower levels of processing have small receptive fields, and neurons in later, higher levels have larger ones. This makes sense because the higher level neurons collect signals from many lower level neurons and pull in information over a larger portion of the visual field. But the larger receptive field also means lower spatial precision, with an effect like placing a large blob of ink over North America on a map to represent New Jersey. In fact, visual processing during perception is a matter of small sharp dots evolving into larger, blurrier, but more meaningful blobs.
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