The classic understanding of brain organization is that perceptual regions of the brain represent the world ‘as it is,’ with the brain’s visual cortex representing the external world based on how light falls on the retina, ‘retinotopically.’ In contrast, it is thought that the brain’s memory areas represent information in an abstract format, stripped of details about its physical nature. Now, a team of neuroscientists from Dartmouth College and the University of Edinburgh has identified a neural coding mechanism that allows the transfer of information back and forth between perceptual regions to memory areas of the brain.
Conventional views of brain organization suggest that regions at the top of the cortical hierarchy processes internally oriented information using an abstract amodal neural code. Despite this, recent reports have described the presence of retinotopic coding at the cortical apex, including the default mode network. What is the functional role of retinotopic coding atop the cortical hierarchy? Steel et al. report that retinotopic coding structures interactions between internally oriented (mnemonic) and externally oriented (perceptual) brain areas. Image credit: Gerd Altmann.
“We found that memory-related brain areas encode the world like a ‘photographic negative’ in space,” said Dr. Adam Steel, a researcher at Dartmouth College.
“And that ‘negative’ is part of the mechanics that move information in and out of memory, and between perceptual and memory systems.”
In a series of experiments, participants were tested on perception and memory while their brain activity was recorded using a functional magnetic resonance imaging (fMRI) scanner.
Dr. Steel and colleagues identified an opposing push-pull like coding mechanism, which governs the interaction between perceptual and memory areas in the brain.
The results showed that when light hits the retina, visual areas of the brain respond by increasing their activity to represent the pattern of light.
Memory areas of the brain also respond to visual stimulation, but, unlike visual areas, their neural activity decreases when processing the same visual pattern.
“The study has three unusual findings,” the researchers said.
“The first is their discovery that a visual coding principle is preserved in memory systems.”
“The second is that this visual code is upside-down in memory systems.”
“When you see something in your visual field, neurons in the visual cortex are driving while those in the memory system are quieted.”
“Third, this relationship flips during memory recall.”
“If you close your eyes and remember that visual stimuli in the same space, you’ll flip the relationship: your memory system will be driving, suppressing the neurons in perceptual regions.”
“Our results provide a clear example of how shared visual information is used by memory systems to bring recalled memories in and out of focus,” said Dr. Ed Silson, a neuroscientist at the University of Edinburgh.
The study was published today in the journal Nature Neuroscience.
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A. Steel et al. A retinotopic code structures the interaction between perception and memory systems. Nat Neurosci, published online January 2, 2024; doi: 10.1038/s41593-023-01512-3
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