Study reveals how brain cells process vision amidst thousands of synaptic inputs
· News-MedicalEven in the primary visual cortex, a brain region named for its specialized role in processing basic features of what the eyes see, not every neuron ends up answering the call to process properties of visual input. Maybe that's because each neuron receives a wide variety of inputs via thousands of circuit connections, or "synapses," and has to opt to respond to the visual information vs. something else.
In a new study in mice, neuroscientists at The Picower Institute for Learning and Memory at MIT reveal how neurons that perform visual processing bring order to this input to get the job done.
Mriganka Sur, Study Senior Author and Newton Professor, Neuroscience, The Picower InstituteThe configuration of inputs, the kind of organization, the assembly of neurons that modulate each other to generate an action potential is the essence of how brain circuits process information. These (visual cortex) cells are a microcosm of this very profound and big picture of neuroscience."
They did this imaging for not only visually responsive neurons, but also for unresponsive neurons that nevertheless have visually responsive spines. That allowed them to analyze many key properties that might influence where a particular synapse forms, and how it influences responses at the cell body.
"This pulls together a lot of things that have been looked at in isolation and looks at them in one collective paper," Jenks said. "We can compare how the neuron and the spines on that neuron respond to the same stimuli, and we can do this for both visually responsive and unresponsive neurons."
Revealing rules
In visual cortex layer 2/3, Jenks and the team genetically engineered neurons such that their individual dendritic spines would glow when surges of calcium indicated increased activity by the synapses on the spines. The scientists did the same for the cell body, or "soma," to keep track of how the cell responded and even signaled its overall responses back out to the synapses.
This way, as the mice watched black and white gratings at varying angles drift by their eyes in different directions, the scientists could keep track of each spine's and each cell's overall response to that patterned visual input.
In all, they tracked 11 neurons that responded to the visual input and 11 others that seemingly ignored it. That enabled them to find several rules:
Orientation selectivity matters most: Jenks, Sur and the team used statistical modeling to determine which of many factors (the stimulus selectivity, reliability of the response, a spine's distance from the soma, apical vs. basal, etc.) most explained how correlated a spine's responsiveness was with that of the soma. By a wide margin, how selective a spine was to the orientation of its preferred grating was the most important single factor.
"Our results reveal that synaptic inputs to excitatory layer 2/3 neurons in mouse (visual cortex) are not randomly arranged, but organized and distributed in a manner that correlates with multiple factors including somatic responsiveness, somatic tuning, branch type, distance from the soma, local correlations, and stimulus selectivity," the researchers wrote.
The team's findings can help advance studies of vision in the brain in multiple ways, Jenks and Sur said. Certain genetic mutations that affect how neurons connect in circuits can affect visual cortex neurons and vision, Sur said. Documenting these rules provides researchers with a baseline to compare against when examining the effects of such mutations. Jenks added that the findings could inform efforts to model how neurons integrate synaptic inputs in their computations.
Source:
Journal reference:
Jenks, K. R., et al (2026) Functional organization of dendritic spines in mouse visual cortex layer 2/3 neurons. iScience. DOI:10.1016/j.isci.2026.115861. https://www.cell.com/iscience/fulltext/S2589-0042(26)01236-8.