Watching and poking neurons
The first part of the study, done with the Allen Institute’s OpenScope program, used high-density silicon probes to record large-scale electrical activity across visual areas of mouse brains, capturing the activity of hundreds of neurons with sub-millisecond precision as the mice viewed stimuli that included illusory contours.
That technique has superb temporal resolution, but it can’t easily distinguish the properties of individual cells. So the team also recorded the activity of thousands of neurons in the upper layers of the visual cortex using a calcium-sensitive fluorescent molecule. This produced what Hillel Adesnik of UC Berkeley calls “dynamic pictures of the brain activity where neurons flash when they fire.”
Together, these approaches not only mapped activity but also identified the specific neurons to target in the next phase with optogenetics. This technique introduces the gene encoding a light-sensitive protein into the genomes of neurons, allowing light to act like an on/off switch. Shine light, and neurons fire. Adesnik’s group took this a step further, using holographic 3D illumination: “We used an optical system that shapes the light in three dimensions that allowed us to target very specific neurons,” he says.
This allowed them to stimulate a tiny, very selective subset: “Out of approximately 5,000 neurons, we were able to selectively target the 20 most responsive to, for example, encoding illusory contours.” By combining this stimulation with cortical recordings, the team could move beyond simply correlating neural activity with a stimulus. As Adesnik puts it: “rather than working with a purely observational approach, we could actually photo-stimulate and activate those particular neurons as a group and observe how that influences neural dynamics.”
By photo-stimulating the IC-encoders, the researchers were able to recreate the same neural activity patterns normally evoked by illusory edges, even in the absence of any visual input. In other words, activating this population was enough to generate the specific ‘border’ signal in the visual circuit, not just a generic burst of activity. This suggests IC-encoders don’t just follow sensory input—they actively build the representation of edges that aren’t physically there.
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