Stanford scientists have found a way to make mice hallucinate, discovering it doesn't take a lot of neurons to create perceptions in the brain.

The new research highlights the potential of influencing animal perception and, maybe someday, human perception.

"Back in 2012, we had described the ability to control the activity of individually selected neurons in an awake, alert animal," study senior author Karl Deisseroth, MD, PhD, recalled in a news release. "Now, for the first time, we've been able to advance this capability to control multiple individually specified cells at once, and make an animal perceive something specific that in fact is not really there — and behave accordingly."

The Magic Of Optogenetics

In a new study published in the journal Science, a group of neuroscientists from Stanford University School of Medicine used a technology known as optogenetics, which allows scientists to stimulate specific neurons in animals using pulses of light.

The team inserted two genes in neurons located in the visual cortex of mouse subjects, one of which encoded a light-sensitive protein causing the neuron to fire as a response to a pulse of light that's a specific color in the infrared spectrum. The other gene encoded a fluorescent protein that glowed green when the neuron was active.

Then, the scientists removed a part of the mice's skulls to expose the visual cortex, so that they can project holograms (three-dimensional configurations of targeted photons) directly onto this part of the brain. Using a device designed specifically for the study, the team projected holograms on precise spots along specific neurons.

During the experiments, the mice were shown random sets of horizontal and vertical bars on a screen. The mice were trained to lick from a tube of water whenever they saw vertical bars.

The study authors, who were monitoring the activities of thousands of neurons in the exposed visual cortex, observed which neurons responded to the different orientations of the bars. From their observations, they could pinpoint the specific neurons that were activated by the horizontal bars and the ones activated by the vertical bars.

To create hallucinations, the scientists shone a hologram on the neurons that responded to the vertical bars, causing them to fire. Incredibly, this prompted the mice to lick the tube of water as if they saw the vertical bars, despite being in a dark room. When the scientists targeted the neurons responsive to horizontal bars, the mice did not lick the tube.

Additionally, the study authors revealed that optogenetically stimulating just 20 or even fewer neurons could prompt the same brain activity or animal behavior as visually displaying the vertical or horizontal bars produces.

"It's quite remarkable how few neurons you need to specifically stimulate in an animal to generate a perception," Deisseroth said, pointing out that a mouse brain has millions of neurons and a human brain has billions. "If just 20 or so can create a perception, then why are we not hallucinating all the time, due to spurious random activity? Our study shows that the mammalian cortex is somehow poised to be responsive to an amazingly low number of cells without causing spurious perceptions in response to noise."

Future Applications

Not only does the new research open up a greater understanding of the brain's natural information processing, but it also could have implications in psychiatric conditions such as schizophrenia. The study authors also brings up the possibility of neural prosthetic devices with a single-cell resolution in the future.

As Inverse suggests, the new findings could someday be used for therapy, such as shining light on the anxiety-activated neurons to reduce anxiety or other similar conditions.

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