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In new spooky research, Stanford University School of Medicine neuroscientists stimulated nerve cells in the visual cortex of mice to induce hallucinations. Surprisingly, the researchers found they needed to stimulate a small number of neurons to generate the false perception.
"Back in 2012, we had described the ability to control the activity of individually selected neurons in an awake, alert animal," said Karl Deisseroth, MD, PhD, professor of bioengineering and of psychiatry and behavioral sciences. "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."
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The researchers used optogenetics for their work, a technology enabling researchers to stimulate particular neurons in animals with pulses of light. Deisseroth and his team inserted a combination of two genes into large numbers of neurons in the visual cortex of lab mice and created cranial windows by removing a portion of the animals' skulls to expose part of the visual cortex.
The mice were then shown random series of horizontal and vertical bars displayed on a screen. The researchers observed and recorded which neurons in the exposed visual cortex were activated by each orientation.
They were then able to "play back" these recordings in the form of holograms. The scientists further trained the mice to lick the end of a nearby tube for water when they saw a vertical bar but not when they saw a horizontal one or saw neither.
The scientists then found that they were able to get the mice to lick the tube of water simply by projecting the "vertical" holographic program onto the mice's visual cortex. Interestingly enough, the mice would not lick the tube if the "horizontal" program was projected.
"Not only is the animal doing the same thing, but the brain is, too," Deisseroth said. "So we know we're either recreating the natural perception or creating something a whole lot like it."
Few neurons stimulated
The researchers were surprised to find that stimulating only about 20 neurons, even fewer in some cases, was enough to generate a hallucination. "It's quite remarkable how few neurons you need to specifically stimulate in an animal to generate a perception," Deisseroth said.
"A mouse brain has millions of neurons; a human brain has many billions," he said. "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."
The study is published in the journal Science.