Stimulating a Single Brain Cell is Enough to Change Behaviour

Scientists have found that stimulating one brain cell is enough to affect learning and behaviour.

The finding contrasts with the long-held notion that thousands of neurons are required to generate a behavioural reaction.

It supports the ‘sparse-coding’ hypothesis of neural networks, which suggests that only a few neurons need to fire to generate a response.

Karel Svoboda, a neurobiologist at the Janelia Farm research campus at Howard Hughes Medical Institute in Ashburn, Virginia, and one of the study's authors, has vehemently debated this theory.

“There are lots of fights about whether or not neural codes are sparse,” Nature magazine quoted him as saying.

He and his colleagues, as well as an independent team of researchers lead by Michael Brecht of the Humboldt University in Berlin, Germany, tackled the debate by studying rodents’ barrel cortex, a region in the brain that receives sensory inputs from the whiskers.

This region is made up of roughly two million neurons. Each whisker transmits signals to a group of cells clustered in the barrel-shaped arrangement that gives the region its name.

According to the report, rats can navigate almost as well with their whiskers as with their eyes.

Brecht said that while experimenting with blind and sighted rats, "You really had to label the cage with the blind rats because they moved so perfectly with their whiskers,” reports Nature magazine.

Both teams stimulated specific sets of neurons. Svoboda’s team created transgenic mice that expressed a light-responsive protein, specifically within the region of the barrel cortex associated with learning.

The protein is naturally found in algae, and responds to blue light by allowing ions to flow through cell membranes, creating an electric current.

After implanting a glass window into the skulls of the mice, the researchers mounted a tiny light-emitting diode onto the subjects’ heads. It enabled them to target individual neurons.

The researchers could vary the intensity of the effect on the cell membranes by dialling the intensity of the light up or down.

Each time the mice correctly selected one of two ports in their cages after stimulation, they were rewarded with a drink of water.

The researchers found that the mice had learnt to respond to pulses of light that activated as few as sixty neurons.

Brecht adopted a different strategy. His team implanted electrodes designed to activate single neurons deep within the barrel cortex, and then trained rats to interrupt a light beam with multiple tongue licks when they sensed the neural stimulation.

The researchers found that on average, the rats responded to the stimulation of a single neuron 5 per cent of the time. However, the extent of that response was highly dependent on which neuron was stimulated.

They revealed that some neurons could provoke a response nearly 50 per cent of the time.

Dirk Feldmeyer, a neurobiologist at Jülich Research Centre in Germany, says that the findings may have a radical effect on how neurobiologists view neural networks.

“Increasing the activity by just a minute amount is actually changing the way the cortex perceives sensory stimuli. It’s really changing the view that the cortex is responding to stimuli with massive activity,” he says.

Both Brecht and Svoboda, however, admit that their findings will not end the sparse-coding debate.

“These experiments show that animals can read out very sparse codes. But they don’t tell you that coding during normal behaviour is sparse,” says Svoboda.

He says that it will require extremely sensitive imaging techniques to probe neural activity to do so.

“That’s not yet been done in a satisfying way. There are technical issues that need to be overcome, but a lot of people are working on that,” says Svoboda.

Source-ANI
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