How the Brain Overrules Our Senses During Faint Stimuli

How do our brains perceive things that aren't really there? Scientists have long known that when sounds are faint or objects are seen through fog in the distance, repetition of these weak sensory inputs can result in different perceptions. Now, scientists are figuring out why that is.

"In everyday life, we experience weak stimuli all the time," said Daniel O'Connor, one of the researchers, in a news release. "Did I hear my name being called? Do you smell smoke? Is that an oasis up ahead or a mirage? When the brain receives weak information through the senses, it can interpret that information in multiple ways, and we wanted to understand what determines the resulting perception."

In this latest study, the researchers used a simple model: the gentle tickling of a single mouse whisker, which is a sensory organ common to many animals. It was faint enough that sometimes the mice perceived it and sometimes they didn't.

The researchers then trained the mice to indicate when they felt tickled. First, the scientists positioned the mice close to a waterspout. After they tickled the whisker, they gave the mouse a "reward," which was a bit of water from the spout. If they didn't tickle the whisker, they didn't give any water.

Then, the researchers used high-resolution microscopy to monitor the activity of hundreds of neurons in the part of the brain responsible for feeling touch. They then paired that data with the animals' licking responses.

The researchers found that when the mice correctly received a tickle and licked the spout, there were higher levels of activity in the cortex than when they didn't perceive the tickle.

But why were there differences in response when the stimulation was the same? The scientists began searching for that answer by monitoring the activity of the neurons that connect to individual whiskers. Those neurons turned out to respond equally to all tickles, whether the mice perceived them or not.

After the first neurons fired in response to a tickle, though, the signal traveled to the brainstem and then onto the thalamus.

The scientists also used a beam of light to artificially enhance the signals sent from the thalamus to the cortex. But even strong light produced only weak activity peaks in the cortex and didn't help the mice perceive whisker tickles.

The findings reveal a bit more about what goes on in the brain when weak perceptions are felt. This, in turn, may tell researchers a bit more about human perceptions.

The findings are published in the journal Nature Neuroscience.

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