How Brains Guesstimate

Experiments show what happens when the mind assumes.

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By Laura Sanders, Science News

SALT LAKE CITY—When the brain can’t nail an answer, it falls back on reasonable guesses. Now scientists have evidence that this strategy may happen very early in the processing of sensory inputs, a study presented February 27 at the Computational and Systems Neuroscience meeting suggests.

The research took advantage of a common misperception of the human brain: People often think that hazy, ill-defined objects are moving more slowly than they really are. The brain’s rationale for this error: “Things in the world don’t tend to move very quickly,” said neuroscientist Ed Vul of the University of California, San Diego, who was not involved in the new study. “They’re not running past you at 60 miles per hour. For the most part, when things are moving, they’re moving slowly.” 

Researchers already knew that the brain relies on assumptions when it has trouble figuring something out, but it wasn’t clear where in the brain—and when—these assumptions get used.

In the new study, Brett Vintch of New York University and Justin Gardner of Riken Brain Science Institute in Japan scanned people’s brains using functional MRI while they judged how fast black and white lines moved on a computer screen. At first, the researchers made the task relatively easy to see how participants’ brains would handle it under normal conditions. Some brain regions grew more active as the volunteers judged speed, and other regions grew less active. The team used a statistical model to decode these brain activity signals and found that some of the most important vision areas in the brain were used to gauge the speed of the object.

“It’s certainly something that’s not been shown before, that you can decode speed,” said Vul.

To find out which parts of the brain were active when more guesswork was involved, the researchers made the test harder by making the lines hazier. As subjects had less information to rely on, the activity shifted within their vision centers to involve parts of the brain that receive input directly from the eyes. This change went along with the subjects’ slower movement estimations.

The results suggest that the brain falls back on its default “most things are slow” reasoning very early in the brain’s vision pathway, Vintch said.

Vul said that the results are interesting, but more studies will be needed to sort out exactly how the brain uses information it already knew. “From this I wouldn’t be entirely convinced” that the brain is incorporating assumptions this early in the vision process, he said. An alternate explanation for the results is that the brain uses assumptions later in the vision process, but then those brain regions send their knowledge back to the frontline vision systems studied by Vintch and Gardner.

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