The researchers conducted two tests of sensory perception. In the first, participants feel two pins against their skin and must distinguish whether they are being touched at one or two points. The closer the pins are to each other, the harder the task. In the second, researchers blow a series of air puffs against the participants' skin, and they must determine how many individual puffs they feel. The faster the puffs, the harder they are to discriminate.
Instead of these weak brain signals translating to poorer sensory perception, people's performance actually improved on both tests.
"Our observations surprised us," Tyler said. "Even though the brain waves associated with the tactile stimulation had weakened, people actually got better at detecting differences in sensations."
What might explain this seeming paradox? The answer might have to do with how neurons function. When brain cells communicate, they can urge their neighbors to become active (excitation) or tell everyone to quiet down (inhibition). The ultrasound may have affected the brain region's balance of excitation and inhibition, Tyler said.
As a result, the excitation impulses may not have spread so far, essentially giving the brain a better triangulation of where the sensory inputs were coming from.
The boost in sensory perception vanished when researchers moved the ultrasound's focus just a half inch (1 centimeter). That means the method is a fine-grained way to "tweak" brain circuits, both to map their activity and potentially to treat brain disorders.
"In neuroscience, it's easy to disrupt things," said Tyler. "We can distract you, make you feel numb, trick you with optical illusions. It's easy to make things worse, but it's hard to make them better. These findings make us believe we're on the right path."
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