ULTRASOUND IMPROVING SENSORY DISCRIMINATION

New research has demonstrated that ultrasound can be employed to modulate brain activity to heighten sensory perception in humans, similar to how bats use ultrasound to help guide them at night.

Virginia Tech Carilion Research Institute (Roanoke, VA, USA; http://research.vtc.vt.edu) scientists have demonstrated that ultrasound directed to a specific region of the brain can improve performance in sensory discrimination. The study’s findings, published online January 12, 2014, in the journal Nature Neuroscience, provides the first validation that low-intensity, transcranial focused ultrasound can modulate human brain activity to raise perception.

“Ultrasound has great potential for bringing unprecedented resolution to the growing trend of mapping the human brain’s connectivity,” said Dr. William Tyler, an assistant professor at the Virginia Tech Carilion Research Institute, who led the study. “So we decided to look at the effects of ultrasound on the region of the brain responsible for processing tactile sensory inputs.”

The scientists delivered focused ultrasound to an area of the cerebral cortex that processes sensory information received from the hand. To stimulate the median nerve, they positioned a small electrode on the wrist of human volunteers and recorded their brain responses using electroencephalography (EEG). Then, right before stimulating the nerve, they began delivering ultrasound to the targeted brain region.

The investigators discovered that the ultrasound both decreased the EEG signal and weakened the brain waves responsible for encoding tactile stimulation. The scientists then administered two classic neurologic tests: the two-point discrimination test, which gauges an individual’s ability to distinguish whether two close by objects touching the skin are truly two distinct points, instead of one; and the frequency discrimination task, a test that measures sensitivity to the frequency of a chain of air puffs.

What the scientists found was unanticipated. The study participants receiving ultrasound showed substantial improvements in their capability to differentiate pins at closer distances and to single out small frequency differences between successive air puffs. “Our observations surprised us,” said Dr. Tyler. “Even though the brain waves associated with the tactile stimulation had weakened, people actually got better at detecting differences in sensations.”

The researchers wanted to know why would brain response suppression to sensory stimulation heighten perception, and Dr. Tyler theorized that the ultrasound affected an important neurologic balance. “It seems paradoxical, but we suspect that the particular ultrasound waveform we used in the study alters the balance of synaptic inhibition and excitation between neighboring neurons within the cerebral cortex,” Dr. Tyler said. “We believe focused ultrasound changed the balance of ongoing excitation and inhibition processing sensory stimuli in the brain region targeted and that this shift prevented the spatial spread of excitation in response to stimuli resulting in a functional improvement in perception.”

To determine how well they could isolate the effect, the researchers moved the acoustic beam 1 cm in either direction of the original site of brain stimulation, and the effect disappeared. “That means we can use ultrasound to target an area of the brain as small as the size of an M&M [a popular US candy about 1 cm in diameter,]” Dr. Tyler said. “This finding represents a new way of noninvasively modulating human brain activity with a better spatial resolution than anything currently available.”

The scientists, based on the findings of the current study and an earlier one, concluded that ultrasound has a greater spatial resolution than two other leading noninvasive brain stimulation technologies – transcranial magnetic stimulation, which uses magnets to activate the brain, and transcranial direct current stimulation, which uses slight electrical currents delivered directly to the brain through electrodes positioned on the head.

“The work by Jamie Tyler and his colleagues is at the forefront of the coming tsunami of developing new, safe, yet effective noninvasive ways to modulate the flow of information in cellular circuits within the living human brain,” said Dr. Michael Friedlander, executive director of the Virginia Tech Carilion Research Institute and a neuroscientist who specializes in brain plasticity.

“This approach is providing the technology and proof of principle for precise activation of neural circuits for a range of important uses, including potential treatments for neurodegenerative disorders, psychiatric diseases, and behavioral disorders. Moreover, it arms the neuroscientific community with a powerful new tool to explore the function of the healthy human brain, helping us understand cognition, decision-making, and thought. This is just the type of breakthrough called for in President Obama’s BRAIN [Brain Research through advancing Innovative Neurotechnologies, also referred to as the Brain Activity Map Project] Initiative to enable dramatic new approaches for exploring the functional circuitry of the living human brain and for treating Alzheimer’s disease and other disorders.”

Image: William Tyler, an assistant professor at the Virginia Tech Carilion Research Institute, studied the effects of ultrasound on the region of the brain responsible for processing tactile sensory inputs (Photo courtesy of Jim Stroup / Virginia Tech)