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)
