Scientists have discovered that ultrasound directed to a specific areas of the brain can enhance sensory awareness and perception in the brain.

Most people are familiar with the use of ultrasound scanners as devices that utilise high frequency sound waves to create images of internal organs. This technology has now been adapted by researchers at Virginia Tech Carilion Research Institute to affect brain performance.

"Ultrasound has great potential for bringing unprecedented resolution to the growing trend of mapping the human brain’s connectivity," said William "Jamie" 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 word "ultrasound", in physics, describes sound waves at a frequency humans outside the range of human hearing. In diagnostic sonography, the ultrasound is usually between 2 and 18 MHz. Higher frequencies yield better quality images, but lower frequencies can penetrate deeper into human tissue.The study, which was published online Jan. 12 in Nature Neuroscience, provided the first evidence that low-intensity, transcranial-focused ultrasound could modulate human brain activity to enhance perception.

During the study, scientists focused ultrasound waves on the area of the cerebral cortex that processes sensory information from the hand. The median nerve, the major nerve passing down the arm and through the carpal was also stimulated by the placement of a small electrode on the wrist of human subjects, and then their brain responses were monitored using electroencephalography, or EEG. Just prior to stimulation of the median nerve, ultrasound was delivered to the cerebral cortex region in some subjects. Two standard neurological tests were then carried out: the two-point discrimination test, which determines whether a test subject can distinguish between two separate pins touching their skin, and the frequency discrimination task that measures responses to a sequence of air puffs, and the results were quite surprising.
Those subjects who had received ultrasound were able to distinguish pins at much closer distances and also sense much more subtle distinctions between the frequency of successive air puffs.

"Our observations surprised us," said Tyler. "Even though the brain waves associated with the tactile stimulation had weakened, people actually got better at detecting differences in sensations."

The scientists discovered that the ultrasound reduced the EEG signal and weakened the brain waves that process tactile stimulation, but it wasn’t clear why suppressing these responses would enhance perception. Tyler hypothesised that the answer might lie in the way brain cells communicate, either by prompting or suppressing activity, so the ultrasound could have affected the delicate balance of excitation and inhibition in the cortex area.

"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," 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 test the effects of the ultrasound, the beam was then re-directed to different regions of the brain, one centimeter (0.5 inches) to either side of the test area, but heightened effect on perception only occurred when the beam was aimed at the original site.

"That means we can use ultrasound to target an area of the brain as small as the size of an M&M," Tyler said. "This finding represents a new way of noninvasively modulating human brain activity with a better spatial resolution than anything currently available."

Other types of non-invasive stimulation technology are known to similarly affect the brain,, such as transcranial magnetic stimulation, or the use of magnets to activate the brain, and transcranial direct current stimulation, the transmission of weak electrical currents to the brain via electrodes on the head, but the latest findings have led scientists to conclude that ultrasound is preferable as it can more accurately pinpoint target areas.

In a statement, Tyler explained: "We can use ultrasound to target an area of the brain as small as the size of an M&M. This finding represents a new way of noninvasively modulating human brain activity with a better spatial resolution than anything currently available."

"Gaining a better understanding of how pulsed ultrasound affects the balance of synaptic inhibition and excitation in targeted brain regions — as well as how it influences the activity of local circuits versus long-range connections — will help us make more precise maps of the richly interconnected synaptic circuits in the human brain," said Wynn Legon, the study’s first author and a postdoctoral scholar at the Virginia Tech Carilion Research Institute. "We hope to continue to extend the capabilities of ultrasound for noninvasively tweaking brain circuits to help us understand how the human brain works."

"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 Michael Friedlander, the 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 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."

These findings, whilst of great value to the medical profession, illustrate just how vulnerable our brains are to subtle external interventions. One wonders how such technology could be developed and adapted for more nefarious means, creating the potential for others to remotely control our decisions and responses.

"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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