"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."
Read full article: unknowncountry