Researchers Control Monkeys' Decisions With Bursts of Ultrasonic Waves

Rhesus macaque monkey.
Rhesus macaque monkey.
Image: Getty

High-frequency sound waves aimed at specific brain regions can influence monkey behavior, according to a new study. The finding complicates our conceptions of free will, but this research could yield new insights into the brain and new treatments for disorders such as addiction.

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New research published today in Science Advances suggests pulses of ultrasonic waves can be used to partially control decision-making in rhesus macaque monkeys. Specifically, the ultrasound treatments were shown to influence their decision to look either left or right at a target presented on a screen, despite prior training to prefer one target over the other.

The new study, co-authored by neuroscientist Jan Kubanek from the University of Utah, highlights the potential use of this non-invasive technique for treating certain disorders in humans, like addictions, without the need for surgery or medication. The procedure is also completely painless.

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Scientists had previously shown that ultrasound can stimulate neurons in the brains of mice, including tightly packed neurons deep in the brain. By modulating neuronal activity in mice, researchers could trigger various muscle movements across their bodies. That said, other research has been less conclusive about this and whether high-frequency sound waves can trigger neuromodulatory effects in larger animals.

The new research suggests they can, at least in a pair of macaque monkeys.

For their experiment, the researchers applied an experimental technique commonly used by scientists to study choice behaviors in humans, such as the effects of brain damage caused by strokes. Two macaque monkeys, with their heads immobilized and sitting in a dark room, were taught to look at a target at the center of a screen. After a few moments, a second target appeared on the screen, either to the right or left of the initial target, and then a third target appeared on the opposite side. The monkeys typically chose to gaze upon the targets in the order they appeared, but they were trained to resist this natural tendency with food rewards.

Graphical representation of the targeted brain stimulation with ultrasound.
Graphical representation of the targeted brain stimulation with ultrasound.
Image: Jan Kubanek

During the experimental phase, the researchers used an ultrasonic transducer to stimulate the monkeys’ frontal eye fields (FEF), the brain region responsible for visual attention and voluntary eye movements. All procedures used in the experiment adhered “to the Guide for the Care and Use of Laboratory Animals and were approved by Stanford University Institutional Animal Care and Use Committee,” according to the paper.

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When the researchers stimulated the left FEFs, the monkeys selected the right target more often, and vice-versa. This effect was better than chance, as the monkeys were twice as likely to choose the target as influenced by the ultrasonic bursts, despite their prior training. Importantly, the ultrasound treatments produced no effect when directed at the motor cortex—the part of the brain responsible for movement—which suggests the scientists weren’t just triggering a physical reflex and that they were actually influencing perceptual choice.

“Brief pulses of low-intensity ultrasound strongly, controllably, and reversibly influenced the subjects’ choice behavior,” wrote the authors in the study.

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Wim Vanduffel, a neuroscientist at Massachusetts General Hospital and an assistant professor at Harvard Medical School, described the new research as an important step forward in this area.

“The non-invasive nature of the method and the fact that deep brain structures can be targeted opens tremendous possibilities for causal research in animals and humans and, possibly, for treating patients in the future,” Vanduffel, who’s not affiliated with the new study, told Gizmodo in an email.As with any development, it will take considerable research effort and time before patients will be helped by this exciting research tool under fast development.”

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What’s more, the observed effects in the study “are entirely consistent with the known properties of the specific brain area that was targeted,” added Vanduffel, who said the results are comparable to experiments involving invasive methods.

This all sounds very magical and even a bit woo, but the high-pitched sound waves, which aren’t audible to the human ear, are actually inducing physical changes in the brain. Specifically, these pulses are causing neurological membranes to vibrate, which kickstarts nearby neurons, thereby influencing their associated behaviors—in this case, the neurons that controlled the monkeys’ impulse to look either at the target on the left, or the target on the right.

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Using this technique, scientists “can change the activity of the neurons and also the connectivity between the stimulated neurons and their neighbors, which has the potential to return malfunctioning neural circuits back to their normal state,” explained Kubanek in a press release.

Indeed, the ability to modulate neural activity in specific brain circuits could be useful in any number of ways. It could help identify brain regions linked to specific diseases or symptoms, for example, or measure the effects of neuromodulation on the brain and how it impacts our behavior. It also opens the possibility of noninvasive, non-drug treatments for addictions, such as alcohol and binge-eating, or for treating compulsive behaviors.

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“Brain disorders should be treated in targeted and personalized ways instead of offering patients cocktails of drugs,” said Kubanek. “But to do that, we need a tool that provides noninvasive, precise, and personalized treatments to address the source of the problem in each individual. This up until now has only been a dream.”

This is all great stuff, but we need to pump the brakes a bit.

These results are very preliminary, as the effects were only observed in two monkeys. Scientists will now need to replicate these results and broaden the scope to see if other parts of the brain are primed for ultrasonic neuromodulation, among other avenues of research. The long-term efficacy of these treatments are not clear; during the experiments, the researchers noticed that “repetitive stimulation diminishes the neuromodulatory effects,” as they explained in the study.

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Vanduffel said neuroscientists still have lots to learn about the specific neuronal mechanisms at play, and these processes need to be understood more fully before we see this technology used in humans. Unintentional damage to targeted brain areas is a possibility, he said.

“Also, it would be interesting to know how much tissue is affected, for example by measuring the local functional consequences of stimulation simultaneously with the behavioral effects,” he added.

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Lots of work still needs to be done, but it’s an exciting result nonetheless. Unless of course, you feel this research diminishes your sense of free will. Science is still undecided on the matter, with evidence suggesting certain aspects of human decision-making are predetermined, while others are not.

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My advice is that you shouldn’t worry about it, as the answer won’t really have a bearing on your daily life. Fact is, we all live with the overwhelming impression that we have free will, whether it’s an illusion or not.

George is a senior staff reporter at Gizmodo.

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Arcanum Five

Is it really wise to teach monkeys that sound can be used to influence behavior?