Maybe it's time to stop laughing at the people who wear tinfoil hats to prevent the government from controlling their brains with radio waves. Scientists have demonstrated they can do something similar to monkeys, although the mechanism they used is high-pitched sound waves rather than electromagnetic. Currently, the process is simple and the reliability not that high, but it's reasonable to assume both will improve with time.

External waves have been used to treat a range of neurological conditions and improve memory with considerable success in recent years. Although this is usually done with magnetic fields, the capacity of ultrasound to bolster or suppress the activity of neurons in specific parts of the brain has also been demonstrated in rodents. The University of Utah's Dr Jan Kubanek sought to take it further, using sound waves to control monkeys' choices.

In Science Advances, Kubanek and colleagues describe showing two macaque monkeys images on the left and right side of a screen. They then recorded which picture they focused on, an approach used to test for brain lesions after strokes as these can create a bias where a person looks. 

Kubanek targeted 270,000 Hz ultrasound waves at the left or right frontal eye field (FEF), starting just before the monkeys were faced with the choice. One image appeared milliseconds before the other and without the stimulation, the monkeys usually focused on the image that appeared first. The larger the time gap, the more likely the earlier one was to be chosen. One monkey was rewarded for fixing its vision on the image that appeared earlier, while the other got a reward either way.

However, the application of the ultrasound caused a shift in target choice, with both monkeys becoming more likely to focus on the image in the direction opposite to the stimulated EFF. Instead of the monkeys usually favoring whichever image appeared first, they looked in the direction counter to stimulation two-thirds of the time. The monkey conditioned to pick the image that appeared first was more resistant to stimulus, but still favored the side opposite the stimulated EFF 55 percent of the time.

“The delicate balance of weighing decision options renders neural circuitry sensitive to perturbation," write the authors. 

When other parts of the brain were stimulated, the outcome was unaffected. The authors suggest external influence could help people make hard choices they know are in their best interests, such as an addict refusing drugs. On the other hand, it's hard to miss the dreadful implications when such technology falls into the wrong hands. Perhaps there is hope, however, in the fact that one of the monkeys often overrode the stimulation, instead choosing to act on its previous inclination. 

The preferences of monkeys have previously been changed using electrical stimulation, but this required invasive surgery, not beaming sound at the unwitting.