The placebo effect can even make us move faster

The placebo effect is a pretty astounding thing. Inert pills can do everything from relieve pain to reduce symptoms of depression, if we expect them to be effective (and sometimes even if we know they are a placebo). But the effect occurs in non-medical contexts too: researchers have found that placebo treatments can enhance creativity, for instance, and even boost cognitive performance.

Now a study in Scientific Reports has shown that a placebo can also make us move faster. After sham electrical stimulation to the arm, participants made faster "goal-directed" movements – a finding that could help develop rehabilitation strategies for people with motor impairments.

Past work has found that placebos can boost athletic performance as well as improve motor function in diseases like Parkinson's. One theory is that after being told that a drug or supplement will improve their motor abilities, people have positive expectations about their performance, and these expectations lead to better "top-down" control of movements – improving the way they plan and predict the outcome of actions, for instance. These are crucial elements of goal-directed movements – the kind of purposeful movements we regularly use to achieve a goal, like reaching for a cup on the table in front of us.

In the new study, Mirta Fiorio and colleagues from the University of Verona examined whether a placebo did indeed affect goal-directed movements. The team recruited 24 participants, who were told that the study was designed to investigate whether electrical stimulation delivered by a TENS machine improved motor performance. They learned that over two separate sessions they would receive different frequencies of stimulation and complete a movement task.

To establish participants' baseline level of performance, at the start of both sessions they did the task without any electrical stimulation. Participants sat in front of a touchscreen, and using their right hand they held a pen so that it was touching a small square on the left on the screen. Then, one to two seconds later, a tone sounded and a rectangle appeared on the right of the screen; participants had to move their arm and hand as quickly as possible to touch the pen to this new rectangle. Participants completed a series of trials in which the size of the target rectangle and its distance from the starting point varied, to change the difficulty of the task.

The team then introduced the electrical stimulation, which was applied to the right arm. In both sessions, the TENS machine was set to an inert mode so that it would not affect participants' motor abilities. However, participants were only told this in one of the sessions. The other session was designed to elicit a placebo effect: participants were told that the machine was set to an active mode, and would stimulate their arm at a frequency known to be effective at improving movement speed and accuracy. After receiving the stimulation, participants completed another set of trials. They then completed another period of stimulation and a third block of trials.

The team found that participants were faster at moving the pen from the starting square to the target rectangle in the placebo condition compared to the control condition. This was true no matter how difficult the trial was, based on how far they had to move and how big the target was. Furthermore, in the placebo condition people were even faster after the second period of electrical stimulation than the first, suggesting that a greater "dose" of the placebo produced even stronger results.

Surveys filled in by the participants revealed that they expected to perform better in the task after receiving placebo stimulation compared to control stimulation, showing that the placebo treatment did indeed boost people's expectations about their own performance. Afterwards, they felt they had done better and were less fatigued in the placebo condition.

Overall, then, when participants were made to expect that electrical stimulation would improve their motor skills, they indeed got faster when making goal-directed movements. This improvement wasn't actually caused by the stimulation, as this was the same in both conditions.

What exactly was going on then? The researchers speculate that people's positive expectations in the placebo condition led them to rely more on their own internal predictions about how and where to move, which meant they could produce movements much faster than if they were relying more on feedback from their senses before making those decisions. Alternatively, positive expectations may have led them to pay more attention to their body and movement, making them perform better.

The results could have implications for rehabilitation after limb injury or stroke. Encouraging patients to have more positive expectations about their motor skills could have tangible effects on their recovery, the team suggests, particularly if they are already suffering from low motivation. "We believe that harnessing the placebo effect could eventually inspire future rehabilitation strategies in the long term", they conclude.