Using a robotic arm or prosthetic hand may seem straightforward, but it actually requires very precise control.
Even a simple task, such as picking up an egg, demands exactly the right amount of force. Too little pressure and the egg slips out of your hand.
Too much pressure and it breaks.
For people who use prosthetic devices or patients recovering from a stroke, this kind of fine control can be especially difficult.
A major reason is that they often receive less sensory feedback than people using their natural limbs. They may not fully feel how much force they are applying or how their movements are changing in real time.
Researchers have been trying for years to solve this problem. Some systems use vibrations, sounds, or visual signals to replace missing sensations. Although these methods can help, they usually require extra equipment and still cannot completely reproduce natural touch.
Now, researchers at the Swiss Federal Institute of Technology Lausanne (EPFL) have tested a much simpler idea. Instead of trying to recreate missing sensations, they asked whether the brain could learn more effectively if it received immediate information about success during a movement.
The researchers conducted five studies involving 106 participants, including 18 people with chronic stroke.
In the experiments, participants were asked to follow a moving target on a screen for seven seconds. They controlled a cursor either by squeezing a force sensor or by contracting their biceps muscles.
As they performed the task, the color of the target changed continuously according to how well they were doing. Green indicated success, while red signaled poor performance. The system adapted as people improved, keeping the challenge appropriate and ensuring that the color feedback remained meaningful.
In some comparison experiments, the colors changed randomly and participants were instructed to ignore them.
The results were striking. Fewer than 20 practice sessions using the color feedback produced immediate improvements in motor control. Even after the colors were removed, participants continued to perform better.
The approach worked particularly well when other forms of feedback were limited. For example, when participants could only see the cursor one-third of the time, the benefits of the color system were about three times greater than when they could see the cursor continuously.
A similar effect was seen in experiments involving muscle-controlled interfaces. The less artificial touch feedback people received, the more helpful the color cues became.
Stroke patients also improved when visual feedback was limited. However, their improvements did not last after training ended. The researchers believe this may be because the training period was relatively short and because brain injuries can affect how motor memories form.
The studies also revealed that not everyone benefited equally. Participants who were naturally more sensitive to rewards showed larger improvements. This finding suggests that researchers may eventually be able to predict which patients are most likely to respond to this type of training.
The researchers believe the method could easily be incorporated into many existing prosthetic devices, rehabilitation systems, and other human-machine interfaces. By taking advantage of the brain’s natural ability to learn from rewards, a simple change in color may help people gain control of prosthetic devices faster, more easily, and at lower cost.
