Led by Dr. Burkhard Pleger of the Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, and the University College London, the researchers have found that the reward effect can be strengthened using dopaminergic compounds.
The international team of German and British researchers point out that targeted manipulation of dopamine levels, thereby enhancing the "teaching signal" in the brain, could open up new possibilities in the treatment of patients, for example, after a stroke.
Studies conducted in the past have demonstrated that if a decision leads to a successful outcome, it is registered in the brain's reward system. The reward stimulus is then relayed to the area of the brain that was responsible for making the decision.
In this way, according to the researchers, the brain optimizes its processes for improved performance each time.
"It was not known until now, however, whether this mechanism also applied to functions of the somatosensory cortex, which process the skin's sense of touch, for instance," says Dr. Pleger.
With an eye on answering that question, the research team designed a "game" for their subjects. They attached electrodes to both index fingers of the subjects.
In each trial, two electric currents, each with a different frequency, were successively applied to the subjects' fingers via the electrodes, and the participants had to decide whether the first or second electric current had a higher frequency.
If they were correct, a monetary reward was displayed on a screen. The amount of the reward was varied from trial to trial.
The researchers observed that depending on the size of the reward, the subjects were able to subsequently make the correct decision with improved accuracy.
"As well as the effects on higher cognitive processes which were already known, it shows that the reward effect also influences somatosensory processes. It turns out to be stronger, the higher the reward," says Dr. Pleger.
The researchers also studied the role of the neural transmitter dopamine, dividing the subjects into three groups before the experiment began.
They administered the dopaminergic compound Levopoda to the first group, and the second group received the dopamine inhibitor Haloperidol. The third group were treated with a placebo as a control.
The effect of the reward was found to be greatest in the group whose dopamine levels had been raised by Levopoda. Subjects in the placebo group also learned with each trial, albeit by less.
The reward effect was totally absent in the group of subjects who received the dopamine inhibitor.
"Apparently, the interaction between the regions of the reward system and the somatosensory cortex are mediated by the transmitter dopamine," says Pleger, adding that this discovery opens up interesting possible uses for medicine.
Based on their findings, the researchers came to the conclusion that, in future, targeted use of dopaminergic compounds could be used to aid the rehabilitation of stroke patients, for example.
Dr. Pleger, however, warns: "A raised dopamine level in the brain has already been identified as the cause of mental illnesses such as schizophrenia, so too much is also not good, and can even be dangerous."