Previous work has shown 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 which was responsible for making the decision. In this way, the brain optimises its processes for improved performance each time. It was not known until now whether this mechanism also applied to functions of the somatosensory cortex, which process the skin’s sense of touch, for instance. To answer this question, the researchers designed a “game” for their subjects. Electrodes were attached to both index fingers of the subjects. In each trial, two electric currents, each with a different frequency, were successively applied to subjects’ fingers via the electrodes. Subjects 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 fact that the reward effect works when it is displayed visually had already been confirmed in an earlier study. The amount of the reward was varied from trial to trial. The result: 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 given.
The researchers were interested in the role of the neural transmitter dopamine. Therefore, subjects were divided into three groups before the experiment began. The researchers administered the dopamine-generating 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 was clear-cut: The effect of the reward was greatest in 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. This discovery opens up interesting possible uses for medicine.
These findings raise the tantalising new possibility that reward manipulations in conjunction with dopaminergic drugs might be used to enhance pathologically deficient or lapsed sensory processes, analogous to how rewards can be used to shape or correct behaviour. In the future, targeted use of dopaminergic compounds could be used to aid the rehabilitation of stroke patients, for example. In theory, applications for pharmaceutical “learning boosters” are also conceivable. However, caution is needed in this. 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.
Influence of Dopaminergically Mediated Reward on Somatosensory Decision-Making. 2009 PLoS Biol 7(7): e1000164 doi:10.1371/journal.pbio.1000164