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Rewards and dopamine affect sensory learning, study shows

Researchers in Germany, Switzerland and the UK have confirmed the relationship between dopamine levels and sensory 'reward-based' learning in adult humans. The findings, published in the journal PLoS (Public Library of Science) Biology, raise the possibility that dopaminergic ...

Researchers in Germany, Switzerland and the UK have confirmed the relationship between dopamine levels and sensory 'reward-based' learning in adult humans. The findings, published in the journal PLoS (Public Library of Science) Biology, raise the possibility that dopaminergic drugs might be used together with reward-based training to treat patients with lapsed sensory processes, such as individuals who have suffered a stroke. Previous research has established that the prospect of a reward enhances learning and decision making, and recent attention has shifted to the question of whether reward affects not only these higher cognitive processes but also basic sensory decision making in humans. In the current study, a team of researchers led by Dr Burkhard Pleger of the Max Planck Institute for Human Cognitive and Brain Sciences in Germany and University College London in the UK examined whether the effects of reward on sensory processing could be influenced by dopamine. When a decision leads to a successful outcome, a reward stimulus is relayed to the area of the brain that was responsible for making the decision; this allows the brain to optimise its performance in the related task. '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,' explained Dr. Pleger. Thirty subjects underwent a series of trials in which small electric currents of varying frequency were applied to an index finger. The subjects were asked to decide whether the first or second current was stronger. If they answered correctly, a monetary reward (which varied between sets of trials) was displayed on a screen. At the same time, they were examined using functional magnetic resonance imaging (fMRI) to see which areas of the brain were activated. After a trial run (with no rewards), subjects were divided into 3 groups of 10 and given levodopa (a drug that enhances dopamine levels), haloperidol (a dopamine suppressor) or a placebo before going through the full protocol. The results were clear: subjects receiving dopamine made significant progress over the course of the trials while those receiving the placebo showed a modest improvement. Decision making did not improve in subjects receiving haloperidol. The size of the reward also had an impact on the accuracy of the subjects' decision making. 'As well as the effects on higher cognitive processes which were already known, it shows that the reward effect also influences somatosensory processes,' said Dr Pleger. 'It turns out to be stronger, the higher the reward.' Importantly, the team observed that after receiving a higher reward on one trial, sensory activations and decisions were enhanced on the next trial. This suggests that receiving a reward can provide a kind of teaching signal that may be fed back to the part of the brain that is relevant to the task. Analysis of fMRI data showed that important signals in two key regions implicated in reward were activated on reward delivery in a way that was dependent on dopamine levels. In addition, the researchers observed dopamine-dependent activations that originated from the primary somatosensory cortex (PSC). These observations confirmed their hypothesis that the behavioural and neural effects of reward on sensory decision making were strongly modulated by the availability of dopamine. 'Apparently, the interaction between the regions of the reward system and the somatosensory cortex are mediated by the transmitter dopamine,' said Dr Pleger. This discovery opens up interesting possibilities for medical applications. Dopamine is implicated in several cognitive functions such as memory formation, reinforcement learning and information processing; the current findings highlight its potential as a learning enhancer. However, as Dr Pleger pointed out, at certain levels it is also implicated in psychotic disorders. 'A raised dopamine level in the brain has already been identified as the cause of mental illnesses such as schizophrenia,' he warned. 'Too much is also not good, and can even be dangerous.' The researchers are hopeful that their findings could lead to new approaches to treating brain injuries. The study concludes: '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.'

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Switzerland, Germany

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