The dopamine system is the brain’s central hub for learning. Dopamine neurons are active when we receive a reward, and also when we perceive a cue predicting that we are about to receive a reward. These signals are known to be critical for reward learning. Dopamine neurons are located deep in the midbrain, and send dopamine signals to different regions of brain, most prominently to the striatum, a large brain region located under the cortex. The dopamine release within the striatum is believed to be responsible for our ability to learn, for instance, making associations between an action and a reward. However, a central question for scientists to answer is how does dopamine help us make decisions based on visual information, when we need to evaluate visual information and act accordingly? We make many visual decisions every day, and often very quickly. For example, when driving at junctions, we need to rapidly decide whether the car is approaching from the left or the right side. Another example is playing tennis, where we rapidly decide whether the incoming ball will land inside the line. Does dopamine play a role in such visually driven decisions? Can dopamine help optimise the actions that we take in response to visual stimuli? Could different groups of dopamine neurons play different roles in this process?
To approach these questions, the Lak Group took advantage of genetic, imaging and behavioural tools in mice. Mice were trained to perform a simple visual decision-making task previously developed by the group and their collaborators. Mice learned to turn a wheel to the left or right in response to visual stimuli appearing on the left or right side of a screen, receiving a small drop of juice when correct. Researchers found that imaging the activity of dopamine terminals in the dorsal striatum during this task lead to new observations that the incoming signals did not respond to visual stimuli on both sides of the body. According to Dr Armin Lak: “Dopamine signals arriving to the dorsal part of striatum signal something special: they rapidly responded to visual stimuli and actions, but they did so only for stimuli appearing on one side of the body, the contralateral side, to be precise. This is particularly interesting because it has been long known that the brain’s visual cortex is also only responding to contralateral stimuli. Thus, the lateralised dopamine signals are perhaps the precise type of signals that striatum needs in order to make simple associations between visual stimuli and actions, for example for teaching the mouse to turn its body towards food when seeing a piece of food.” Further experiments showed that this lateralised signalling is relatively exclusive to dopamine in the dorsal striatum: measuring dopamine in other regions of striatum, such as the ventral striatum, showed far fewer of these lateralised signals. Dr Lak said: “These results are beginning to uncover the roles dopamine across the brain plays in shaping and optimising our decision-making process.”
The full paper “Dopamine axons in dorsal striatum encode contralateral visual stimuli and choices” is available to read in The Journal of Neuroscience.