Farrell, Karolina J. H.; (2022) The role of midbrain dopaminergic neurons in goal-directed navigation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Successful goal-directed navigation requires learning to accurately estimate location and select optimal actions in each location. Midbrain dopamine neurons are known to be involved in reward value learning, and have also been linked to reward location learning. Dopamine neurons are therefore ideally placed to provide teaching signals for goal-directed navigation. To test this, we imaged dopamine neural activity as mice learned to navigate in a closed-loop virtual reality corridor and lick to report the reward location. Across learning, phasic dopamine responses developed that resembled reward prediction errors and indicated the animal’s estimate of the reward location. We also observed the development of pre-reward ramping activity, the slope of which was modulated by both learning stage and task engagement. We devised a Q-learning model incorporating subjective position inference and an eligibility trace which recapitulated our behavioural and physiological findings. The model further predicted a teaching effect of the ramp in improving task performance, which we also found in our experimental data, indicating that the ramp played a teaching role in the selection of accurate location-specific action during navigation. Our results indicate that midbrain dopamine neurons, through both their phasic and ramping activity, encode reward prediction error to provide teaching signals for improving goal-directed navigation.

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