Abstract
Optogenetic stimulation of specific types of medium spiny neurons (MSNs) in the striatum has been shown to bias the selection of mice in a two choices task. This shift is dependent on the localisation and on the intensity of the stimulation but also on the recent reward history. We have implemented a way to simulate this increased activity produced by the optical flash in our computational model of the basal ganglia (BG). This abstract model features the direct and indirect pathways commonly described in biology, and a reward prediction pathway (RP). The framework is similar to Actor-Critic methods and to the ventral/dorsal distinction in the striatum. We thus investigated the impact on the selection caused by an added stimulation in each of the three pathways. We were able to reproduce in our model the bias in action selection observed in mice. Our results also showed that biasing the reward prediction is sufficient to create a modification in the action selection. However, we had to increase the percentage of trials with stimulation relative to that in experiments in order to impact the selection. We found that increasing only the reward prediction had a different effect if the stimulation in RP was action dependent (only for a specific action) or not. We further looked at the evolution of the change in the weights depending on the stage of learning within a block. A bias in RP impacts the plasticity differently depending on that stage but also on the outcome. It remains to experimentally test how the dopaminergic neurons are affected by specific stimulations of neurons in the striatum and to relate data to predictions of our model.
Highlights
In situations where multiple choices are available, selection might rely on the relative estimated value of each possible action
reward prediction error (RPE) is sent from reward prediction (RP) to the states-actions connections in the direct and indirect pathways, and to projections from the states and the actions to RP
We were able to reproduce quantitatively in our model the shift in action selection observed in mice subjected to optogenetic stimulation
Summary
In situations where multiple choices are available, selection might rely on the relative estimated value of each possible action. Optogenetic studies, where a specific type of dopamine receptor expressing MSN was infected with channelrhodopsin 2 (ChR2), have brought support to the dual pathways categorisation. Stimulations of the D1 MSNs in dorsomedial striatum (DMS) have been shown to increase motor activity [23] and the probability of selecting the contra lateral side out of two lateralised options [26], and to reduce Parkinson’s disease motor symptoms in animal model [27]. We implemented in our abstract model of the BG the possibility to selectively increase the action value in one of the direct or indirect pathway and in the reward prediction (RP) system. We discuss the possible causes and consequences on the reward prediction of stimulations in striatum, based on the results of the model
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