Abstract
The Basal Ganglia (BG) is a central structure involved in multiple cortical and subcortical loops. Some of these loops are believed to be responsible for saccade target selection. We study here how the very specific structural relationships of these saccadic loops can affect the ability of learning spatial and feature-based tasks. We propose a model of saccade generation with reinforcement learning capabilities based on our previous BG and superior colliculus models. It is structured around the interactions of two parallel cortico-basal loops and one tecto-basal loop. The two cortical loops separately deal with spatial and non-spatial information to select targets in a concurrent way. The subcortical loop is used to make the final target selection leading to the production of the saccade. These different loops may work in concert or disturb each other regarding reward maximization. Interactions between these loops and their learning capabilities are tested on different saccade tasks. The results show the ability of this model to correctly learn basic target selection based on different criteria (spatial or not). Moreover the model reproduces and explains training dependent express saccades toward targets based on a spatial criterion. Finally, the model predicts that in absence of prefrontal control, the spatial loop should dominate.
Highlights
IntroductionThe basal ganglia (BG) participate in purely subcortical loops (Groenewegen and Berendse, 1994; McHaffie et al, 2005, 2006; May, 2006), which are wired a bit differently as the input to the BG is relayed through the thalamus and the BG output projects directly to the considered subcortical structures (Figure 1B), and which rely on different thalamic nuclei (pulvinar, lateral posterior, rostral, and caudal intralaminar)
If we look at the details of neural activity in normal and express saccades (Figure 13), what appears for the spatial task is that direct retinal input induces activity in the spatial loop, which is quickly dis-inhibited by the basal ganglia (BG) and activates the SCi motor map
We described a model of the saccadic system with some very specific structural features:
Summary
The BG participate in purely subcortical loops (Groenewegen and Berendse, 1994; McHaffie et al, 2005, 2006; May, 2006), which are wired a bit differently as the input to the BG is relayed through the thalamus and the BG output projects directly to the considered subcortical structures (Figure 1B), and which rely on different thalamic nuclei (pulvinar, lateral posterior, rostral, and caudal intralaminar) They do, in particular, participate in loops with the superior colliculus (SC), well-known for its laminar structure, its mapping of the visual field and its involvement in gaze orientation movements, including saccadic eye movements (Moschovakis et al, 1996; Lynch and Tian, 2006). The SC (and the tecto-basal loop) is a bottleneck receiving all this information for the final decision, it receives target location information earlier than the cortically processed information, through direct projections from the retina
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