Abstract
Electrophysiological studies in monkeys have shown that dopaminergic neurons respond to the reward prediction error. In addition, striatal neurons alter their responsiveness to cortical or thalamic inputs in response to the dopamine signal, via the mechanism of dopamine-regulated synaptic plasticity. These findings have led to the hypothesis that the striatum exhibits synaptic plasticity under the influence of the reward prediction error and conduct reinforcement learning throughout the basal ganglia circuits. The reinforcement learning model is useful; however, the mechanism by which such a process emerges in the basal ganglia needs to be anatomically explained. The actor–critic model has been previously proposed and extended by the existence of role sharing within the striatum, focusing on the striosome/matrix compartments. However, this hypothesis has been difficult to confirm morphologically, partly because of the complex structure of the striosome/matrix compartments. Here, we review recent morphological studies that elucidate the input/output organization of the striatal compartments.
Highlights
Reinforcement learning mechanisms have been recently proposed to be based in circuits of the basal ganglia, assuming that the dopamine nigrostriatal projection acts as a reinforcement signal pathway (Sutton, 1988; Schultz et al, 1997, 1998; Sutton and Barto, 1995; Bayer and Glimcher, 2005; Cohen et al, 2012; Hart et al, 2014)
We found that all single dopaminergic neurons innervated both striosome and matrix compartments, projections from dorsal substantia nigra pars compacta (SNc) neurons favored the matrix compartment and those from ventral SNc neurons favored the striosome compartment (Matsuda et al, 2009)
Takahashi et al (2008); Takahashi et al (2011) reported that information about task structure is represented in the orbitofrontal cortex and that it influences the computation of reward prediction error in ventral tegmental area (VTA) dopaminergic neurons
Summary
Reinforcement learning mechanisms have been recently proposed to be based in circuits of the basal ganglia, assuming that the dopamine nigrostriatal projection acts as a reinforcement signal pathway (Sutton, 1988; Schultz et al, 1997, 1998; Sutton and Barto, 1995; Bayer and Glimcher, 2005; Cohen et al, 2012; Hart et al, 2014). Reynolds et al (2001) have reported that synaptic potentiation in striatal neurons receiving dopamine projections depends on the input from the cerebral cortex and on the dopaminergic input from the substantia nigra. These reports have led to the hypothesis that synaptic plasticity in the striatum is under the influence of the reward prediction error, and that the striatum conducts reinforcement learning throughout the basal ganglia circuits (Barto, 1995; Montague et al, 1996; Doya, 1999, 2000a,b; Crittenden and Graybiel, 2011; Takahashi et al, 2011). Its principal action will be at those cortical and thalamic synapses that are “active,” aiding the “selection” of striatal neurons to be fired (see Bolam et al, 2006; Arbuthnott and Wickens, 2007)
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