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Genela Morris"to decide : the role of dopamine in planning and action".

Abstract :


A
ccording to current models of the basal-ganglia as the neural substrate underlying reinforcement learning, dopamine neurons provide a reinforcement signal reporting the change in prediction of reward.
In the first study we report, we recorded midbrain dopamine neurons in monkeys performing a probabilistic instrumental conditioning task. We found that dopamine neurons accurately report the mismatch between the monkeys’ expectations and reality, closely resembling the TD error signal. Yet, this study, and other physiological studies are limited to classical conditioning (CC) and instrumental conditioning (IC), avoiding the question of decision-making. We therefore examined behaviour and dopamine activity in a 2-armed bandit task vs. a reference IC task involving the same stimuli.
The place of dopamine in decision behaviour was explored on two time-scales. First, we show that while the monkeys’ long term decision policy follows models of reward reinforcement, behaviour is best explained by its neural correlate, the dopamine responses in the reference task. Secondly, we demonstrate that in the decision task, future action is already encoded in the dopamine responses when the stimuli are presented. This locates the dopamine neurons upstream or at the centre of the decision junction. We further infer that the effect of dopamine on behaviour is one of long-term reinforcement, rather than immediate action selection.
Finally, our results call for an update of computational basal-ganglia models to implement learning of state-action values, rather than state alone. 

Selected publications

Morris G, Hershkovitz Y, Raz A, Nevet A, Bergman H.
Physiological studies of information processing in the normal and Parkinsonian basal ganglia: pallidal activity in Go/No-Go task and following MPTP treatment.
Prog Brain Res. 2005;147:285-93.
Arkadir D, Morris G, Vaadia E, Bergman H.
Independent coding of movement direction and reward prediction by single pallidal neurons.
J Neurosci. 2004 Nov 10;24(45):10047-56.
Morris G, Nevet A, Bergman H.
Anatomical funneling, sparse connectivity and redundancy reduction in the neural networks of the basal ganglia.
J Physiol Paris. 2003 Jul-Nov;97(4-6):581-9. Review.

Morris G, Arkadir D, Nevet A, Vaadia E, Bergman H.
Coincident but distinct messages of midbrain dopamine and striatal tonically active neurons.
Neuron. 2004 Jul 8;43(1):133-43.
Nevet A, Morris G, Saban G, Fainstein N, Bergman H.
Discharge rate of substantia nigra pars reticulata neurons is reduced in non-parkinsonian monkeys with apomorphine-induced orofacial dyskinesia.
J Neurophysiol. 2004 Oct;92(4):1973-81. Epub 2004 Apr 28.

Bar-Gad I, Morris G, Bergman H.
Information processing, dimensionality reduction and reinforcement learning in the basal ganglia.
Prog Neurobiol. 2003 Dec;71(6):439-73. Review.

Thomas Boraud