ACQ and the Basal Ganglia

1. ACQ and the Basal Ganglia Jimmy Bonaiuto USC Brain Project 6/26/2007

2. Actor-Critic Learning • Actor – learns action policy • Critic – learns value functions • Different actor-critic architectures have been proposed for learning different value functions: • V(s) = State values (most common) • V(a) = Action values • Q(s,a) = State, action pair values

3. Actor-Critic Architecture • Core Data – recording of midbrain dopaminergic neurons in appetitive learning tasks (Schultz, 1992; Schultz, 1998) (from Barto, 1995)

4. Critic – V(s), V(a), or Q(s,a)? • How do dopamine cells know about reward value? • Largest striatum input is from cortex (Haber and Gdowski, 2004) • V(s) and Q(s,a) learning may require the ventral striatum, SNc, and/or VTA to receive a copy of the same cortical projections that the dorsal striatum receives (state information) • V(a) may only require a projection from the dorsal striatum or globus pallidus (actor) to the ventral striatum, SNc and/or VTA (critic) • Largest forebrain input to dopamine neurons is striatum (Haber and Gdowski, 2004) • V(a) may be more biologically plausible in terms of connectivity

5. Actor-Critic in the Basal Ganglia • Dopamine targets (striatum) are site of value and policy learning (Suri & Schultz, 2001) • The striatum split into dorsal and ventral divisions (some say dorsolateral and ventromedial) (Voorn et al., 2004) • Ventral striatum – inputs from limbic structures (critic?) • Dorsal striatum – connected with motor and associative cortices (actor?)

6. Role of Dopamine • (Joel & Weiner, 2000) Dopamine neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) • VTA projects to ventral striatum – learning state values • SNc projects to dorsal striatum – policy learning • Little difference in VTA and SNc firing (Schultz et al., 1993) • Predicted by TD learning equation since the policy and values are both updated using TD error

7. ACQ • Reinforcement learning should maximize total utility, not necessarily total reward. Motivations map outcomes to utilities (Niv et al., 2006) • Multiple critics – one for each dimension of interoception (hunger, thirst, etc.) • Q(s ,a), s =internal state, a=action • Actor • Composite policy • Desirability – based on internal state • Executability – based on environmental state • Eligibility trace from mirror and canonical motor signals i i

8. ACQ – Actor/Multiple Critics x=executed action x=recognized action ^

9. ACQ - Eligibility Trace • = executed action (from efference copy) • = recognized action (from mirror system) ^ Idealized situations (perfect recognition) Realistic implementation would have confidence values between 0.0 and 1.0 for x and x, but the pattern of values for ε would be the same ^

10. ACQ - Weight Modification • Desirability and Executability updated using same eligibility and reinforcement signals • Requires different weight change rules: • Desirability • Executability Don’t update the value of the last action unless some action is currently recognized Step function of eligibility trace – Makes sign of weight change depend on r(t) ^ Tonic dopamine level, d, added to TD error – Makes sign of weight change depend on ε(t)

11. Multiple Critics – Q(s ,a) i • Is there evidence for multiple critics gated by interoceptive information? • The lateral hypothalamus does project to the SNc, VTA, and the ventral striatum (Saper et al., 1979; Fadel & Deutch, 2002; Brog et al., 1993) • The accumbens shell of the ventral striatum is reciprocally connected with the lateral hypothalamus and has been called a “sensory sentinel” or “visceral striatum” (Kelley, 1999, 2004) • Motivational state, such as food deprivation can influence the magnitude of dopamine release in the ventral striatum (Wilson et al., 1995; Ahn & Phillips, 1999) • Sexual satiety is signaled by serotonin from the lateral hypothalamus to the ventral striatum, which reduces dopamine levels (Lorrain et al., 1999)

12. Internal State-Dependent Policy • Is there evidence for internal state-dependent policies? (Kelley et al., 2005) • Information from the lateral hypothalamus reaches the dorsal striatum through the paraventricular nucleus • Hypothalamic-midline thalamic-striatal projections carry internal state information to cholinergic interneurons of the dorsal striatum • These are thought to modulate dorsal striatal output neurons

13. Eligibility Trace from the Mirror System • What is the evidence for an eligibility signal from mirror neurons? • People can implicitly learn sequences through action observation (Bird et al., 2005) • The striatum is consistently implicated in implicit sequence learning and the magnitude of activation is correlated with reaction time improvement (Rauch et al., 1997, 1998) • The basal ganglia is active during action observation (Frey & Gerry, 2006) • Projection from ventral premotor cortex (including the arcuate sulcus) to dorsal and ventral striatum in the macaque (McFarland & Haber, 2000)

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