Abstract
Most reinforcement learning models assume that the reward signal arrives after the activity that led to the reward, placing constraints on the possible underlying cellular mechanisms. Here we show that dopamine, a positive reinforcement signal, can retroactively convert hippocampal timing-dependent synaptic depression into potentiation. This effect requires functional NMDA receptors and is mediated in part through the activation of the cAMP/PKA cascade. Collectively, our results support the idea that reward-related signaling can act on a pre-established synaptic eligibility trace, thereby associating specific experiences with behaviorally distant, rewarding outcomes. This finding identifies a biologically plausible mechanism for solving the 'distal reward problem'.
Highlights
Spike timing-dependent plasticity (STDP) is a physiologically relevant form of Hebbian learning (Caporale and Dan, 2008)
In its classic form, STDP depends on the order and precise timing of presynaptic and postsynaptic spikes: pre-before-post spike pairings induce timing-dependent longterm potentiation (t-LTP), whereas post-before-pre pairings induce timing-dependent long-term depression (t-LTD) (Markram et al, 1997; Bi and Poo, 1998)
We first sought to corroborate the shape of the STDP induction curve by varying the time interval between the presynaptic and postsynaptic activity (Δt; Figure 1A,B,G)
Summary
Spike timing-dependent plasticity (STDP) is a physiologically relevant form of Hebbian learning (Caporale and Dan, 2008). It is well established that reward-motivated behavior depends on the activity of DA neurons (Schultz et al, 1997; Suri and Schultz, 1999; Pan et al, 2005), the mechanisms that associate specific experiences with rewarding outcomes, which typically occur after a delay, are not well understood. This is referred to as the distal reward problem (Hull, 1943).
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