# Theta sequences as eligibility traces: a biological solution to credit assignment

arXiv: https://arxiv.org/abs/2305.08124 · HTML: https://mlsanity.com/paper/2305.08124/

## Key facts

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- First posted on arXiv 14 May 2023.

## Abstract (arXiv)

Credit assignment problems, for example policy evaluation in RL, often require bootstrapping prediction errors through preceding states textitor maintaining temporally extended memory traces; solutions which are unfavourable or implausible for biological networks of neurons. We propose theta sequences -- chains of neural activity during theta oscillations in the hippocampus, thought to represent rapid playthroughs of awake behaviour -- as a solution. By analysing and simulating a model for theta sequences we show they compress behaviour such that existing but short mathsfO(10) ms neuronal memory traces are effectively extended allowing for bootstrap-free credit assignment without long memory traces, equivalent to the use of eligibility traces in TD(λ).