Eligibility Traces and Plasticity on Behavioral Time Scales: Experimental Support of neoHebbian Three-Factor Learning Rules

TL;DR

This paper reviews experimental evidence supporting the role of eligibility traces and a third factor in synaptic plasticity, bridging the gap between rapid neuronal activity and slower behavioral learning on the scale of seconds.

Contribution

It provides experimental support for neoHebbian three-factor learning rules, linking eligibility traces with neuromodulatory signals in biological plasticity.

Findings

Experimental evidence for eligibility traces lasting seconds.

Support for third factors like neuromodulators in synaptic plasticity.

Validation of neoHebbian three-factor learning models.

Abstract

Most elementary behaviors such as moving the arm to grasp an object or walking into the next room to explore a museum evolve on the time scale of seconds; in contrast, neuronal action potentials occur on the time scale of a few milliseconds. Learning rules of the brain must therefore bridge the gap between these two different time scales. Modern theories of synaptic plasticity have postulated that the co-activation of pre- and postsynaptic neurons sets a flag at the synapse, called an eligibility trace, that leads to a weight change only if an additional factor is present while the flag is set. This third factor, signaling reward, punishment, surprise, or novelty, could be implemented by the phasic activity of neuromodulators or specific neuronal inputs signaling special events. While the theoretical framework has been developed over the last decades, experimental evidence in support of eligibility traces on the time scale of seconds has been collected only during the last few years. Here we review, in the context of three-factor rules of synaptic plasticity, four key experiments that support the role of synaptic eligibility traces in combination with a third factor as a biological implementation of neoHebbian three-factor learning rules.