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QUESTION-311: Do synaptic eligibility traces on the scale of seconds carry learning from delayed outcomes in behaving animals, so that interfering with the trace between action and outcome abolishes learning from the delayed outcome but not from immediate ones?

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Properties4
StatusOpen
Tagsneuroscience, learning-and-conditioning
Linewill-organization
Cited as “bears on” byTHEORY-1178 — In slices of striatum, visual and prefrontal cortex and hippocampus, Hebbian co-activation leaves a synaptic eligibility trace, lasting about 1 s in striatum, 3 to 10 s in cortex and about 2 s in CA1, that a later neuromodulator or plateau potential converts into a weight change and whose sign it can set: experimental support for three-factor learning rules on behavioural time scales, THEORY-909 — As reviewed by Gerstner et al. (2018) from five papers, mostly in slices and one laboratory per region, pairing pre- and postsynaptic activity changed the synapse when a neuromodulator or plateau potential followed within about 1 s in striatum, within 5 s (noradrenaline, LTP after pre-before-post pairing) or 2.5 s (serotonin, LTD after post-before-pre pairing) in visual and prefrontal cortex, and within about 2 s in CA1, and not at longer delays; the reviewers read this as a decaying synaptic eligibility trace and as experimental support for three-factor learning rules on behavioural time scales

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Why it is a question

THEORY-909 holds that, in five papers reviewed by Gerstner et al., mostly slice experiments with one laboratory per region, a pairing changed the synapse when a neuromodulator or plateau potential followed within one to five seconds and not later. The reviewers grant that persistent network activity or short-term plasticity could bridge the same gap. CLAIM-443’s schematic three-factor rule would need such a trace to learn from delayed outcomes, though it claims no neural implementation.

What would answer it

The seconds-scale trace shown in behaving animals, not only in slices: an experiment in vivo in which interfering with the trace’s mechanism (for example CaMKII in striatal spines) during the interval between action and delayed outcome abolishes learning from that outcome while leaving learning from immediate outcomes intact, and independent replication of at least the striatal and cortical time constants. Refuted if learning from delays of seconds survives the abolition of synaptic traces, carried instead by persistent network activity or short-term plasticity. Further slice experiments with the same protocols cannot settle it.

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