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Discovering plasticity rules that organize and maintain neural circuits
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Meta-learns local plasticity rules (CMA-ES over basis-function coefficients and time constants) that self-organize and maintain sparse sequential activity in a threshold-linear HVC-like network under synaptic turnover, including E→E and inhibitory plasticity.
Problem
How local, unsupervised plasticity rules can organize and maintain sequence-generating (timing) dynamics in neural circuits despite biological noise such as synaptic turnover and cell death, inspired by zebra finch HVC.
Approach
Parameterize plasticity as a weighted basis of pre/post activity filtered by decaying exponentials and synapse size, and meta-learn coefficients and time constants via CMA-ES with a loss that measures decodability of elapsed time, with synaptic turnover injected during meta-training; extend to separate rules for E→E, E→I and I→E synapses.
Key results
- Meta-learning without perturbation found a temporally asymmetric generalization of Oja's rule (three dominant terms) that reliably organizes sparse sequences; term-dropout analysis showed a sharp elbow at 3 terms.
- Meta-learned rules trained with and without synaptic turnover outperformed a rigid-bounds Hebbian sequence rule under turnover (p = 2.5e-8, Cohen's d = -0.45), but not distinct without perturbation after Bonferroni correction.
- Learning plasticity on E→I and I→E synapses improved time decoding over E→E-only rules, especially at high synaptic turnover rates; learned recurrent inhibition was homeostatic after afferent perturbations.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Use the meta-learned rule trained with synaptic turnover instead of the rigid-constraint sequence rule | Sequence-generating circuit · Time decoding after synaptic turnover | decoder loss: p = 2.5 x 10−8, Cohen’s d = −0.45 | Helped |
| Use the meta-learned rule trained without synaptic turnover instead of the rigid-constraint sequence rule | Sequence-generating circuit · Time decoding after synaptic turnover | decoder loss: p = 7.2 x 10−7, Cohen’s d = −0.44 | Helped |
| Use the meta-learned rule trained with synaptic turnover instead of the rigid-constraint sequence rule | Sequence-generating circuit · Time decoding without synaptic perturbation | decoder loss: p = 0.016, Cohen’s d = −0.29 | No clear effect |
| Use the meta-learned rule trained without synaptic turnover instead of the rigid-constraint sequence rule | Sequence-generating circuit · Time decoding without synaptic perturbation | decoder loss: p = 0.029, Cohen’s d = −0.30 | No clear effect |
| Add temporally asymmetric Hebbian terms ˜xᵢxⱼ and xᵢ˜xⱼ (ablation) | Meta-learned E→E plasticity · Sequence organization without perturbation | loss | Helped |
| Add the postsynaptic activity bound wᵢⱼ˜xⱼxⱼ (ablation) | Meta-learned E→E plasticity · Sequence organization without perturbation | loss | Helped |
| Add constant synaptic potentiation and a postsynaptic activity bound independent of synapse size (ablation) | Meta-learned E→E plasticity trained with turnover · Sequence organization with synaptic turnover | loss | Helped |
| Add temporally asymmetric Hebbian learning ˜xᵢxⱼ and the postsynaptic activity bound wᵢⱼ˜xⱼxⱼ (ablation) | Meta-learned E→E plasticity trained with turnover · Sequence organization with synaptic turnover | loss | Helped |
| Add E→I and I→E plasticity to E→E plasticity | Meta-learned sequence-generating circuit · Time decoding across synaptic turnover rates | decoder loss | Helped |
| Add the presynaptic activity term ˜xᵢxᵢ to E→I plasticity (ablation) | Meta-learned E→I plasticity · Sequence organization | loss | Helped |
Surprising findings
- Without perturbation, the meta-learned rule did not clearly outperform the rigid-bound baseline after multiple-comparison correction, so the advantage appears mainly under synaptic turnover.
- Learned plasticity rules were dense rather than sparse in the basis, so interpretation required perturbation-based term dropout analysis.
Limitations
- Meta-learning is computationally expensive and CMA-ES may need many epochs; different seeds yield different solutions.
- The chosen basis is dense and limits interpretability; it also excludes feedback-modulated plasticity.
- Initial connectivity strongly affects which plasticity rules succeed; only a limited number of seeds were tested.
Experimental setup
Benchmarks: time decoding loss in simulated HVC-like networks under synaptic turnover
Metrics: decoding loss (linear decoder) median loss across 100 test networks Kruskal-Wallis H test Cohen's d
Baselines: multiplicative asymmetric Hebbian rule with rigid synapse bounds (Fiete/Jun-styl meta-learned E→E-only rules no plasticity
Builds on: Confavreux et al. meta-learning of plasticity rules Oja's rule Nicola and Clopath sequence model Bengio et al. learning rule meta-learning
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