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Leveraging Low-Rank and Sparse Recurrent Connectivity for Robust Closed-Loop Control
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Parameterizes RNN recurrent weights as a random-sparse, low-rank product and shows that, for closed-form continuous-time networks (CfCs), low-rank sparse connectivity improves closed-loop robustness under distribution shift on Arcade Learning Environment and MuJoCo imitation tasks.
Problem
Recurrent agents fit offline on expert demonstrations often fail when deployed closed-loop under distribution shift; it is unclear how recurrent connectivity structure (rank and sparsity) affects this robustness.
Approach
Parameterize recurrent weights as a rank-r factorization multiplied elementwise by a fixed random sparsity mask at initialization, and analyze how rank and sparsity modulate spectral radius, spectral norm, singular value decay and state-space dimensionality. Low-rank connectivity is found to improve robustness under distribution shift for CfCs, whereas sparsity helps LSTMs in-distribution.
Key results
- Under distribution shift, low-rank, low-sparsity CfCs are the best performing models in Seaquest, and low-rank models perform on par with or better than higher-rank ones across architectures.
- In-distribution, LSTMs tolerate high sparsity well, whereas CfCs and RNNs degrade at high sparsity relative to LSTMs.
- CfCs learn lower recurrent spectral norms and lower-dimensional state-space trajectories than LSTMs and RNNs.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Use low-rank, low-sparsity recurrent connectivity in CfCs (ablation) | CfC · Seaquest, online under distribution shift | normalized reward | Helped |
| Reduce recurrent rank (ablation) | recurrent architectures · Seaquest, online under distribution shift | normalized reward | Helped |
| Increase recurrent sparsity (ablation) | recurrent architectures · Seaquest, online under distribution shift | normalized reward | Hurt |
| Increase recurrent sparsity (ablation) | LSTM · Seaquest, online in-distribution | normalized reward | Helped |
| Increase recurrent sparsity (ablation) | CfC · Seaquest, online in-distribution | normalized reward | Hurt |
| Reduce recurrent rank (ablation) | LSTM · Seaquest, online in-distribution | normalized reward | Hurt |
| Use CfC instead of LSTM or RNN | recurrent agents · online under distribution shift | robustness | Helped |
| Reduce recurrent rank (ablation) | CfC, RNN, and LSTM · online in-distribution | recurrent state-space dimensionality | Helped |
| Increase recurrent sparsity (ablation) | CfC, RNN, and LSTM · online in-distribution | recurrent state-space dimensionality | Hurt |
| Increase recurrent rank (ablation) | CfC · trained model | time-constant deviation from 0.5 | Hurt |
Surprising findings
- Sparsity and low-rank pruning have opposing effects: high sparsity hurts robustness under distribution shift while low rank improves it.
- Extending the rank/sparsity parameterization to input weights was found to be detrimental to performance (results not shown).
Limitations
- Theoretical results on spectral radius and norm are proven only for some initialization schemes; other cases are empirical.
- Spectral analyses are used as proxies for robustness and comparisons across architectures are acknowledged as potentially limited.
- Input weights are not parameterized by rank and sparsity, since doing so hurt performance.
Experimental setup
Datasets: Seaquest expert trajectories (from DQN) Alien expert trajectories (from DQN) HalfCheetah expert trajectories (from DQN)
Benchmarks: Arcade Learning Environment Seaquest Arcade Learning Environment Alien MuJoCo HalfCheetah
Metrics: normalized online reward in-distribution normalized reward averaged across distribution shifts spectral radius spectral norm singular value decay effective dimensionality (PCA)
Baselines: RNN LSTM GRU CNN full-rank fully-connected CfC
Builds on: closed-form continuous-time neural networks low-rank RNN echo state networks LSTM
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