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Learning Options for Compositional Motor Control with Adapter Banks
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Proposes CABRA, a shared LSTM core modulated by a bank of ten residual adapters selected by a CompILE-style discrete latent code learned from expert demonstrations of closed-loop biomechanical arm control, and shows emergent low-rank adapter perturbations and out-of-distribution compositional transfer.
Problem
How motor primitives implemented as low-rank perturbations of a shared recurrent network can be learned end-to-end from demonstrations, and whether the resulting primitives recombine to support out-of-distribution compositional motor transfer.
Approach
A CompILE-style encoder segments compound expert demonstrations into discrete codes that select residual adapters (full-rank, unconstrained) modulating a shared LSTM core; low-rank structure emerges from training, and a frozen-network soft policy over codes sequences the adapters for novel trajectories.
Key results
- Student reaches hand L1 approximately 0.005 on the two-petal transfer versus about 0.04 for the multitask teacher, an order-of-magnitude gap.
- Learned adapter perturbations have effective rank about 5-6 each, while the core hidden-state manifold has effective rank about 30 out of 256 units.
- Student task subspaces show mean principal-subspace overlap 0.18 versus 0.69 for the teacher.
Surprising findings
- Despite full-rank adapter parameterization, individual adapter perturbations converge to low effective rank (about 5-6).
- Teacher and student dynamics differ: the teacher's task representations overlap heavily in one shared subspace, while the student's are highly separated.
Limitations
- Training uses task-identity supervision on the encoder's segment-1 representation, so the purely unsupervised variant is untested.
- Student is evaluated only on a single biomechanical arm environment with five compound tasks and two OOD shapes.
- Forced-code analysis shows partial sharing among retraction codes, so retraction primitives are less one-to-one than extension primitives.
Experimental setup
Datasets: closed-loop teacher-generated demonstrations of compound biomechanical movements
Benchmarks: held-out compound task hand-trajectory L1 two-petal OOD trajectory butterfly OOD trajectory
Metrics: hand trajectory L1 error effective rank (95% variance) principal-subspace overlap DSA distance
Baselines: task-input-conditioned multitask RNN teacher single shared adapter ablation no-adapter ablation
Builds on: CompILE DIAYN thalamocortical low-rank perturbation framework LoRA Dynamical Similarity Analysis
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