Limits of Trial-Adaptive Neural–Language Fusion Across Large Language Models in P300 Brain–Computer Interfaces
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Objective
Published P300-speller fusion schemes fix prior trust regardless of trial reliability; we tested whether a reliability estimate improves on it.
Methods
We reanalyzed 3,373 archived P300-speller selections from 47 people with ALS (BigP3BCI). A fair, matched-search-space comparison, tuning both a fixed weight and an adaptive policy out-of-fold, was evaluated across 22 evaluable language-model priors up to 46.7B parameters. Two representative priors, GPT-2 and a classical 5-gram, additionally received detailed naive and mechanistic analyses.
Results
No prior’s 95% CI favored adaptive fusion under the fair comparison, despite unexploited oracle headroom at every scale. Under GPT-2, the naive comparison was significantly worse for adaptive fusion; both anchors converged to a degenerate or near-degenerate fair-comparison solution. For the representative anchors, three further controllers failed to convert that headroom into benefit; the fixed-fused posterior’s output probability outperformed the best controller for flagging errors (2.8- to 3.8-fold enrichment).
Conclusion
A tuned fixed weight is a difficult-to-beat default across the tested scale range; reliability estimation gave no deployable adaptive advantage.
Significance
Adaptive weighting should be validated against a fairly tuned baseline across model families and scales; in this dataset, the fused output’s confidence identified high-risk selections better than the tested purpose-built ranker.
