What Are We Actually Decoding? Source Attribution for Non-Invasive Brain-to-Language Retrieval

• URL: http://arxiv.org/abs/2605.24524v1
• Date: Sat, 23 May 2026 11:23:39 GMT
• Title: What Are We Actually Decoding? Source Attribution for Non-Invasive Brain-to-Language Retrieval
• Abstract summary: We recast stimulus-locked MEG-to-audio retrieval as an auditing framework.<n>We use structural shortcuts, window-level stimulus-locked evidence, and cross-window contextual aggregation.<n>These results suggest that brain-to-language performance should be source-attributed, not merely reported.
• Score: 42.66754319854329
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: In non-invasive neural language decoding, results can be inflated by sources that are not stimulus-evoked neural evidence: decoder priors, embedding-based metrics, and non-neural structural nuisances such as signal duration. The methodological challenge is therefore attribution: a reported gain is more informative when it can be traced to a specific source. We recast stimulus-locked MEG-to-audio retrieval as an auditing framework that separates apparent performance into three sources - structural shortcuts, window-level stimulus-locked evidence, and cross-window contextual aggregation - and provides a diagnostic for each. Signal-blind Gaussian noise reaches 66.3% Rank@1 (R@1) under variable-length decoding but collapses to near chance once fixed-duration windows and stimulus-identity splits are enforced, isolating structural leakage. Under these controls, fixed-window retrieval recovers measurable MEG-audio discriminability, while an oracle sentence-bucket diagnostic shows that 95.7% of Top-1 errors select the wrong sentence, localising the residual bottleneck to sentence-level competition. We audit this contextual source with Group Context Bias (GCB), an inference-time additive logit bias that pools sentence-consistent evidence across windows while leaving the base retrieval scores and candidate pool fixed. Used as a score-space intervention, GCB makes the contextual source measurable: R@1 shifts from 44% to 52% on Gwilliams and from 22% to 29% on MOUS under the same fixed setting. GCB is auditable under this design: its effect collapses under random-grouping perturbations and vanishes when local evidence is attenuated in MEG or is near chance in EEG, supporting its use as a controlled source-attribution intervention. These results suggest that brain-to-language performance should be source-attributed, not merely reported.

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