A Negative-Control Protocol for Clinical EEG Foundation-Model Benchmarks: Dataset Identity and External-Cohort Stress Testing

EEG foundation-model gains may depend on cohort, montage, or probe design. We evaluated five models on five tasks across four benchmark datasets plus Korean CAUEEG, using subject-disjoint validation where identifiers exist. CAUEEG is recording-level with an annotated no-overlap held-out sensitivity. On matched CAUEEG normal/mild cognitive impairment/dementia classification (1,187 recordings), classical features reached 0.734 macro-AUROC (enhanced sensitivity: 0.736), versus BIOT-bipolar16 0.677, CBraMod 0.669, and REVE 0.568. The annotated no-overlap held-out subset preserved the classical-over-REVE ordering (0.717 versus 0.565). All five encoders decoded dataset identity at 1.000 before and after in-fold PCA-50; label permutations collapsed to chance and balanced subsamples remained at 1.000. This establishes dataset membership, not a causal site, geography, or population effect. A matched fully randomly initialized encoder was descriptively higher than pretrained REVE on CAUEEG (0.667 versus 0.570), and correct- versus scrambled-source-label LoRA runs yielded numerically similar AUROCs in unmatched descriptive sensitivities, not label-effect estimates or equivalence tests. On CHB-MIT cross-subject ictal detection, REVE reached 0.793 AUROC, versus 0.739 for the best tested enhanced nonlinear comparator, 0.691 for fully random initialization, and 0.505 for raw-signal random features. The paired REVE-minus-enhanced-comparator difference was +5.38 percentage points (95% CI -0.36 to +11.22), so comparator superiority remains unresolved; an amplitude-aware comparator also cannot be reconstructed from the retained normalized inputs. We distill montage matching, patient-overlap checks, stronger comparators, and representation controls into a reporting protocol for clinical EEG foundation-model studies.