Early Post-Stimulus Activity Negatively Predicts P300 Amplitude: A Single-Trial Analysis of the Auditory Oddball Task

The P300 event-related potential is a core index of attention and context updating, yet the trial-by-trial factors that modulate its amplitude remain incompletely characterized. This study tested whether early post-stimulus signal magnitude (0–150 ms) predicts subsequent P300 amplitude (300–600 ms) at the single-trial level. Using data from the ERP CORE auditory oddball dataset (N = 40 participants; 1,661 trials), early activity was quantified as root mean square (RMS) amplitude at electrode Fz. A linear mixed-effects model with full model diagnostics and post-hoc power analysis revealed a statistically reliable negative association between early RMS and P300 amplitude (β = −0.064, SE = 0.0245, z = − 2.61, p = 0.0085, 95% CI [− 0.112, − 0.016]). However, the effect size was minimal (R² = 0.0042), explaining less than 0.5% of trial-level variance. Notably, the stimulus condition effect (Target vs. Standard) was approximately 13 times larger, indicating that early signal magnitude provides modest modulation rather than decisive control over P300 generation. Model diagnostics confirmed adequate assumptions (Shapiro-Wilk W = 0.9995, p = 0.97; achieved power = 0.85). Exploratory complexity measures (Permutation Entropy, Lempel–Ziv) were non-predictive, suggesting amplitude-dependent rather than complexity-based coupling. The most conservative interpretation is that early RMS reflects momentary neural state that weakly biases P300 amplitude, possibly through resource competition or refractory-like effects. These findings establish a quantitative constraint on early–late ERP coupling, demonstrate that P300 is predominantly endogenously driven, and highlight the importance of distinguishing amplitude-based measures from trial-to-trial variability. Future work should decompose these components and incorporate pre-stimulus state covariates to clarify mechanisms.