Signal-specific performance of in-ear EEG: strengths and limitations.

Fully in-ear Electroencephalography (EEG) configurations prioritize wearability and rapid setup but may constrain spatial sampling and signal-to-noise characteristics. We conducted a domain-specific evaluation of a generic-fit, fully intra-auricular dry-electrode EEG system benchmarked against simultaneous 32-channel BioSemi scalp EEG in 19 healthy adults. Rather than testing equivalence, we mapped strengths and boundary conditions across (i) resting-state alpha modulation (eyes open vs. eyes closed), (ii) auditory event-related-potentials (ERP; N1-P2 complex), and (iii) alpha ERSP during speech-in-noise. The in-ear configuration robustly captured canonical resting-state alpha increases during eyes-closed periods, despite reduced absolute amplitude relative to scalp EEG. In contrast, reliable N1-P2 responses were observed only in conventionally average-referenced scalp EEG; in-ear and adapted referencing configurations showed reduced component detectability with lower component SNR. During speech-in-noise, significant alpha ERSP deviations from baseline emerged in scalp EEG under less adverse listening conditions, whereas comparable deviations were not consistently detected in the minimal in-ear configuration. These findings indicate that signal detectability depends strongly on signal class, effect size, and recording geometry, providing configuration-aware guidance for study design and future in-ear EEG development.