Involuntary facial muscle activity during imagined vocalisation contaminates EEG and enables emotion decoding

Objective. Decoding imagined speech from electroencephalography (EEG) recordings is potentially useful for brain-computer interfaces. Previous studies have focused on decoding semantic information from EEG, leaving the decoding of emotion - an important component of human communication - largely unexplored. Approach. Here, we report two experiments involving participants tasked with overt (n = 14) or imagined (n = 21) emotional vocalisation in five different categories: anger, happiness, neutral, sadness, and pleasure. Throughout, we recorded 64-channel EEG; we computed time-frequency features and used a logistic-regression classifier to evaluate emotion decoding accuracy. In five participants, we also recorded facial surface electromyography (sEMG) during imagined vocalisation, and studied the contamination of EEG by sEMG. Main results. Our results show that emotion can be decoded from single-trial EEG recordings of both overt (78.1%, chance = 20%) and imagined vocalisation (36.4%). The high-gamma band (50 to 100 Hz) and lateral EEG channels (T7, T8, and proximal) were important for decoding. sEMG analysis indicated that involuntary facial muscle activity contributed to these spectral and spatial patterns during imagined vocalisation. We identified a prominent muscle-induced pattern over lateral EEG channels - the railroad cross-tie pattern - showing greater activation during imagined happy vocalisations. We conclude that involuntary facial muscle activity is associated with certain emotion categories (i.e., happiness) and contributes to the decoding of emotion from single-trial EEG recordings. Significance. This study provides converging EEG and sEMG evidence demonstrating that involuntary facial muscle activity occurs during imagined vocalisation and is modulated by emotion, producing emotion-specific EEG contamination patterns. Although we focused on imagined vocalisations, the EMG patterns we observed closely resembled EEG patterns commonly reported in emotion decoding studies, raising concerns about involuntary facial muscle activity in such work.