Decoding semantics from intermodulation responses in frequency-tagged stereotactic EEG

Humans perform object recognition using holistic processing, which is different from computers. Intermodulation responses in the steady-state visual evoked potential (SSVEP) of scalp electroencephalography (EEG) have recently been used as an objective label for holistic processing. Using stereotactic EEG (sEEG) to record SSVEP directly from inside of the brain, we aimed to decode Chinese characters from non-characters with activation from multiple brain areas including occipital, parietal, temporal, and frontal cortices. Semantic categories could be decoded from responses at the intermodulation frequency with high accuracy (80%−90%), but not the base frequency. Moreover, semantic categories could be decoded with activation from multiple areas including temporal, parietal, and frontal areas. Previous studies investigated holistic processing in faces and words with frequency-tagged scalp EEGs. The current study extended the results to stereotactic EEG signals directly recorded from the brain. The human brain applies holistic processing in recognizing objects like Chinese characters. Our findings could be extended to an add-on feature in the existing SSVEP BCI speller. •Frequency-tagged sEEG responses to real-, and non-Characters were recorded.•Intermodulation responses can be observed throughout the brain.•Semantic categories can be successfully decoded from intermodulation responses with high accuracy, but not from base-frequency responses.