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Abstract

Introduction: Sensory stimuli that escape conscious perception can still modulate brain function. For imperceptible electrical finger nerve stimulation, we observed an early event-related potential (ERP) in extracranial EEG at 60 ms (but no later ERPs) and increased alpha power over the primary somatosensory cortex (S1; Nierhaus et al., 2015). However, the sensitivity of extracranial EEG might be insufficient for observing later ERPs and the exact localization of multiple alpha rhythm generators.

Methods: Here, we recorded electrocorticography (ECoG) data at 2048 Hz from one epilepsy patient while we applied single electrical pulses to her right index finger. The intracranial electrodes were placed over S1 and the secondary somatosensory cortex (S2). The pulse intensities were either imperceptible (subthreshold) or clearly perceptible (suprathreshold) but below the pain threshold. The trials were segmented from 0.5 before to 1.0 s after the stimulus onset and bandpass filtered (1-45 Hz).

Results: We observed ERPs both in S1 and S2 together with an alpha desynchronization over the posterior parietal cortex following suprathreshold stimulation. For subthreshold stimulation, we observed next to the early ERP (P60) also a later ERP (N136) in S1, accompanied by an alpha desynchronization over the posterior parietal cortex, as well as a synchronization over the premotor cortex.

Discussion: Most likely unconscious somatosensory processing ends in primary somatosensory cortex but not at 60 ms. Furthermore, the modulation of alpha rhythmic activity (synchronization and desynchronization) does not seem to arise in S1. Further research is necessary to investigate which cognitive processes result in the later ERPs (N136) for subliminal stimuli and which mechanisms underlie the alpha effects. However, our results support the proposal that S2 is necessary for conscious tactile perception.