Development and Validation of a Concurrent AM-tACS — EEG Protocol Investigating Nociceptive Processing: A Proof-of-Concept Study

Pain perception is a multidimensional experience mediated by both the ascending and descending neuroanatomical pathways. The present thesis is a proof-of-concept study aimed at developing and validating a concurrent amplitude-modulated transcranial Alternating Current Stimulation (AM-tACS) - EEG protocol for investigating phase dependent modulation of nociceptive processing. A single-session within-subject design involving a baseline and an online stimulation part was conducted with five healthy right-handed adult participants. Real and dummy sensory stimuli were delivered via thermonociceptive heat stimuli and median nerve stimulation to the dominant volar forearm. Electrophysiological responses were recorded using cortical contact heat-evoked potentials (CHEPs) and cervical spinal somatosensory evoked potentials (SEPs). CHEPs and SEPs were explored as neurophysiological indices of nociceptive processing. Modulation of the descending pain pathways was targeted via AM-tACS. Non-conventional AM-tACS was delivered at a carrier frequency of 25 Hz with a 2.5 modulation envelope over sensorimotor-parietal electrode montage targeting the hand representation of the left primary motor cortex (M1), at an intensity of 4 mA peak-to-peak, with thermal and electrical sensory stimuli time-locked to the peak or trough of the modulation envelope. Real and dummy sensory stimuli were interleaved and temporally matched, enabling the isolation of genuine stimulus-evoked neural activity through subtraction of the corresponding trigger categories. Several preprocessing pipelines designed for concurrent AM-tACS recordings were evaluated before event-related potential extraction. Signal-to-noise ratio was assessed across preprocessing stages as a primary feasibility index. The primary outcome measure is CHEP N2–P2 amplitude. Statistical analyses primarily aimed to provide preliminary estimates of effect size and variability using repeated-measures models, with inferential statistics interpreted cautiously. Exploratory analyses include examination of other SEP components, namely N9 and N20, and correlation assessment of pain ratings. Brain stimulation safety and tolerability measures were collected and assessed for each participant. Preliminary findings showed an attenuation of CHEP N2 amplitude during Peak stimulation across all four participants, however no significant pattern was found for spinal N13 responses or pain ratings. Nevertheless, an interesting inverse relationship emerged between peripheral and spinal response magnitudes. Findings are interpreted in terms of protocol feasibility, effect size estimates, and suitability for a larger confirmatory study targeting phase-dependent descending pain modulation.