Evaluating the Ear for Monitoring Somatosensory Evoked Potentials

Short-Iatency somatosensory evoked potentials (SSEPs) serve as a biomarker for recovery of consciousness from coma but often require bulky and time-consuming measurement setups such as scalp-based electroencephalography (EEG), cer-vical recording, or invasive in-vivo electrodes, which limits their deployment to clinical environments. To extend the utility of SSEPs in ambulatory monitoring and acute care, here we present a pilot study recording SSEPs from the inside of the ears of three rat subjects tested using non-invasive silver / silver-chloride (Ag / Agel) electrodes. Simultaneous recording from stainless steel electrodes surgically implanted at the cortical surface served as ground truth. The ground electrode for both modalities is shared and placed at 2 mm behind the lambda. The rat subjects under anesthesia using isoflurane were stimulated at the center of the gastrocnemius muscle in the left or right hindlimbs with a pulse width of 200 μs and amplitude of 1 mA every 2 s. Time domain analysis was performed to get epoch-averaged SSEP waveforms from the ear and cortical surface electrodes. Frequency domain analysis reveals that the ear EEG recording is able to capture most characteristics of neural activities below 100 Hz but with a shorter latency of SSEP peaks, warranting further evaluation of ear SSEPs to study if they can reflect experimentally induced changes in the subject state.