Right or left ear reference changes the voltage of frontal and parietal somatosensory evoked potentials

Abstract

Short-latency cortical somatosensory evoked potentials (SEPs) to left median nerve stimulation were recorded with either the left or right earlobe as reference. With a right earlobe reference the voltage of the parietal N20 and P27 was reduced while the voltage of the frontal P20 and N30 was enhanced. The effects were consistent, but their size varied with the SEP component considered and also among the subjects. Analysis of SEPs at different scalp sites and at either earlobe suggested that the ear contralateral to the side stimulated picked up transient potential differences, depending a.o. on side asymmetry and geometry of the neural generators as disclosed in topographic mapping. For example, the right ear potential can be shifted negatively by the right N20 field evoked by left median nerve stimulation. The changes involve the absolute potential values, but not the time features or the gradients of potential fields. Scalp current density (SCD) maps are not affected. The results are pertinent for current discussions about which reference to use and document the practical recommendation of recording short-latency cortical SEPs with a reference at the ear ipsilateral (not contralateral) to the side of stimulation.

Cortical Processing of Noxious Somatosensory Stimuli in the Persistent Vegetative State

“…Further leads explored the spinal potentials at the C-6 level (bipolar derivation) and the brachial plexus potentials on Erb's point. The ear lobe ipsilateral to the stimulated side served as reference for cephalic and brachial plexus electrodes (Tomberg et al, 1991). Stimuli consisted of 0.2-ms electrical square-wave pulses delivered at 5.1 Hz to the median nerve at the wrist.…”

Inhibitory action of forearm flexor muscle afferents on corticospinal outputs to antagonist muscles in humans

“…SEPs evoked by median nerve stimulation at the elbow were recorded with a standard P3-ipsilateral earlobe montage (Tomberg et al 1991) in each subject by standard Ag-AgCl skin surface disc electrodes (diameter, 9 mm). One recording electrode was placed over the left parietal cortex (P3), the other over the ipsilateral earlobe.…”

Central scalp projection of the N30 SEP source activity after median nerve stimulation

“…Disk recording electrodes (impedance below 5 K⍀) were placed at 19 locations of the 10-20 system (excluding Fpz and Oz). The reference electrode was at the lobe of the ear ipsilateral to stimulation, as suggested by a previous report, 24 and the ground was at Fpz. The analysis time was 64 ms, including also 5 ms of prestimulus delay, with a bin width of 250 µsec.…”