Scalp distribution of human auditory evoked potentials. I. Evaluation of reference electrode sites.

In an attempt to settle the long-standing controversy about the relative activity of nose, sternovertebral (SV), and other commonly used AEP reference sites, we recorded AEPs from a coronal chain of electrodes from the vertex to the tragus, extending down the neck, and from the nose, ear, mastoid process, knee and ankle. All electrodes were referred to a sternovertebral reference balanced to minimize EKG. Voltage gradients and wave forms corresponding to other references were derived by computer. Stimuli were presented at fixed durations after the R wave of the EKG and averages with no stimulus were subtracted from averages with click stimuli to minimize synchronized EKG activity in non-cephalic derivations.

Because the activity at a given electrode site cannot be determined in absolute terms, alternative reference sites must be compard by examining the relative location of each site in the total potential field. An indifferent electrode site defined in this manner is one at which the spatial and temporal voltage gradients are minimal over the duration of the activity of interest. AEP voltage gradients in this experiment were steepest in the temporal region, became extremely shallow at locations on the upper neck, and showed no detectable gradients below that point attributable to AEP activity. A gradient of this form was obtained in all subjects, in spite of significant intersubject differences in the magnitude of potentials in nose-SV derivations. Commonly used reference sites on the head such as the nose, ear and mastoid process, were shown to lie in regions of the AEP field characterized by significant voltage gradients over time and spatial location in most subjects. The balanced sternovertebral reference of Stephenson and Gibbs (1951) appears to be the best general choice for AEP recordings.