The auditory brainstem response (ABR) is an auditory evoked potential extracted from ongoing electrical activity in the brain and recorded via electrodes placed on the scalp. The measured recording is a series of six to seven vertex positive waves of which I through V are evaluated. These waves, labeled with Roman numerals in Jewett and Williston convention, occur in the first 10 milliseconds after onset of an auditory stimulus. The ABR is considered an exogenous response because it is dependent upon external factors.
The auditory structures that generate the auditory brainstem response are believed to be as follows:
• Wave I through III – generated by the auditory branch of cranial nerve VIII and lower
• Wave IV and V – generated by the upper brainstem
• More in depth location – wave I originates from the dendrites of the auditory nerve fibers, wave II from the cochlear nucleus, III showing activity in the superior olivary complex, and wave IV–V associated with the lateral lemniscus.
When interpreting the ABR, we look at amplitude (the number of neurons firing), latency (the speed of transmission), interpeak latency (the time between peaks), and interaural latency (the difference in wave V latency between ears). The ABR represents initiated activity beginning at the base of the cochlea and moving toward the apex over a 4ms period of time. The peaks largely reflect activity from the most basal regions on the cochlea because the disturbance hits the basal end first and by the time it gets to the apex, a significant amount of phase cancellation occurs.