Sensory evoked potentials (SEP) are recorded from the
central nervous system following stimulation of
sense organs, for example,
visual evoked potentials elicited by a flashing light or changing pattern on a monitor,
auditory evoked potentials by a click or tone stimulus presented through earphones), or tactile or
somatosensory evoked potential (SSEP) elicited by tactile or electrical stimulation of a sensory or mixed nerve in the
periphery. Sensory evoked potentials have been widely used in
clinical diagnostic medicine since the 1970s, and also in intraoperative neurophysiology monitoring (IONM), also known as surgical neurophysiology. There are three kinds of evoked potentials in widespread clinical use: auditory evoked potentials, usually recorded from the scalp but originating at
brainstem level; visual evoked potentials, and
somatosensory evoked potentials, which are elicited by electrical stimulation of peripheral nerve. Examples of SEP usage include: were the first investigators to report the abnormal brainstem auditory evoked potentials (BAEPs) in an alcoholic woman who recovered from
acquired central hypoventilation syndrome. These investigators hypothesized that their patient's
brainstem was poisoned, but not destroyed, by her chronic alcoholism.
Visual evoked potential Visual evoked potential (VEP or EVP or EVR) is an evoked potential elicited by presenting light flash or pattern stimulus which can be used to confirm damage to visual pathway including
retina,
optic nerve,
optic chiasm,
optic radiations, and
occipital cortex. One application is in measuring infant's visual acuity. Electrodes are placed on infant's head over
visual cortex and a gray field is presented alternately with a checkerboard or grating pattern. If the checker's boxes or stripes are large enough to be detected, VEP is generated; otherwise, none is generated. It's an objective way to measure infant's visual acuity. VEP can be sensitive to visual dysfunctions that may not be found with just physical examinations or MRI, even if it cannot indicate etiologies. It can be used to examine infant's visual impairment for abnormal visual pathways which may be due to delayed maturation. In 1934, Adrian and Matthew noticed potential changes of the occipital EEG can be observed under stimulation of light. Ciganek developed the first nomenclature for occipital EEG components in 1961. During that same year, Hirsch and colleagues recorded a visual evoked potential (VEP) on the occipital lobe (externally and internally), and they discovered amplitudes recorded along the
calcarine fissure were the largest. In 1965, Spehlmann used a checkerboard stimulation to describe human VEPs. An attempt to localize structures in the primary visual pathway was completed by Szikla and colleagues. Halliday and colleagues completed the first clinical investigations using VEP by recording delayed VEPs in a patient with retrobulbar neuritis in 1972. A wide variety of extensive research to improve procedures and theories has been conducted from the 1970s to today and the method has also been described in animals.
VEP Stimuli The diffuse-light flash stimulus is rarely used nowadays due to the high variability within and across subjects. However, it is beneficial to use this type of stimulus when testing infants, animals or individuals with poor visual acuity. The checkerboard and grating patterns use light and dark squares and stripes, respectively. These squares and stripes are equal in size and are presented, one image at a time, via a computer screen.
VEP Electrode Placement Electrode placement is extremely important to elicit a good VEP response free of artifact. In a typical (one channel) setup, one electrode is placed 2.5 cm above the
inion and a reference electrode is placed at Fz. For a more detailed response, two additional electrodes can be placed 2.5 cm to the right and left of Oz.
VEP Waves The VEP nomenclature is determined by using capital letters stating whether the peak is positive (P) or negative (N) followed by a number which indicates the average peak latency for that particular wave. For example, P100 is a wave with a positive peak at approximately 100 ms following stimulus onset. The average amplitude for VEP waves usually falls between 5 and 20 microvolts. Normal values are depending on used stimulation hardware (flash stimulus vs.
cathode-ray tube or
liquid-crystal display, checkerboard field size, etc.).
Types of VEP Some specific VEPs are: • Monocular pattern reversal (most common) • Sweep visual evoked potential • Binocular visual evoked potential • Chromatic visual evoked potential • Hemi-field visual evoked potential • Flash visual evoked potential • LED Goggle visual evoked potential • Motion visual evoked potential •
Multifocal visual evoked potential • Multi-channel visual evoked potential • Multi-frequency visual evoked potential • Stereo-elicited visual evoked potential •
Steady state visually evoked potential Auditory evoked potential Auditory evoked potentials (AEP) can be used to trace the signal generated by a sound through the ascending auditory pathway. The evoked potential is generated in the cochlea, goes through the
cochlear nerve, through the
cochlear nucleus,
superior olivary complex,
lateral lemniscus, to the
inferior colliculus in the midbrain, on to the
medial geniculate body, and finally to the
cortex. Auditory evoked potentials (AEPs) are a subclass of
event-related potentials (ERPs). ERPs are brain responses that are time-locked to some "event", such as a sensory stimulus, a mental event (such as recognition of a target stimulus), or the omission of a stimulus. For AEPs, the "event" is a sound. AEPs (and ERPs) are very small electrical voltage potentials originating from the brain recorded from the scalp in response to an auditory stimulus, such as different tones, speech sounds, etc.
Brainstem auditory evoked potentials are small AEPs that are recorded in response to an auditory stimulus from electrodes placed on the scalp. AEPs serve for assessment of the functioning of the
auditory system and
neuroplasticity. They can be used to diagnose learning disabilities in children, aiding in the development of tailored educational programs for those with hearing and or cognition problems.
Somatosensory evoked potential Somatosensory evoked potentials (SSEPs) are EP recorded from the brain or spinal cord when stimulating peripheral nerve repeatedly. SSEPs are used in
neuromonitoring to assess the function of a patient's
spinal cord during
surgery. They are recorded by stimulating peripheral nerves, most commonly the
tibial nerve,
median nerve or
ulnar nerve, typically with an
electrical stimulus. The response is then recorded from the patient's
scalp. Although stimuli such as touch, vibration, and pain can be used for SSEP, electrical stimuli are most common because of ease and reliability. Because SSEP with latency less than 50 ms is relatively independent of consciousness, if used early in comatose patient, it can predict outcome reliably and efficiently. For example, comatose patients with no responses bilaterally has 95% chance of not recovering from coma. But care should be taken analyzing the result. For example, increased sedation and other CNS injuries such as the spinal cord can affect SEP. == Motor evoked potentials ==