Evoked Potentials

Evoked potentials ( Latin evocare , "bring about", "cause" and potentia , "strength", "power") are potential differences in the electroencephalogram (EEG), which are triggered by stimulation of a sensory organ or peripheral nerve . In a broader sense, all specifically triggered electrical phenomena in the EEG can be understood as evoked potentials.

Measurement

Every sensory stimulus triggers changes in electrical potential in the sensory areas of the cerebral cortex . These evoked potentials can only be detected with the usual technology with significantly smaller amplitudes than the spontaneously occurring EEG signal (1–15 µV instead of 50–100 µV). In order to be able to measure and represent the evoked activity, several realizations of an evoked potential are averaged . The EEG is regarded as a stochastic interference signal that is independent of the stimulus and whose mean value is zero, while the evoked potential of interest is temporally linked to the stimulus. It shows the same course after each stimulus. As a result of the repeated presentation of a stimulus and the averaging of the subsequent EEG segment, the stimulus-independent activity tends towards zero, while the stimulus-related evoked potential is added up. The number of realizations necessary in practice depends on the signal-to-noise ratio and is different depending on the sensory modality and physical characteristics. With a potential triggered by light flashes, around 50 stimuli are sufficient, while around 1000 to 2000 stimuli have to be presented to measure the early acoustic brainstem potentials.

The evaluation takes into account the shape of the wave, the wave height ( amplitude ) and the transit time ( latency ).

Examples

• VEP - visually evoked potentials allow an assessment of the optic nerve and the visual pathway , especially in the diagnosis of the course of optic neuritis in multiple sclerosis . During the examination, the patient looks at a checkerboard pattern that reverses its colors every second (black becomes white and vice versa). Both eyes can be stimulated simultaneously or individually. The resulting electrical potentials in the visual cortex ( occipital cortex ) are recorded.
• AEP - acoustic evoked potentials enable an assessment of the auditory pathway: from the cochlea to the auditory nerve to the brain stem (early AEP ( FAEP )), further via the midbrain (medium AEP ( MAEP )) to the auditory cortex (late AEP ( SAEP or CERA (cortical evoked response audiometry))). The FAEPs are z. B. used for the diagnosis of acoustic neuromas .
• SEP - somatic evoked potentials, also SSEP - somatosensitive evoked potentials (as anglicism also imprecise "somatosensory evoked potentials") enable an assessment of the central somatosensitive pathway and peripheral sensory nerves. Repeated electrical stimuli are applied via a stimulation electrode near a sensitive nerve. The measuring electrodes are attached to the scalp. Additional measuring electrodes can be attached along the peripheral nerves and over the stimulus-processing centers of the spinal cord and brain and measure their electrical response and the transit time of the signal. By comparing the transit times and the distances, it can then be determined in which section a possible delay in the signal has occurred ( floor diagnostics ). Typical stimulus locations are the tibial nerve on the leg or the median nerve on the hand, but also the facial nerve . The method is also suitable for people with pacemakers .
• MEP - Motor evoked potentials serve the functional state of the in diagnostics, especially the determination cortikospinalen tract , i.e. the web from the used in the execution of voluntary movements primary motor cortex to, for example, to the motor neurons in the Spinalmark and further over the peripheral motor nerves to Muscle. MEP are used, among other things, to diagnose amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS). The method uses transcranial magnetic stimulation (TMS). As a rule, it should not be used on people with pacemakers.

See also

• Event-related potentials

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1. ^ Roche Lexicon Medicine, 5th edition (online version) Search termsomatosensorisch e. P.
2. ↑ Pschyrembel Clinical Dictionary Version 2002 (electronic version of the 258th edition), under the headingPotentiale, somatosensibel evozierte