Evoked potential

Evoked potentials ( " summon " latin evocare, " cause ", " potential ", see there) are potential differences in the electroencephalogram (EEG ), which are triggered by a stimulation of a sense organ or peripheral nerve. In a broader sense, all specifically induced electrical phenomena in the EEG can be understood as evoked potentials.

Measurement

Each sensory stimulus triggers in the sensory areas of the cerebral cortex of electrical potential changes. These evoked potentials are, however, increases with the conventional technique, only with much smaller amplitudes than the spontaneous running EEG signal (1-15 uV instead of 50-100 microvolts ). In order to measure and display the evoked activity can, several realizations of an evoked potential are averaged. The EEG is doing. Than stochastic noise signal, which is independent of the stimulus and its mean value of zero is considered, while the interest evoked potential is temporally coupled to the stimulus It shows after each stimulus the same course. Due to the repeated presentation of a stimulus and the subsequent averaging of the EEG segment seeks the charming independent activity towards zero, while the charming -related evoked potential is summed. The number of the necessary implementations, in practice depends on the signal -to-noise ratio, and is different depending on the modality and physical characteristics. When triggered by light flashes potential satisfy about 50 stimuli, while about 1000 to 2000 stimuli must be presented for measurement of early acoustic brainstem potentials.

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

Examples

  • VEP - Visual evoked potentials allow an assessment of the optic nerve and visual pathways, especially in the course of diagnosis of optic neuritis in multiple sclerosis. On examination, the patient viewed a checkerboard pattern that turns its colors in seconds intervals ( black becomes white and vice versa). It can also be stimulated both eyes at the same time or individually. This triggered electrical potentials in the visual cortex ( occipital cortex) are recorded.
  • AEP - Auditory evoked potentials allow an assessment of the auditory pathway from the cochlea via the auditory nerve to the brain stem ( Early AEP ( ABR ) ), continue along the midbrain (middle AEP ( MAEP ) ) to the auditory cortex ( Late AEP ( SAEP or CERA ( cortical evoked response audiometry ))). The FAEPs are essential for the diagnosis of acoustic neuromas. There are (amplitude) and distance judged each other a total of five waves with respect to height.
  • Sep - Somatic evoked potentials, also SSEP - somatosensory evoked potentials (as anglicism also imprecise " Somatosensory evoked potentials " ) allow an assessment of the central somatosensory pathway and peripheral sensory nerves. About a stimulation electrode in the vicinity of a sensitive nerve repeated electrical stimuli to be set. The measuring electrodes are attached to the scalp. Other measuring electrodes can be placed along the peripheral nerves and the stimulus -processing centers of the spinal cord and brain and measure the electrical response and the duration of the signal. Can then be determined by comparing the running times and distances in which section the possible delay of the signal occurred (bunk diagnostics). Typical stimulus locations are the posterior tibial nerve in the leg or the median nerve in the hand, but also the facial nerve. The method is also suitable for wearers of pacemakers.
  • MEP - Motor evoked potentials are used in the diagnosis, especially in determining the functional state of the cortikospinalen tract, ie, the path used in the execution of voluntary movements of the primary motor cortex to, for example, to the motor neurons in the Spinalmark and continue on the peripheral motor nerves to muscle. MIPs are used, among other things, for the diagnosis of amyotrophic lateral sclerosis ( ALS) and multiple sclerosis (MS). The method used transcranial magnetic stimulation ( TMS). They should not be used with cardiac pacemakers usually.
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