Evoked Potential

Evoked Potential

An evoked potential (EP), also known as an evoked response, refers to an electrical potential recorded from the nervous system of a human or animal following the presentation of a stimulus. This distinguishes it from spontaneous potentials detected by electroencephalography (EEG) or electromyography (EMG).

EP amplitudes are typically low, ranging from less than a microvolt to several microvolts. This is in contrast to EEG, which measures tens of microvolts, EMG, which measures millivolts, and ECG, which measures close to a volt. To distinguish these low-amplitude potentials from the background of ongoing EEG, ECG, EMG, and other biological signals, as well as ambient noise, signal averaging is often necessary. The signal is time-locked to the stimulus, and most of the noise occurs randomly, allowing it to be averaged out with repeated responses.

Signals can be recorded from various parts of the nervous system, including the cerebral cortex, brainstem, spinal cord, and peripheral nerves. Typically, the term "evoked potential" is used for responses involving the central nervous system structures, whether recorded from or stimulated by them. Therefore, evoked compound motor action potentials (CMAP) or sensory nerve action potentials (SNAP) used in nerve conduction studies (NCS) are generally not considered evoked potentials, despite meeting the above definition.

Sensory evoked potentials (SEP) are recorded from the central nervous system following stimulation of sense organs. For example, visual evoked potentials are elicited by flashing lights or changing patterns, while auditory evoked potentials result from clicks or tone stimuli. These tests have been widely used in clinical diagnostic medicine since the 1970s, as well as in intraoperative neurophysiology monitoring (IONM), also known as surgical neurophysiology.

There are three main types of evoked potentials in clinical use: auditory evoked potentials, typically recorded from the scalp but originating at the brainstem level; visual evoked potentials; and somatosensory evoked potentials, which result from electrical stimulation of peripheral nerves.

Regan utilized an analogue Fourier series analyzer to record harmonics of the evoked potential to flickering light. Instead of integrating the sine and cosine products, he fed them to a two-pen recorder via low-pass filters. This enabled him to demonstrate that the brain reached a steady-state regime where the amplitude and phase of the response's harmonics remained relatively constant over time. By analogy with the steady-state response of a resonant circuit, he defined an idealized steady-state evoked potential (SSEP) as a form of response to repetitive sensory stimulation, where the constituent frequency components remained constant in amplitude and phase over time.

While it is sometimes suggested that SSEPs are only elicited by stimuli of high repetition frequency, this is not entirely accurate. In principle, even stimuli with low repetition frequency can elicit SSEPs. Repetitive sensory stimulation generates a steady-state magnetic brain response that can be analyzed similarly to SSEPs.