What are Evoked Potentials

Evoked Potentials (EPs), or Event-Related Potentials (ERPs), are changes in the electrical activity of the nervous system (‘potentials’) that occur at a particular time before, during or after (‘related’) a change in the external physical world and/or the internal mental/cognitive state of the subject (‘event’). From this definition, it follows that ERPs are distinguished according to whether the potentials are evoked by external events (exogenous EPs) or by an internal mental process (endogenous ERPs).

EPs within the first 50 msec after a sensory stimulus are typically exogenous in nature, reflecting the physical characteristics of the stimulus (type, intensity, rate of presentation) and the sensory system’s functional state and integrity. Hence their clinical utility in diagnosis and monitoring of sensory pathways in particular as well as the nervous system in general.

EPs are classified according to the sensory modality used to evoke them (auditory, visual, somatosensory) and their site of generation along the sensory pathway and timing following stimulus onset: subcortical short latency, primary cortical middle-latency, cortical long latency.

The exact latency ranges vary between sensory modalities according to their physiological and anatomical characteristics. Thus, for example, short latency auditory evoked potentials include cochlear, auditory nerve and brainstem evoked potentials within the first 10 msec after onset of a brief stimulus such as a click. In comparison, short latency visual evoked potentials include retinal and optic nerve and tract potentials within the first 50 msec following an abrupt visual stimulus such as a flash. The auditory middle latency evoked potentials reflect thalamo-cortical activity in the latency range of 10-60 msec.
 

EPs with latencies exceeding 200 msec are most often endogenous in nature. Thus, the very same physical stimulus, or its absence, may or may not evoke a potential change, irrespective of the physical properties of the stimulus. The potential depends on the stimulus context to the task performed (e.g., target or non-target in target detection) or to the subject (subject’s name or other neutral word). The potentials within the latency range of 50 to 200 msec are mixed in nature, reflecting both exogenous and endogenous activity. For example, auditory long latency evoked potentials reflect cortical activity including auditory cortex and associative areas involved in endogenous activity.