Evoked Potentials
An evoked potential (or "evoked response") is an electrical potential recorded from the nervous system. Signals can be recorded from 
Cerebral Cortex, Brain Stem, spinal cord and peripheral nerves. Usually the term "evoked potential" is reserved for responses involving either recording from, or stimulation of, Central Nervous System structures.
Evoked Potentials Procedure
Electrodes need to be attached to various points of on your scalp. Your head is measured using a standardized EEG measurement technique to determine the right spots (each spot corresponding to a type of EP that will be measured - e.g. the two locations on the back of the skull for the visual cortex, etc.), which are marked with a writing implement akin to a very thick pencil. Each of these spots is rubbed with an oil-removing scrub to get rid of the skin oil, then an electrode dipped in a liberal quantity of conductive gel (approximately the consistency of soft butter) is applied and pressed to each spot, and affixed with a strip of adhesive tape.
In order to improve the efficacy of this technique, you are advised to relax and not move, so as to reduce the noisiness of the signal and make the averaging technique more effective with fewer iterations of the stimulus.
Why is this test used?
The evoked potential test can be used to determine areas controlled by an individual nerve. They may serve as a guide during the surgical removal of tumours growing around important nerves.
Sources:
ABTA's Brain Tumour Primer
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