Written by Travis M. Moore
Last edited 18-Jun-2020
There are several companies that manufacture clinical electrophysiology equipment, but they all share the same basic components. Any system will need transducers to present stimuli, electrodes to record voltage from the scalp, a place to plug in electrode wires, a preamplifier, an amplifier and a computer. Typical evoked potential equipment is shown in Figure 1. We have already covered the basics of electrodes in the Recording Evoked Potentials, so that information is not repeated here.
Evoked potential (EP) systems use insert earphones to deliver stimuli, as opposed to supra-aural headphones; can you guess why? Think about where the transducer really is with a pair of inserts: it's in the little box that you clip to the patient's collar. All the electronics are in that box, and the rest of the setup, the tubing, adaptor, and foam eartip, are just there to carry the sound from the transducer box to the ear canal. That means any electrical interference from the stimulus presentation will be far removed from the sensitive electrodes place around the ear. Supra-aural headphones contain the electronics in the casing behind the rubber earcups, putting electrical noise too close to the recording electrodes.
With electrodes placed and leads attached, the other end of those wires need to go somewhere! That somewhere is a plastic box filled with terminals for the leads to plug into. By plugging in different numbers of leads, you can change your electrode montage to include more or fewer electrodes. From this terminal box, all the wires are sent to the preamplifier bundled together so cords aren't flying everywhere.
When the bundled cable leaves the terminal box, after a short distance it encounters the preamplifier. The purpose of this device is to amplify the voltage from the electrodes enough to overcome any interference that might occur on the journey to the actual amplifier. This might seem like redundant system, but remember that the cable from the electrodes needs to be long enough to reach your patient comfortably from the computer. The the main amplifier is big enough that it needs to sit near the computer. That means we are forced to use a rather long cable by necessity, and long cables carrying small voltages are susceptible to electrical interference - even from the electricity in the room running through the light fixtures, computers, monitors, or anything else plugged in! So it is necessary to have a small, portable amplifier, a preamplifier, to make sure the tiny scalp voltage survives the journey to the amplifier.
This is where the gain of the voltage is increased thousands of times (100,000 times amplification is common for the auditory brainstem response). The amplifier also houses the analog to digital converter (ADC). This device is discussed in detail in the Instrumentation module (not yet available online). Suffice it so say the ADC turns the raw voltage picked up from the electrodes into computer-friendly code. Even though computers run on electricity and can control electrical devices, they cannot use raw electrical inputs.
It might be obvious, but it bears saying outright that the computer is responsible for digital filtering, averaging, artifact rejection, controlling the gain of the amplifier, displaying the waveforms and allowing the clinician to label peaks and prepare reports. Basically, just about everything that doesn't have to do with recording is handled by the computer.