Evoke Potentials

two graphs with blue lines showing the evoke potential going up then down across time with the abnormal time highlighted in red

You may have heard of an evoke potentials test in relation to diagnosing Multiple Sclerosis (MS). The evoke potentials test for brain injury is similar. The eVox system “acquires biomarkers based on electroencephalography (EEG) and Event-related Potentials (ERP).” It measures the rate of speed your neurons communicate with each other in reaction to visual, auditory, and cognitive stimuli. It differentiates between internal communication — that is, within the brain from one neuron to another — and external — that is, from inside the brain to the external world. An example would be translating thought to speech.

The idea is this. When you receive a stimulus, your nerves and neurons respond before you are conscious of seeing the stimulus. That response is intrinsic and cannot be altered by your conscious control (though brain injury can change your intrinsic response); whereas when you see something, you can choose whether or not to react to it. So an evoked potential is a way to evoke or stimulate that subconscious, intrinsic physiological nerve response, which the computer records. It also evaluates cardiac function. And like with qEEG, some tests are done with eyes open, others with eyes closed to differentiate between the two states.

The Electrodes

To measure your brainwaves, the tester pulls a stretchy cap with nineteen electrodes, like grommets, over your head. The nineteen electrodes correspond to the 10-20 system and is the same cap as used for qEEG. There are different sized caps for different sized heads so that the electrodes end up over specific pre-mapped points on the head.

The tester pastes and clips the ear electrodes on. Like the third prong in a plug, your ears are the grounding wires, so to speak. Then with a large plastic syringe with a blunt tip, the tester squirts electrogel into each of the nineteen points and uses the tip to create a gel connection between the scalp and the electrodes. (The electrogel stays liquid till you take the electrodes off, wipe all the gel off, and then it turns into a dry film. Ack!). The wires from the electrodes are gathered together into a cable, which plugs into hardware that’s attached to the computer.

The Software

Once fitted, the tester tests the impedance through the computer software. Impedance here is a measure of how well the electrodes are reading the brainwaves, that is, how well the electrical waves are getting through to the electrodes.

Like with qEEG, the tester brings up the impedance screen on the monitor. A line drawing represents the head with two ears and the nose from the top view. Twenty-one circles — two on the ears, the rest on the head — represent the electrodes and grounding clips. As the tester connects the scalp to each electrode via the electrogel, each circle changes from green to black as connection is achieved.

This procedure takes a few minutes, and during that time, you may be given a paper questionnaire to fill out about memory and concentration. Once the tester achieves good impedance, the test begins.

The Test

For part one, the BrainCheck Screener by Evoke Neuroscience, you’re given a mouse and the computer shows you the questions on your monitor. Each question has multiple answers. As you click each answer, the computer moves through the questions. Unlike with qEEG where the tester guides you, in this one the computer guides you.

For part two, you’ll be given a clicker that looks like a small black handheld microphone with a lit red button on top. You will be asked to practice clicking that button. Next, they will test the volume of the auditory stimulus – a burst of scratchy white noise. They want a startle reflex without a physical response. For someone with sensitive hearing, normal volume will make you almost hit the ceiling. That’s why it’s important for the tester to take the time to ensure the volume is low (or loud) enough to startle but not make you physically jump. Lastly, the tester will show you the visual startle stimulus, a black-and-white checkerboard pattern that fills the screen.

The tester leaves the room, and the computer guides you through the test. The ten-minute test comprises three images flashed very briefly in seemingly random order against a white background on the monitor located directly in front of you. A burst of sound through earplugs intersperses the images. You have to practice distinguishing between the first two images, which are a small blue circle and a large blue circle because they are not that different in size. And you have to learn which stimulus you’re to click the clicker in reaction to. The computer only proceeds to the test once you’ve achieved a high enough percentage of correct responses. This system is standard for all computer-guided tests, where the computer takes you through a practice session to ensure you know what you’re supposed to do before it tests you.

At the end of the ten minutes, you may be exhausted, and you’ll be divested of all electrodes. Prepare to look like you have Frankenstein hair. Bring a cap or hat to cover up until you can shower it off. For a detailed description of the combined qEEG and evoke potentials tests, see the blog post on my website.

The Results

The report covers several pages, showing brainwaves, brain maps, cardiac function, and problem areas.

P300 is a measure of brain speed. The P300 used to measure attention through P3a and working memory through P3b (see top image).

“The P3a, or novelty P3,[7] has a positive-going amplitude that displays maximum amplitude over frontal/central electrode sites and has a peak latency in the range of 250–280 ms. The P3a has been associated with brain activity related to the engagement of attention (especially the orienting, involuntary shifts to changes in the environment), and the processing of novelty.[8]

The P3b has a positive-going amplitude (usually relative to a reference behind the ear or the average of two such references) that peaks at around 300 ms, and the peak will vary in latency from 250 to 500 ms or more, depending upon the task and the individual subject response.[3] Amplitudes are typically highest on the scalp over parietal brain areas.[3] The P3b has been a prominent tool used to study cognitive processes, especially psychology research on information processing. Generally speaking, improbable events will elicit a P3b, and the less probable the event, the larger the P3b amplitude.[9] This was shown to be true both for the overall probability and for the local probability.[2] However, in order to elicit a P3b, the improbable event must be related to the task at hand in some way (for example, the improbable event could be an infrequent target letter in a stream of letters, to which a subject might respond with a button press). The P3b can also be used to measure how demanding a task is on cognitive workload.[9]

Since the initial discovery of the P300, research has shown that the P300 has two subcomponents. The subcomponents are the novelty P3, or P3a, and the classic P300, which has since been renamed P3b.[10]


After a stimulus, the computer times how long it takes the brain to process that information. 300 ms is about how long data gets from inside to outside, meaning processing speed. The evoke potentials results report will highlight speeds that are abnormally slow. That’s an objective measure of how much brain injury impacted processing speed. Standard medical care of brain injury relies on observation or oblique methods such as a tester timing how long it takes to do a vocabulary test.

But observational measures are prone to human error, and even a computer timing a vocabulary test does not show what’s actually happening in the brain.

Brain injury is located in the brain; therefore, good diagnostics measure the brain directly through non-invasive methods. That’s why an evoke potentials test along with a qEEG is superior. Evoke potentials and qEEG are direct measures, objective, and not subject to human error nor observational bias.

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