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Visual Evoked Potential Test Query

Had various Evoked Potential tests Tuesday. Am curious especially about the Visual Evoked Potentials test. I experienced the test very differently with each eye: with the right eye, the moving black and white checkerboard pattern on the second test run soon began to grey out, losing almost all contrast so that it was all more or less alternating shades of grey; In addition it became very difficult to spot the yellow centre square which became feint and blurred. With my left eye, these things did not happen at all and the moving checkerboard remained at constant contrast and sharpness. I still dont know if the fading was ‘real’ or meant to happen or if it was just my perception, as the tester evaded answering my query mid test.

Also on the leg test, my little left toe would not start to twitch until the dial was turned right up and I couldnt feel the pulse unless it was somewhat higher than for the other leg. Anyone know how these tests work, particularly the visual one?

Sorry Bob, don’t know how they work but just wanted to mention my experiences with them. I found that the chequerboard pattern was the same for both eyes and I have had no eyesight problems with my MS. However, I had exactly the same experience as you with my left toe. The guy testing me had to turn the dial so far up to make my left toe twitch that the electric shocks through my left leg were really uncomfortable. Teresa xx

Hi Bob,

I have only a limited layperson’s understanding of the VEPs, but as I understand it, they measure the delay between the signal entering your eye (i.e. when it appears on the screen) and arriving at the visual cortex, which is at the back of the brain.

There is a “normal” range of times for this to take. By comparing your actual results with the norm, they can work out if yours are unexpectedly slow. This would be a strong indicator that there’s inflammation of the optic nerve, which is impeding the signals.

As far as I know, your subjective experience of seeing (whether it looks blurred or whatever) is not what is being measured in this test. They are only interested in electrical responses, and how long they take. If you are experiencing sight deficit, visual distortions or whatever, these may be because of damage to the optic nerve, in which case you would expect VEP results probably also to be abnormal.

But you can have ordinary sight problems that are related only to the functioning of the eye, and nothing to do with the optic nerve at all (typical reason people wear glasses).

If you are short-sighted, for example, in one or both eyes, I don’t think it will show on a VEP test, because that’s not what it’s looking for. It’s only looking at the speed of the signals.

Tina

Hi Bob,

I had my VEP on Tuesday as well and I have to say it was similar on both eyes. However, many months ago I got my eyes tested at the opticians and I had a very similiar result to you. I was to Identify when I could see a light flashing. While I could still make out the light everything in mmy left eye went to shades of grey and black. Strangely it doesn’t do it any more.

I agree with Tina though. It is my understanding that they are purely measuring the speed of the impulses along the optic nerve.

Graene

Thanks

Yes thats what i thought, and presumably why they dont ask you what you are seeing. However, I’m thinking I should have mentioned it if it is in fact unusual because it might have changed the electrical responses they were expecting to find.

VEPs work by measuring the time and the speed of the response to a known visual stimulus. The checkerboard pattern elicits responses from all over the visual cortex in a very particular and well established way. The same stimulus is used for each eye so the fact that you saw different things means that it is your vision differs between your eyes. In which case, your results will also differ between eyes. I don’t know anything about SSEPs, but I wouldn’t have thought it was normal to have to turn the stimulus up so high. Hth. Karen c

No, it won’t have any effect, as they don’t have any expectation of what to find (if they did, that in itself would be unscientific, and might skew the results).

They are only looking for differences between your responses, and the norm.

If your optic nerve is damaged, then indeed they should get an abnormal response. That’s why you don’t need to report what you’re seeing; the measurements should speak for themselves.

Thanks guys, really helpful to have someone to bounce thoughts off. I’m wondering if the stimulus is so strong that the optic nerve of my right eye gets fatigued by it and starts to fall down on the job; but then wouldnt the white signal degrade towards black so the whole thing tended to become dark rather than pale grey, given that black is an absence of light reception? Also, it is the sight of my left eye which has obviously deteriorated in recent months.

got a question to you all.

i read that someone wrote that evoked potentials tests are not being done anymore.

is it true??

I had also the evoked-potential tests, and its nice to shere theimpressions of from the test

If someone has more articles like this on the subject, would love to have

Thanks
http://www.medoc-web.com/applications/pain-research/ep-(evoked-potential)