In VEP 4Hz means 4 images per second or really 4Hz?

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mike@outofthisworld.com - 08 Oct 2007 14:03 GMT

Hi there!

when one reads that in VEP a checkboard is shown as positive/negative image
at a 4Hz rate, it is ambiguous. Which of the following two loops is right?

a) show positive, wait 250ms, show negative, wait 250ms, again..

b) show positive, wait 125ms, show negative, wait 125ms, again..

Really, "a" should be 2Hz, because a whole cycle lasts 500ms, but someway I
think that when they say "4Hz" they may mean "4 different images per second"
(which is 2Hz, not 4Hz, because one cycle means a whole positive+negative!).

Thanks for removing this ambiguity from my wannabe medical culture! (I am not
a pro of course, just passionate about the matter).

Kindest regards,
Mike

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otisbrown@pa.net - 09 Oct 2007 18:09 GMT

Dear Mike,

Don't know if this will help.

The original term for Hertz was "cycles per second".

Thus you have to complete one cycle, and repeat.

For instance, if the period of "show positive" lasted 250 mseconds,
and then wait 250 mseconds, then show-negaitve for 250 mseconds,
the wait 250 msec, then REPEAT, the complete cycle would
take 1 second, and that would be 1 hertz.

Best,

Otis

On Oct 8, 9:03 am, m...@outofthisworld.com wrote:

> Hi there!
>
[quoted text clipped - 14 lines]
> Kindest regards,
> Mike

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Mike Tyner - 09 Oct 2007 18:58 GMT

> Don't know if this will help.

Then it won't.

> the wait 250 msec, then REPEAT, the complete cycle would
> take 1 second, and that would be 1 hertz.

Yes, it would. But the OP already knows the definition of Hertz. You'd need
some actual experience in a real laboratory to answer the question.

I have some experience, but it was 25-30 years ago.

At that time, I think the convention was to call each change a cycle, so 4
changes per second were called "4 Hz."

It's a misnomer, yes, but the printed output does not distinguish between
black-on-white vs white-on-black. Each change produces the same signal along
the visual pathway, so done your way, the printed tracing would have to show
two complete wave complexes. They don't.

Another clue is that the normal window for a VEP tracing is about 250 msec.
The wave isn't complete until about 200 msec, so testing at 125 msec each
stimulus would tread on the tail of the previous one.

But we're so glad you had the opportunity to cloud things up.

-MT

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