EEG Pulse Duration with Onsetsignal

[Nate]

Hi-

I am trying to clear something up. Right now I am sending triggers to
Biosemi via parallel port using the onsetsignal and offsetsignal
commands. Please do not tell me to read the documentation on such and
such! I have done so...

So here is the situation:
In one instance, trigger 5 is sent at the onset of the presentation of
a text slide (which is synced to the vertical refresh of the monitor,
with a refresh rate of 85hz). The slide is displayed for 1500ms.
On the offset of the slide (after 1500ms) the offsetsignal sends
trigger 0 to clear the port.

My question then is how long is the duration of the pulse sent to
Biosemi? Is it 1500 ms, or is it the pulse duration equivalent to the
refresh rate of the monitor (11.76ms)? Or something else...?

[David McFarlane]

Fair question. But I will still give my stock reminder: 1) I do not
work for PST. 2) PST's trained staff takes any and all questions at
http://support.pstnet.com/e%2Dprime/support/login.asp , and they
strive to respond to all requests in 24-48 hours (although current
estimates are more like 10 days) -- this is pretty much their
substitute for proper documentation, so make full use of it. 3) If
you do get an answer from PST Web Support, please extend the courtesy
of posting their reply back here for the sake of others.

On top of that, I will tell you to not take my word for
this. Ideally, you will hook up a voltmeter or oscilloscope to your
paralell port and measure the actual duration of your "pulse". That
would settle the question.

But now my answer. E-Prime will set the OnsetSignalPort to the value
in OnsetSignalData at the onset time of your Slide object. That
value will remain on the port until it is replaced with something
else. In particular, EP will set the OffsetSignalPort to the value
in OffsetSignalData at the *offset* time of your Slide object; as
long as OffsetSignalPort = OnsetSignalPort (and OffsetSignalData <>
OnsetSignalData), this means that the "pulse" will last from the
onset time of your Slide object until the offset time of the
Slide. Now note that the target offset time of your Slide will be
onset time - PreRelease. So, putting this all together, the pulse
will last for more or less Duration - PreRelease ms, depending on
onset and offset delays; if PreRelease = 0, then the pulse should
last about Duration ms.

So let's illustrate this with some numbers. You have
Slide.OffsetSignalPort = Slide.OnsetSignalPort, Slide.OnsetSignalData
= 5, Slide.OffsetSignalData = 0, Slide.Duration = 1500, and
Slide.PreRelease = 0. In this case, the value 5 should go out to the
port and stay on for more or less 1500 - 0 = 1500 ms (depending on
onset & offset delays), and then go to 0. If you set
Slide.PreRelease to, say, 1400, then the pulse will last more or less
1500 - 1400 = 100 ms. (We do exactly this sort of thing to
manipulate pulse durations in our programs, but before you mess with
PreRelease please note carefully the caveats in Chapter 3 of the User's Guide.)

I hope this helps.

-- David McFarlane, Professional Faultfinder

[Nate]

This is indeed VERY helpful. If I was to measure the duration of the
pulse (the trigger sent is actually '9') using a voltmeter, I would
then connect the negative wire to the incoming PCI-DIO cable ground
hole (which one is this?) and the positive lead would then connect
to...hmm the chart below does not show 9....

This shows the pin mapping for this, but there is an error on the page
and I also can't tell which chart is actually for eprime.
http://www.biosemi.com/faq/presentation.htm




Onto the other method/explanation: in this example then, I do not have
a prerelease for this particular slide, so the pulse duration is
probably1500ms. Other triggers/slides are of different durations and
are hopefully not relevant....

However, now comes the potential problem of analysis in Brain Vision
analyzer, though I am not sure this is the correct forum for this (but
then again, I have not found a BV forum anywhere). Anyways, if anyone
is familiar with this, I would really appreciate some input, as I am
kind of lost:

I have been told that the triggers/markers (or only the one marker of
interest?) need to be moved based on the length of the pulse. So
in this example the 5 (or 9 in my instance) marker would need to be
moved with an offset of 1500 ms. Anyone know if this is true?

[David McFarlane]

Wow, BioSemi wants $100 for a simple 25-pin D-sub to 37-pin D-sub
cable? I could build one from parts for a lot less than that, and
then I would know exactly what it does. But anyway... I would just
work on the 25-pin connector on the E-Prime end (and in fact I would
add a straight-through 25-pin cable just to make my work
easier). That pinout remains the same for Presentation and EP. Then
yes, your negative lead goes to ground (pin 25; you can Google to get
several pinout diagrams, pay attention to gender). Then you have to
know how to translate decimal to binary, and be aware that we count
bits from low to high. The value 9 comes to binary 00001001, which
means that it would raise a signal on pins 2 and 5. So put the
positive lead of the meter on either of these (you will of course try
both those pins plus a few others just to complete the exercise and
to see if I really know what I am talking about). If you then still
wanted to see how the signal makes it through the BioSemi cable, then
it looks like you put leads from pin 37 to pins 1 or 4 at the end of
the BioSemi cable.

Sorry, I can't help at all with the Brain Vision issues.

-- David McFarlane, Profesional Faultfinder

[Nate]

Yes it's ridiculous. It's even more ridiculous that our lab paid for
one...but we don't really have a dedicated tech staff. (Have you seen
how much the electrode caps cost?! I would be willing to quit my job
and sit at home and make them myself for that price.)

Thanks for the help on the pin placement- I'll check this out. If
anything it is a good lesson in parallel port communication.

[Nate]

So to close this- the pulse duration was the duration of the stimulus
in this instance. This was actually not a problem, as Brain Vision
only looks at the onset of the pulse, not the duration of the pulse.
So, all is well!