[visionegg] Re: visionegg for fMRI experiments
- From: pallier <pallier@xxxxxxxxxxxxx>
- To: visionegg@xxxxxxxxxxxxx
- Date: Thu, 28 Nov 2002 09:42:39 +0100
>
>
> I foresee a potential problem if, for example, the MRI pulse triggers
>your visual stimulus application, which may get the pulse quite quickly
>(within 1-2 msec, I'd guess), but then it's some unknown time (maximum
>one vertical retrace interval) between then and when the video card
>actually draws a new frame. Perhaps this isn't actually an issue: is
>the inter-frame interval of sufficient precision for the temporal
>alignment of the stimulus and the MRI device?
>
Yes: given the sluggishness of the hemodynynamic response measured by
fMI, an error of even 100 ms is probably of little consequence if one
only care for the response to a given stimulus (moreover the sampling
rate of fMRI typically is 1~2 seconds).
(However, frames staying displayed for too long can ruin, for example,
subliminal perception experiments...)
I imagine that with electro- or magneto-encephalography, an imprecision
of 16 msec may pose problems (some components are about 10 msec wide).
>>
>>
>
>I haven't kept up with linux much as of late because I found that the
>Vision Egg was never skipping frames at 200 Hz under Windows 2000 Pro
>(as long as the stimuli weren't too complex) and I have stuck with that
>for now. If I remember correctly, the latest kernels are much better
>with respect to latency and don't need patches (and the 2.2.16 kernel
>is quite old). I would be interested to hear reports of frame skipping
>(or not) under the recent linux kernels.
>
When I tried the demo provided with visionegg on a Pentium800 with a
GeForce 2mx,
and linux kernel 2.4.18 - redhat 7.3 updated, there were some
latencies. I will try to give you more detail reports when I get more
recent hardware.
Apparently, there are things that can be done to standard 2.4 kernels to
improve the responsiveness (change the HZ value, apply the low-latency
patches), but I have not tried them yet.
Christophe Pallier
www.pallier.org
www.unicog.org
note: I am not (yet) using visionegg for my experiments, just evaluating it.
My fMRI experiments are still run by a DOS program which I wrote 10
years ago. The main reason is that I have complete control of the
timing, but I am stuck with very old hardware. As I do everything else
under Linux, I started to try and program my experiments in C with the
SDL library, and was thinking of using pygame when I discovered
visionegg. It really seems a very neat program, and I congratulate you
for making it a free project.
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