[visionegg] Re: visionegg for fMRI experiments

>
>
>  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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