[visionegg] Re: time and OS es

> What has not been done by anyone testing these issues, and what really needs to be published, is a paper on the odds of actually >losing a critical millisecond. I suggest, based on casual observation that, A: it is very rare, B: it only adds a small amount of variance, and > C: even if it does occur it is catchable through the same method as used to verify the machine timing.


Based on non casual observations, I can tell you it is too frequent , at least on my hardware and for some experiments.
(My PC is a 2 Ghz pentium 4/Nvidia Gforce4 and with all the (unpatched) linux kernels I have tried over the years, I get several latencies above 30 msec every minute. To my dismay, VisionEgg demos almost always skip a few frames even in realtime mode. Maybe I should change my computer...).

Any Linux user can perform latency measurements with latencytest (available at http://www.gardena.net/benno/linux/audio/), and check his own configuration.

Yet, the results are obsviously dependent on hardware, version of kernel, and the load while testing (for example, the BRMIC paper you cite, if I remember correctly, ran the test on a machine which did not do any stimulus presentation. I was not impressed.).

The only sound approach is the one you advocate: log the timing of all critical events in the real experiment and check.

I have never implied that Linux sucks for psychology experiments and I agree that, in many experiments occasional latencies are not a problem.

But let me just give some real life cases where occasional latencies could be a problem:

- The stimulation PC must not miss the 5 millisec-long TTL signals sent from a MRI scanner to synchronize trials with brain scans.

- In ERP experiments, subliminal stimuli are displayed for 2/3 video frames, several hundreds of times in 10 minutes runs. It is very important that the subject does not see consciously any of the subliminal images because it could change his strategy. So one would like to have frame-by-frame accuracy for several minutes.

For such experiments, we use DOS programs that work.

But again, I would be sorry if people deduced from my previous posts that Linux is generally bad for psychology.
On the contrary, I am a big Linux fan and encourage people to use it, especially now that there are programs like the VisionEgg.

Good experimenters must know the limits of their tools, and how to deal with them.
I have encoutered more than one who tells you that nowadays machines are fast enough and who does no check the timing.

Cheers & Happy Easter!

Christophe Pallier

======================================
The Vision Egg mailing list
Archives: http://www.freelists.org/archives/visionegg
Website: http://www.visionegg.org/mailinglist.html

Other related posts: