bci2000.org BBS

Hello,

I am currently working with the P300 Speller and I was wondering which is the best monitor, at least, which monitors (CRT, LCD, LED) are not suitable for the experiments. I know it depends on the refresh rate, we have several CRT monitors at 60 Hz and LCD at 60 Hz as well. Do they work for the experiments? Could be a delay on the screen image for ISI >= 100ms?

I have been reading on the literature but I could get a good answer for my question. I have read in the forum as well that CRT are much better than LCD but you talk about CRT with >=100Hz. For the same refresh rate, which is the advisable monitor?

What do you think about the projectors?


Thanks.

I would advise against using a CRT at 60Hz. Its strong flickering will be quite distracting and annoying for your subject, especially in a dimly lit room as is best for P300 speller experiments.

The absolute time delay between the time when the computer presents a stimulus, and when it actually appears on the screen, is rather irrelevant. This is because a classifier training procedure will adapt to a time-delayed P300 as well as to a non-delayed one. Thus, the monitor's "reaction time" does not matter much as long as it's in a reasonable range, which is the case for current LCD monitors.

More problematic is a variable delay because it cannot be accounted for in data analysis. The only relevant contribution to delay variability comes from the fact that screen updates are limited by frame rate, and is proportional to frame duration. Each stimulus will be time-delayed by an unknown delay between 0 and a frame's duration, which amounts to 16,7ms for a refresh rate of 60Hz. Still, this is not too big a problem for a P300 speller because the width of the P300 is much larger than that (about 200ms). Usually, one will also downsample P300 responses to a sampling rate of 20Hz prior to classifier training, which means that one of the resulting classifier samples corresponds to three screen updates. So unless you are going to explore high-precision classification, including the _huge_ amount of training data required to train such a classifier, there is not much to worry about in terms of timing precision.

More important than timing is stimulus contrast (both in space and in time) -- adjust monitor brightness and ambient light for optimal contrast while taking care not to stress the subject by too strong flickering, which may be tiring.

A projector will be fine in terms of time delay and contrast. However, the speller matrix should not exceed the subject's visual field, so there is probably no advantage in using a projector over a monitor.

Regards,
Juergen

Thanks a lot Juergen.

I will follow your advise.

Regards