fast priming procedures

[Yoav Bar Anan]

Hi

I'm trying to show a stimulus for a very short period of time, but it doesn't work too good.
I have the following script (both dotCnvs and stimCnvs are already drawn):
cnvs.copy dotCnvs, scrRect , scrRect
sleep(200)
'Show Stim
Display.waitforverticalBlank
cnvs.copy stimCnvs, scrRect , scrRect
sleep(1)
'Clear
cnvs.clear

But I can still see the stimCnvs. Furthermore, sometimes I see it and sometimes not. I guess my problem is related to refresh-rate issues, but I don't really know much about screens. 
So could someone explain to me what I'm doing wrong, and more important - how can I fix it?

Thanks

Yoav

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