synchronize event markers with refresh rate

[David McFarlane]

Hank,

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strive to respond to all requests in 24-48 hours -- this is pretty 
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others.  (Flash -- As noted in the most recent PST e-mail Newsletter, 
we can all now access the E-Prime Knowledge Base without having to 
log in.  Woo hoo!)

That said, here is my take...

Hmm.  Could you post some details on how you measured this 
discrepancy?  Are you using the "OnsetSignal..." properties of the 
stimulus to send your signals to your device?  And are you sure that 
you have Onset Sync set to "vertical blank" for your stimulus?

I have to admit that the documentation in the online E-Basic Help 
remains vague on when exactly OnsetSignal is supposed to send its 
signal -- At the StartTime of the 
stimulus?  TargetOnsetTime?  OnsetTime?  ActionTime?  (See the EP 
manuals for explanation of these items, or my own discussion at 
http://groups.google.com/group/e-prime/browse_thread/thread/39e899d3457d4917 ).

But let's suppose that OnsetSignal acts at the OnsetTime.  Then, as I 
understand it, as long as Onset Sync is set to "vertical blank", 
then, once EP reaches the TargetOnsetTime for the stimulus, it 
further withholds the stimulus until the next vertical blank, then 
presents the stimulus (which should now appear almost simultaneously 
on the screen), sends the OnsetSignal, and considers this the actual 
OnsetTime.  Thus, there would be no way for the OnsetSignal to not be 
synchronized with the vertical blank, unless you do not have Onset 
Sync set to "vertical blank", or I am just wrong about how 
OnsetSignal works.  So again, we need to know  (1) whether the 
OnsetSignal really does always coincide with the stimulus OnsetTime, 
and  (2) whether you have Onset Sync set to "vertical blank".

Although I have measured many of these things in detail myself, I 
have not yet done so for this exact issue, so I will be interested to 
learn of your methodology and measurements.

-- David McFarlane, Professional Faultfinder


>I am trying to run a stop signal response task while recording
>electrophysiological activity on another setup. In order to get
>synchronization of behavior with the timing of my recordings, I have E-
>prime sent out event markers via the parallel port to an input of my
>recording device. It seems that this works well responses, but it
>seems that E-prime sends out the event markers for the display of
>stimuli before the stimulus actually appears on the screen: the
>eventmarker/timestamp is sent when the code issues the request for the
>stimulus to appear, rather than when the stimulus actually appears on
>the screen (i.e at the vertical blank/screen refresh after the
>stimulus.onset delay). Is there a way to get the event marker signal
>to synchronize with the actual appearance of the stimulus or is this
>something that can only be corrected offline by correcting the
>timestamps for stimulus appearance with their onset delay ?
>
>Thanks very much for your help.
>
>Hank

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