OnsetDelay query

[River]

Hi,

I'm curious about my experiments OnsetDelay. As i understand it,
OnsetDelay is the difference between when the image should be shown
according to your design and when it is actually shown and that these
delays are often caused by the computers OS doing stuff you can't stop
as well as (possibly) the stimuli prep time. At present participants
see a bunch of stimuli in this structure:

1) fixation cross-500ms
2) 1280x1024 32 bit colour image- 2500ms, which then clears to a blank
white screen for 2000ms giving a total response window of 4500ms (the
reponse does not terminate the image or the blank screen).
Structurally, this is followed by a Wait object of 2000ms to
accomodate the blank white screen following the image.

So therefore the OnsetDelay in this instance describes the delay
between the end of the fixation duration and the start of the image
duration. Would this be correct?

My monitors sampling rate is 60Hz (as listed in the eprime data files)
and so my sample refresh duration is 16.67ms.

I noticed that the fixation cross or the blank screen occasionally
appeared for longer than specified so added a PreRelease of 200ms to
the fixation (but not the Wait as this caused a failure to record
responses in that period), as I thought the large bitmaps were taking
a while to load.

I also changed all my durations to accomodate the refresh duration:

1) 501ms
2)2505ms, 2004ms, giving a total of 4509ms

Despite this, my OnsetDelays for the colour image still average around
1000ms across all the stimuli. The DurationError is 0, I'm running no
other applications and I've tried increasing the PreRelease to 400ms
and 450ms with no obvious change. I also placed a PreRelease on the
Wait Object to determine if the delay was coming from the prep time
for the fixation and this didn't improve the OnsetDelay either so now
I'm stumped!

I noticed that on another version where smaller images were being used
(1024 x 768, and the display size was changed to accomodate) the Onset
delay was around 600ms with no PreRelease on the fixation, and 400ms
with the PreRelease. When the screen is physically smaller (i.e. the
experiment is being run on a small laptop instead of a large screen
desktop) it seems as if this delay is reduced again.

Is it possible that the image size is the problem and if so, is there
a way to fix this or is it just a symptom of my systems processing
power? Or is it the OS doing stuff and therefore it can't be fixed?

I've attempted to guage my screen refresh rate using the Refresh
detector attached to the SRBox but cannot get it to work (the sensor
seems to be working when its waved around and i'm testing in 'ambient
lighting conditions' as prescribed).

I'm using Eprime v1.

Any help would be greatly appreciated! Perhaps i'm just
misinterpreting the definition of OnsetDelay...or any of the other
things mentioned here!

River
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