Millisecond Forums

Hi,

I understand from Inquisit documentation that specifying 'stimulusonset' in the columns attribute of the data element will write the "actual" stimulus presentation time for each trial, in milliseconds.  I have used 'stimulusonset' and I am familiar with how its output appears, but I have two questions about it:

1.  How does Inquisit determine the "actual" presentation time?  I imagine that it somehow extracts this information from the DirectX controller, but I really have no idea.  I do trust the developers' claim that 'stimulusonset' tells me the actual onset time, but I would feel much better if I knew how this was computed and how reliable it is so that I could justify it to colleagues.

2.  When I specify the beginning of the allowed response window to occur before all stimuli are presented (using the responsetime attribute), Inquisit will not output stimulusonset values for stimuli that are presented after the beginning of the response window.  For example:

<trial straightGS>
/ validresponse = ("a", "l")
/ correctresponse = ("l")
/ stimulustimes = [0 = fixation; 1000 = noreplace(ASM); 1060 = mask; 1100 = blank]
/ responsetime = 1000
/ trialdata = [ASM]
</trial>

This design results in all 0's being recorded when I specify stimulusonset four times in the columns attribute of the data element.  When I remove the responsetime attribute from the trial element, but do not change anything else, then stimulusonset is recoreded correctly for each of the four items.  In another example:

<trial straightGS>

/ validresponse = ("a", "l")

/ correctresponse = ("l")

/ stimulustimes = [0 = fixation; 1000 = noreplace(ASM); 1060 = mask; 1100 = blank]

/ responsetime = 1060

/ trialdata = [ASM]

</trial>

This results in correct recording of the stimulusonset of 'fixation' and 'ASM', but 0's are recorded for 'mask' and 'blank' when I specify stimulusonset four
times in the columns attribute of the data element.  As in the first example, when I remove the
responsetime attribute from the trial element, but do not change
anything else, then stimulusonset is recoreded correctly for each of
the four items.

The problem is that I would like to allow responses to be entered before the conclusion of stimulus presentation and I would like to record stimulusonset.  Is there some work-around here?

Many thanks for everyone's help.