Stim timing adjustment in a continous loop

[Becky Prince]

Dear Forum,

I have to create a task where participants hear two tones in a loop
and adjust the spacing between the tones (that is, they move the
second tone forward or backward in time) until the participant
perceives the tones to be rhythmic or isochronous.  We'd like to use
two keyboard keys as "+" and "-", which when pressed would move the
second tone forward or backward (say, 50ms) within the continous loop
of the two tones.

To illustrate, let's say the tones are "A" and "B", and the dashes
represent some unit of time:
A---B-----------A---B-----------A---B-----------
The duration of the loop (from onset of A to onset of A) remains the
same, but the subject uses two keyboard keys to adjust B until the
tones sound isochronous:
A-------B-------A-------B-------A-------B-------

I'm not new to E-Prime, but in the past I've always created standard
standard probe-response type tasks.  I have not tried to create a
program like this one before, where the participant has dynamic
control over the stimuli.

So my questions are the following:

1) Is it even possible to create this task in E-Prime?

2) Have you done something at all similar, and if so, how did you do
it?

3) If you haven't done anything like this, do you have any ideas about
how it could work?

My only thought is that perhaps I could set this up so that the two
tones are not actually 'continous' but presented in pairs over many
trials, and the temporal properties of trial n would be based on the
response to trial n-1.  However, I think this would create a delay
between the end of one 'loop' and the beginning of the next, which
would certainly affect the subject's perception of a continous
rhythm...

Any thoughts or suggestions would be greatly apprectiated.
Thanks!
Becky

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