Randomising the Fixation duration

Thu Jun 9 08:33:47 UTC 2011

Hi all,

I ment to react back for a while but kept forgetting. I stand
corrected on the use of randomizing fixation durations, I forgot the
ERP rationale that Michiel explained.

Still for the use that Krupa described I'd say that a 3500 ms cross
defies it's own purpose and in such a design I would give a larger
range of durations to a blank intertrialinterval display and a smaller
durations range to the cross.

David and Miciel, also thanks for pointing out the drawbacks of using
the random(value,value) inline.

I like it when such discussions arise.

Best,

AW

On Jun 1, 12:33 pm, Krupa Sheth <krupa.d.sh... at gmail.com> wrote:
> Hi!
>
> We are also using this experiment to assess for inhibition in children
> with neurodevelopmental disorders (we used the anti-saccade task).
> Therefore by randomising the fixation duration it allows a) to ensure
> that their gaze is constantly at the centre of the screen ensuring
> that  study participants are motivated and attentive throughout the
> testing session and more importantly b) prevents participants from
> utilizing any anticipatory strategies while executing the task.
>
> Hope this answers your question.
>
> --
> Best Wishes,
>
> Krupa Sheth
> Research Associate
> Cerebra Centre of Neurodevelopmental Disorders
> University of Birmingham
> Edgbaston
> B15 2TT
>
> On May 31, 5:46 pm, liwenna <liwe... at gmail.com> wrote:
>
>
>
>
>
>
>
> > Glad you got it to work, but if you don't mind me asking: why would
> > you want your fixation cross to have a random duration?
>
> > I've seen people ask for this before, but as far as I can see it makes
> > no sense at all.
>
> > The fixation cross serves the purpose of fixating the gaze and warning
> > that the new trial is too start. It exerts it's attention capturing
> > 'quality' only at it's onset and should therefore appear always about
> > 500 ms prior to the start of the trial, instead of having a random
> > duration. Especially with a fixation cross as long as 3500 changes are
> > pretty high that your participant's gaze has wandered elsewhere after
> > 3500 ms when the trial starts, in which case the fixation cross
> > achieved the exact opposite from what it is intended to do. The cross
> > intends to indicate the start of the trial to the pp and therefore it
> > should be predictable in it's duration.
>
> > You do want to have a random duration inter trial interval in order to
> > avoid automation of the timing of the motor response over trials, but
> > rather use a separate blank slide (inter trial interval slide) with
> > it's duration set to random in between the response slide and the next
> > fixation cross (i,e, either at the total end or right at the start of
> > the trial procedure), instead of randomizing the duration of the
> > cross.
>
> > best,
>
> > liw
>
> > On May 31, 6:18 pm, Michiel Spape <Michiel.Sp... at nottingham.ac.uk>
> > wrote:
>
> > > Hiya,
> > > There's indeed nothing wrong with the code, but yes, it's more elegant to do David's suggestion, because you're at least sure it's also logged. There is also good reason to do the nested list with Duration, because you're never quite sure whether "random" isn't too random for your purposes. Suppose, for instance, that you constantly show each and every subject really short fixations? That will make an utter mess of your jittering purposes! It's unlikely, but still possible nonetheless - the joy of randomising with replacement. So, indeed, if you have a nice nested list with every random value (you can safely skip a few, because of screen refresh rates), you can be safe in the knowledge that at least you're sure no value is used twice (depending on the length of the list and the number of trials, of course).
>
> > > Best,
> > > Mich
>
> > > Michiel Spapé
> > > Research Fellow
> > > Perception & Action group
> > > University of Nottingham
> > > School of Psychologywww.cognitology.eu
>
> > > Michiel Spapé
> > > Research Fellow
> > > Perception & Action group
> > > University of Nottingham
> > > School of Psychologywww.cognitology.eu
>
> > > -----Original Message-----
> > > From: e-prime at googlegroups.com [mailto:e-prime at googlegroups.com] On Behalf Of David McFarlane
> > > Sent: 31 May 2011 15:57
> > > To: e-prime at googlegroups.com
> > > Subject: RE: Randomising the Fixation duration
>
> > > Stock reminder:  1) I do not work for PST.  2)
> > > PST's trained staff takes any and all questions
> > > athttp://support.pstnet.com/e%2Dprime/support/login.asp
> > > , and they strive to respond to all requests in
> > > 24-48 hours -- this is pretty much their
> > > substitute for proper documentation, so make full
> > > use of it.  3) If you do get an answer from PST
> > > Web Support, please extend the courtesy of
> > > posting their reply back here for the sake of others.
>
> > > That said, here is my take...
>
> > > First, I agree that you would do well to work
> > > through Mich's E-Primer, as well as the Getting
> > > Started and User's Guides that came with E-Prime.
>
> > > Now, the inline code method allows you to easily
> > > randomize down to the ms, and without using
> > > attribute references.  OTOH, the lack of an
> > > attribute reference means that the data log
> > > contains no record of the randomized fixation
> > > duration.  So I would modify Mich's example as follows:
>
> > > c.SetAttrib "FixDur", Random(1500,3500)
>
> > > Then, in my fixation object, I would set Duration to "[FixDur]".  Done.
>
> > > But if you don't mind (or even prefer) a coarser
> > > grain (say, 1500 - 3500 ms in steps of 100 ms)
> > > then you can accomplish this without any inline
> > > code.  Just use a List, or perhaps a nested List,
> > > to hold all the possible fixation durations, and
> > > set the List Selection to Random.  Suppose we
> > > call that attribute (i.e., column)
> > > "FixDur".  Then, as before, in your fixation
> > > object set Duration to "[FixDur]".  Done.  (For
> > > more on nested Lists in particular, see Appendix
> > > C of the User's Guide that came with E-Prime.)
>
> > > -- David McFarlane, Professional Faultfinder
>
> > > At 5/31/2011 10:26 AM Tuesday, Michiel Spape wrote:
> > > >It's actually somewhere in the E-Primer:
> > > >http://www.cognitology.eu/pubs/AnE-Primer2009.pdf
> > > >  as an example of what to do with line-in
> > > >stuff... I suggest going through the last 2 chapters, or earlier.
> > > >But, then again, if you have a textdisplay
> > > >showing just a +, called FixationDisplay, and a
> > > >short inline before that:
> > > >FixationDisplay.Duration = Random(1500,3500) - you're done.
> > > >Best,
> > > >Mich
>
> > > >Michiel Spapé
> > > >Research Fellow
> > > >Perception & Action group
> > > >University of Nottingham
> > > >School of Psychology
> > > >www.cognitology.eu
>
> > > >-----Original Message-----
> > > >From: e-prime at googlegroups.com
> > > >[mailto:e-prime at googlegroups.com] On Behalf Of Krupa Sheth
> > > >Sent: 31 May 2011 14:34
> > > >To: E-Prime
> > > >Subject: Randomising the Fixation duration
>
> > > >Hi! I been totally baffled by this and I am sure there is a way where
> > > >to over come this.
>
> > > >Is there any function in E-Prime that allows the fixation point ("+")
> > > >to appear for a random duration between 1500ms to 3500ms. I am sure it
> > > >is possible but am so clueless how to do it.
>
> > > >Many Thanks!!
>
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