Randomising the Fixation duration

Tue May 31 17:03:31 UTC 2011

Hiya,
I can see loads of reasons, but my favourite is the jittering one. That is, if you have a paradigm like fixation(500 ms)-->stimulus(500ms)-->response(500 ms) (I guess this accounts for a lot of them), and you do EEG, then chances are that whatever you measure as peaks after stimulus (say, an N1), are actually not in the 100 ms after stimulus, but 600 ms after the onset of your fixation. By blurring the duration of your fixation, you'd blur the peak, which can - if adequately done - result in a flat-line. Other reasons I'll leave for others (e.g. I get a lot of anticipation responses if I do not randomise the fixation at least somewhat) - nice question anyway, it's good to sometimes discuss more theoretical issues here!
Best,
Michiel

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 liwenna
Sent: 31 May 2011 17:46
To: E-Prime
Subject: Re: Randomising the Fixation duration

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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