Hi Tom, thanks for the reply. That's helpful, but perhaps I should
better describe the paradigm. An example of the sequence of trials,
each of which the subjects respond as same or different, would include
three kinds of trial: some in which the two stims are the same, some
in which they different colors, and some in which they are moving
differently, as:
dif_move (x)
same
same
dif_color (switch) (x)
same
dif_color
dif_color (x)
dif_color
same
same (x)
same
dif_color (x)
same
dif_move (switch) (x)
same
dif_move
same (x)
dif_move
dif_move
dif_move (x)
same
same
dif_color (switch) (x)
dif_color
same
same (x)
dif_color
same
We wish to compare switch trials to non-switch same, dif_color, and
dif_move trials.
So, I hope I'm correct in thinking if we select the trials marked (x),
this will let us do so, and if we select these trials so that there is
jitter (how much is needed?), jittering trials is also accomplished. My
question is, am I wrong, for example, because the intervening (non-x)
trials will impact BOLD such that my
approach will not work?
Again, many thanks...
Jim
----- Original Message -----
From: Tom Eichele <[log in to unmask]>
Date: Sunday, November 19, 2006 3:59 am
Subject: SV: [SPM] Minimum spacing and jitter in event related designs
To: "'Dr James K. Kroger'" <[log in to unmask]>
> Hi Jim,
>
> I am not familiar with task-switching experiments so this may be
> off, but
> when I get your question correctly you have series of trials with
> T1 (task 1), T2 (task 2) and the sparse event S (Switch) is the
> transitionbetween T1 and T2 at variable intervals (= jitter), is
> it? Possibly there
> T0, where there is no task (off-block).
> My guess is that this would be conceived as a 'mixed design' with both
> block- and event-related features. So, rapid SOAs between trials,
> say 1-2
> secs is not much of a problem wrt SNR/Power as long as the SOA
> between your
> events of interest, i.e. transitions (switches) between blocks
> occur at
> variable intervals, are spaced reasonably far apart (say 10-30
> secs), T1 and
> T2 are perceptually similar, and some areas x,y,z respond
> selectively to the
> switches. Then you'd model the activation in this area with the
> timing of
> the switches, and possibly with other switch-related info, e.g.
> responsetime slowing, Accuracy... I guess relevant control events
> would be
> transitions from e.g. T0 to T1 and so forth
> Areas involved in task processing per se you'd model with block
> functionsfor T1 and T2, and specify contrasts against T0 and so
> forth.
> So, you may have a design like
> T0: 1 1 1 1 0 0 0 0 0 0 0 0 ...
> T1: 0 0 0 0 1 1 1 1 0 0 0 0 ...
> T2: 0 0 0 0 0 0 0 0 1 1 1 1 ...
> S: 0 0 0 0 0 0 0 0 1 0 0 0 ...
> RT: ....
> ACC: ...
>
> Hope this is helpful,
> Tom
>
> --
> Tom Eichele
> Department of Biological and Medical Psychology, University of Bergen
> Jonas Liesvei 91, N-5009 Bergen, Norway
> Tel: +47-55586290
>
>
> -----Opprinnelig melding-----
> Fra: SPM (Statistical Parametric Mapping)
> [mailto:[log in to unmask]] På
> vegne av Dr James K. Kroger
> Sendt: 19. november 2006 02:12
> Til: [log in to unmask]
> Emne: [SPM] Minimum spacing and jitter in event related designs
>
> Dear SPMers,
>
> I'm somewhat of a novice at SPM and hope you don't mind an
> elementary
> question on how closeness and jitter of intermixed events affects
> differential sensitivity to their BOLD effects. We wish to conduct
> an
> event-related experiment to look at task switching. Switch effects
> diminish as interstimulus intervals increase so rapid trials (2
> seconds) provide the strongest psychological effect. From what I've
> read, the ability to see the separate BOLD effects of switch and
> non-switch trials in a design with trials this closely spaced will
> be
> low. How will the ability to detect separate BOLD effects be
> impacted
> as the interval varies between 2 and 5 seconds? Also, I know that
> jittering will be desirable.....if our interval is X seconds, what
> percentage of this interval is necessary to form helpful jitter in
> this
> scenario?
>
> Many thanks in advance for any help or references.
> Jim
>
>
>
>
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