Dear Claus,

There are a some of potential improvements I can suggest to your 
experimental design and analysis.

1. The first concerns the comparison B > A. The differential effects of 
mental rotation in B vs. no mental rotation in A are confounded by the 
presence of visual stimulation in A vs. no stimulation in B. Your present 
design cannot rule out the possibility that activations are due to 
differences in visual stimulation rather than to differences in mental 
rotation. May I suggest that you have another trial type which is identical 
to the present one, but in which subjects are not required to mentally 
rotate. You can then compare B in rotate vs. no rotate (see below). Since 
there will be no visual stimulus present in either, activity can only be 
ascribed to mental rotation.

2. The second concerns the timing of conditions A, B and C. You are right 
to be concerned about carry over effects between these since these events 
occur closely in time and are timelocked to each other. In its present 
form, your design is not able to disambiguate haemodynamic activity that is 
timelocked to A, B and C since  these conditions are all timelocked to each 
other. You might consider randomly varying the time between A, B and C from 
trial to trial, over 1 TR. Toni et al. (Cereb Cortex. 1999 
Jan-Feb;9(1):35-49) and Ramnani et. al. (HBM abstract in NeuroImage, 
http://www.apnet.com/www/journal/hbm2001/10757.html ) have implemented this 
approach successfully.

Combining both of these suggestions will allow you to compare rotate vs. no 
rotate in B, without the contaminating effects of A and C.

I hope this is helpful.

With best wishes,

Narender Ramnani




At 16:06 02/10/2001 +0100, you wrote:
>Dear community,
>
>I would be happy to get some comments and advice concerning our analysis of
>the following fMRI paradigm.
>
>Subjects processed a mental rotation task. Each trial consisted of the
>following three steps/“conditions”:
>A) Presentation of a geometrical object (duration 3 seconds)
>B) Mental Rotation of the stimulus (duration 5 seconds), no visual input
>during this period
>C) Presentation of either a mirror-reversed or a matching version of the
>geometrical object presented in A) (duration 2 seconds)
>
>The task of the subjects was to decide whether the object presented in A)
>was identical to the object presented in C). This had to be done within the
>last 2 seconds. Each trial was preceded and followed by the presentation of
>a fixation cross (so to say “the baseline period”).
>
>There was no temporal gap between the three steps/conditions of a trial.
>The intertrial interval was randomly varied having a mean around 10.5 s. TR
>was 1.007 s, 18 EPI slices were acquired, 488 volumes/session, 24
>trials/session.
>
>The main question we are interested in is whether brain activation during
>step/condition B is higher than during A. To this end, we chose the
>following approach (after standard preprocessing) using the fMRI models
>setup option in SPM99:
>
>The model consists of 3 predictors (the three conditions, present-rotate-
>match/respond), each with variable stimulus onset times and durations
>(because of the random ITI).
>For each condition, we specified the exact onset and duration (in TR units)
>The three conditions were modeled as events convolved with the canonical
>hrf.
>
>Low-pass and high-pass filtering were also used/incorporated.
>
>After model estimation, we used a t-contrast (­1 1 0) to identify voxels
>with higher activation during the rotation condition.
>
>Basically, I’d like to know whether you think this is ­ in general ­ a
>correct approach, or whether anybody comes up with a better or more
>elaborated idea. More specifically, the main thing I am concerned about is
>the rapid succession of different “events” (i.e. conditions) and
>potential “carry over effects” this might cause. For example, I’d like to
>know whether it would make sense (and why) to add the temporal derivative
>to our model, whether I should use (and why) Volterra interactions (up to
>now, I did not) etc.
>
>Any help, comments, criticism…. will be gratefully appreciated,
>
>
>-Claus

********************************************************************
Dr Narender Ramnani

Sensorimotor Control Group
Department of Physiology
University of Oxford
Parks Road
Oxford OX1 3TP
UK

Oxford University Centre for
Functional Magnetic Resonance Imaging of the Brain,
John Radcliffe Hospital,
Headington,
Oxford OX3 9DU
UK

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