Dear Narender,
thanks for the help.. This is very similar to the suggestion I've just 
received from Richard Perry. He suggested specifying one condition (task) 
and including parametric modulation over time.  I am unsure, however, how to 
separate a positive linear expansion (areas with increasing activation over 
time) from a negative one (areas with decreasing activation over time). Is 
there some way to put a sign on the parametric modulation? Whether I weight 
it with a [1] or a [-1] it seems to yield the same map..
Regards,
Rachael
Rachael D. Seidler, PhD
University of Minnesota Dept of Neuroscience
and Brain Sciences Center (11B)
Veterans Affairs Medical Center
Minneapolis, MN  55417
Phone: 612 725-2000 x.1765
Fax: 612 725-2291
Email: [log in to unmask]

>From: Narender Ramnani <[log in to unmask]>
>To: "Rachael Seidler" <[log in to unmask]>
>CC: [log in to unmask], [log in to unmask]
>Subject: Re: contrasts, activations and deactivations
>Date: Mon, 14 Aug 2000 18:10:08 +0100
>
>Dear Rachael,
>
>If I understand correctly, you are primarily interested in
>time-by-condition interactions, where you have a control and a task
>condition that alternate over the time-series.
>
>I would set up the design matrix in the following way.
>
>Your design matrix should have four othogonal covariates:
>
>i) Control condition (control condition not modulated over time)
>ii) Task condition (task condition not modulated over time)
>iii) Control condition x time (control modulated by time)
>iv) Task condition x time (task modulated by time)
>
>(iii) and (iv) are simply (i) and (ii) multiplied by a trend (linear or
>non-linear - see below). SPM99 implements this in the following way. If I
>remember correctly, after you have entered the details of your variables,
>it will ask about parametric specifications. The choices offered are
>"none", "time" or "other". Your choice should be "time". The next choice
>will relate to the nature of the expansion ("linear", "polynomial" or
>"exponential"). I would choose "linear"as a first pass. You will then be
>asked which trial types to apply the expansion to. Select both your trial
>types.
>
>If you choose to modulate both your conditions, there will be four
>covariates of interest in your design matrix (first four columns). Your
>contrasts will be:
>1. Task > control (not over time) -1 1 0 0
>2. Control > Task (not over time) 1 -1 0 0
>3. Task > control (interaction with time) 0 0 -1 1
>4. Control > task (interaction with time) 0 0 1 -1
>
>This provides a flexible design matrix in which you may wish to compare
>(iv) either with (iii) (control and task both change over time).
>Alternatively, if there is good reason to believe that the control
>condition itself remains unchanging, the time-by-condition interaction may
>be found by -1 0 1 0 and 1 0 -1 0 (compare (i) with (iii)).
>
>Best wishes,
>
>Narender Ramnani
>
>
>
>At 14:16 11/08/00 -0500, you wrote:
>>I'm still struggling a little with understanding how contrasts need to be
>>specified in spm99... The timecourse does not always look like I expect it 
>>to!
>>I have the following blocked fmri design:
>>control task control task control task
>>If a set it up as one condition and use the contrast [1] then I see areas
>>that are activated with respect to the control.  The contrast [-1] reveals
>>areas that are deactivated with respect to control.
>>However, I'm interested in the task vs. control (activation) x time
>>interaction. So I specified the design as 3 conditions.  I thought that I
>>could mask [1 1 1] (task activated wrt control) with [1 2 3] (increasing
>>trend over time) or [3 2 1] (decreasing trend over time).  However, the [1
>>1 1] contrast appears to show regions that are both activated and
>>deactivated for task wrt control. Can anyone explain this?
>>thanks in advance,
>>Rachael
>>Rachael D. Seidler, PhD
>>University of Minnesota Dept of Neuroscience
>>and Brain Sciences Center (11B)
>>Veterans Affairs Medical Center
>>Minneapolis, MN  55417
>>Phone: 612 725-2000 x.1765
>>Fax: 612 725-2291
>>Email: [log in to unmask]
>>________________________________________________________________________
>>Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com
>
>********************************************************************
>Dr Narender Ramnani
>
>Sensorimotor Control Group
>Department of Physiology
>University of Oxford
>Parks Road
>Oxford OX1 3TP
>
>Oxford University Centre for
>Functional Magnetic Resonance Imaging of the Brain,
>John Radcliffe Hospital,
>Headington,
>Oxford OX3 9DU
>
>Tel.	01865 222704 (Direct)
>	01865 222729 (Admin)
>mob. 	0771 2632785
>Fax. 	01865 222717
>email [log in to unmask]
>*******************************************************************
>

________________________________________________________________________
Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%