Dear Jack and J-B (hope you dont mind me answering this one),
>Dear Jean Baptiste and SPMers
>Do you mean that you can not test for an interaction using T-contrast ?
>The only trouble could be in the interpretation of the result or is there
some deeper reasons ?
t-contrasts may indeed be used to asses interactions. What J-B was alluding
to was simply the fact that a t-contrast poses a highly specific question
(e.g. [-1 1 1 -1 0 0] in my previous email which will show areas exhibiting
a larger activation due to generation in happy mood than in sad mood, or
alternatively a larger deactivation in sad mood than in happy mood).
However, say that for example activations due to word generation is equal
in happy and sad mood, but in both cases larger than in neutral mood we
would miss that with the t-contrast above, the proper t-contrast in that
case being [-1 1 -1 1 2 -2].
Hence, if you have a strong a-priori hypothesis regarding the nature (not
location) of your interactions you may/should use t-contrasts. If not you
may use e.g. the F-contrast suggested by J-B which answers the question
wether there is any interaction of any kind between mood and word generation.
In order to understand the nature of the interactions displayed by the
resulting F-map you may then plot the "contrast of parameters estimates",
which you would interpret much the same way as any user specified contrast
vector. However, note that the interpretation of one blob in your F-map may
be completely different from that of another blob.
Good luck Jesper
>De: Jean-Baptiste Poline [SMTP:[log in to unmask]]
>Date: mardi 7 decembre 1999 18:06
>A: [log in to unmask]
>Objet: Re: advice on contrasts
>Just a small add on to Jesper email : the "global" mood
>by verbal fluency interaction contrast would be
>an F-contrast of the form
>-1 1 1 -1 0 0
>0 0 -1 1 1 -1
>and the "global" effect of mood
>1 1 -1 -1 0 0
>0 0 1 1 -1 -1
>In general, an interaction of effect1 X effect2 where either
>effect1 or effect2 has more than 2 level will express itself
>as an F-test, as in classical analysis of variance.
>------------- Begin Forwarded Message -------------
>Date: Tue, 07 Dec 1999 16:33:43 +0000
>Subject: Re: advice on contrasts
>From: Jesper Andersson <[log in to unmask]>
>To: "McBride, Alan" <[log in to unmask]>, "[log in to unmask]"
><[log in to unmask]>
>X-Unsub: To leave, send text 'leave spm' to [log in to unmask]
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>Dear Alan and David,
>>Dear Spm experts,
>>We have performed an fMRI expt to examine the effects of experimental mood
>>induction on verbal fluency activations on bipolar patients and normal
>>controls. Three blocks of scans are obtained, one in each mood state (
>>happy, sad, neutral)while subjects perform a verbal fluency task constisting
>>of epochs of generation repetition and rest.
>>We have specified two conditions: repetition and generation, and examined
>>contrasts -1 1 in all three mood conditions, to examine main effect of
>>verbal fluency.( ie. -1 1 -1 1 -1 1)
>> To examine the modulation of fluency activations by mood, we have compared
>>contrast -1 1 for scans obtained in the individual mood states (eg -1 1 0 0
>>0 0, if the first series of scans is obtained in Happy mood, and compared
>>the activations with those associated with contrast (0 0 -1 1 0 0) where the
>>second set of scans were obtained following sad mood induction.
>What you describe here seem to be some form of anecdotal description of the
>task_by_mood interaction. While this mode of analysis may be helpful in
>understanding the different activation patterns, you may want to use the
>proper interaction contrast for the inferential assesment. This, as you
>probably know, is affected by e.g. the contrast (-1 1 1 -1 0 0) which will
>show areas exhibiting a larger activation due to generation in happy mood
>than in sad mood.
>> To attempt to identify the main effect of mood, we have compared all scans
>>obtained in one mood state vs other mood states, eg ( 1 1 -1 -1 0 0). This
>>contrast does not identify any regional differences- we have used scaling to
>>remove global effects, and wonder whether this may have anything to do with
>I dont think global scaling effects this. I think your problem may be that
>the main effect of mood has a very low frequency content (i.e. it changes
>very slowly with time). Either this has prevented you from using a high
>pass filter (HPF) in which case your sensitivity may be a bit poor.
>Alternatively you may have (erroneously) used an HPF wich may then have
>removed the main effects of mood.
>I the above I have assumed that you have scanned all moods in the same
>"session" such that mood effects and session effects are orthogonal.
>>Could you advise us on how best to examine the main effect of mood in this
>I think I would need some additional information about your design (e.g.
>length of epochs of both verbal task and mood) and analysis (e.g. HPF cut
>off frequency) to be able to give any advice.
>>We would also be interested in performing a further analysis, constraining
>>the comparison of fluency activations across all mood states to areas
>>activated by verbal fluency following neutral mood induction, could you also
>>help with advice about how best to do this.
>I suspect that what you want to do here is to mask the ineraction contrast
>(e.g. [-1 1 1 -1 0 0]) with the main effect contrast (i.e. [-1 1 -1 1 0
>0]). The resulting (uncorrected) p-level is simply the product of the
>p-level threshold of the mask, and the display p-level of the resulting
>masked (interaction) contrast. You are likely to be most sensitive if you
>use a high threshold (small p-value) for the mask and a more liberal
>criterion for the interaction contrast itself.
>>With thanks, David Barbenel, Alan McBride
> Good luck Jesper
>Wellcome Dept. of Cognitive Neurology
>12 Queen Square
>London WC1N 3BG
>phone: 44 171 833 7484
>fax: 44 171 813 1420
>------------- End Forwarded Message -------------
Wellcome Dept. of Cognitive Neurology
12 Queen Square
London WC1N 3BG
phone: 44 171 833 7484
fax: 44 171 813 1420