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(-: Thanks for your reply. On 12/5/06, PESENTI Mauro <[log in to unmask]> wrote: > > LLN, le 5/12/06 > > Dear Christina, > > Cyril is right: concatenation is not recommended. If the scanner > stopped and restarted between runs, yet you modelled your conditions > as a single session, then your data no longer represent a continuous > timeseries, and several aspects of SPM will be disrupted, eg.: > > - highpass filtering, > - temporal autocorrelation estimation, > - grand-mean scaling, > - session-meaning. > > (I quote Rik Henson, from this list). > > Note that I once compared "concatenated runs + constant" to > "separated sessions" and the results were quite similar, though not > identical. > > > >I will need to reanalyse 15 peoples data . . .). > > Hmmmm, what are 15 individual analyses in the life of a brain-mapper...? > > Hope this helps, > > Mauro > > > >Dear Cyril, > > > >Thanks for your quick reply. > >Each session starts and ends with a 'baseline' condition to prevent > >problems due to the scanner starting, and also the first two volumes > >are discarded. Do you think that there is still a problem with that? > >(well, conditions order was different between sessions, but if there > >is a problem with that there is no question I will need to reanalyse > >15 peoples data . . .). > > > >Thanks so much, > >Christina > > > > > >On 12/5/06, cyril pernet > ><<mailto:[log in to unmask]>[log in to unmask]> wrote: > > > >Hi Christina: > > > >>Dear SPMers, > >> > >>I am new to SPM and I would appreciate any help on the following > question. > >>I have a four session design. Each session contains all the conditions I > >>want to compare later on through contrasts. When presented, the sessions > >>follow each other and there is a very short break between sessions > >>presentation (30sec approx.).Participants stay still and they only get > out > >>of the scanner after they have completed all the sessions. I have > analysed > >>the data as one single session (instead of 4) as this made contrast > >>definition easie .... I am now unsure about how correct this is. Do I > >>violate any assumptions? Is it a common procedure to regard multiple > >>sessions as one? Is this ‘eligible’? > >> > >> > >I think that's a bad idea - each time you stop start the scanner you > >have variations. Therefore you should use 4 sessions in your design, > >each one having its' own cste. Contrasts are not more complicated, if > >you enter the conditions the same way you should be able to use exactly > >the same contrasts. > > > >Best > >Cyril > > > -- > _____________________________________ > > > Help fighting hunger: http://www.hungersite.com > > Just click your mouse and sponsors of The Hunger Site donate a > serving of food to a person in need - at no cost to you. > > ______________________________________ > > Mauro PESENTI > Research Associate, National Fund for Scientific Research (Belgium) > Unite de Neurosciences Cognitives > Departement de Psychologie > Universite Catholique de Louvain > Place Cardinal Mercier, 10 > B-1348 Louvain-la-Neuve > > tel.: +32 (0)10 47 88 22 > fax: +32 (0)10 47 37 74 > E-mail: [log in to unmask] > http://www.nesc.ucl.ac.be > http://www.nesc.ucl.ac.be/mp/pesentiHomepage.htm >