 |
 |
LLN, le 16/09/05 Dear Nici, I asked the same question about concatenated runs filtering some time ago but did not get any answer. I thus cannot help you with this. You should however be aware that 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If the scanner stopped and restarted between runs, yet you modelled your conditions as a single session, then your data no longer represent a continuous timeseries, and several aspects of SPM will be disrupted, eg.: - highpass filtering, - temporal autocorrelation estimation, - grand-mean scaling,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. Hope this helps, Mauro >Dear SPM experts, > >I’ve checked the spm archives but I couldn’t find an answer to the >following question: > >I’m using an event-related design, in which some events depend on >the behavior of the subjects such that there are sometimes only 2 >per run. Since I can not acquire the data in one long run, I would >like to concatenate 5 runs (lets say each contains 200 images) to >one long containing then images 1 to 1000. Right now I’m adding a >constant for each run to correct for session specific effects as >well as one extra covariate modeling the transition from one run to >the next (not sure whether this is really necessary). I would like >to highpass-filter the data and use the default SPM2 value of 128 s. >If I apply this filter to the concatenated run, the “session >specific-constants” are filtered too. This is clearly visible in the >design matrix. My first question is: is this a problem?????? > >In the meanwhile, I’ve changed the spm code a little bit, such that >the filtering is applied for each original run, i.e the filter is >applied to image 1-200 of the concatenated data, then to image >201-400, then to 401-600 and so on. If I check the design matrix the >constants look fine. Now question number 2: is this correct or do I >overlook something important here? > >Thanks a lot, >Nici > >_________________________________________________________________ >Free blogging with MSN Spaces http://spaces.msn.com/?mkt=nl-be -- _____________________________________ 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