Right. If there was a fairly long interval, there should be no problem. I assume, however, in most such designs the interval won't be very long (on the order of 1-3 sec). In such situations it's my strong impression that it's hard to get around the collinearity of the cue and response, as a practical matter. -----Original Message----- From: S.F.W. Neggers [mailto:[log in to unmask]] Sent: Wednesday, November 09, 2011 12:21 PM To: Fromm, Stephen (NIH/NIMH) [C] Cc: [log in to unmask] Subject: Re: [SPM] Assessing Multicollinearity from a Review Perspective Dear Andrew, Stephen, when one models both factors (cue and response), with some time in between (several seconds), one should be able with T or F tests to create a contrast testing the variance uniquely attributable to either cue or response, right? Provided such unique variance exists of course. So the question to my opinion really is whether there is enough time between cue and response, and whether the onsets of both are jittered substantially. Namely, when there is a lot of time in between cue and response, say 12 seconds, no one would doubt, even when there is still a little collinearity, that one can attribute variance to one event or the other. How long is the interval in this design, and the jittering? My 2 cts... Cheers, Bas Op 09-11-11 16:16, Stephen J. Fromm schreef: > Did they model both cue and response in the subject-level GLM? Or did they just model cue? > > From my experience with such models, it's impossible to disambiguate one part of the trial (e.g. cue) from the other parts of the trial (response, etc), at least with fMRI. Because response must always follow cue, you can't get sufficient jittering to separate them. (That is, the usefulness of jittering is limited when you can't randomize the order of the events.) If they model multiple parts of the trial (e.g. cue, response and maybe other phases), they might get something that survives some measure of statistical significance, but it's hard to see how it would be a meaningful measure of anything. > > I did see one paper that claimed to separate the trial parts by using incomplete trials. So if you have a trial with cue that's truncated, you could claim that the cue-only trials measure cue; and you could get response by comparing the parameter estimates for complete trials with parameter estimates for the truncated trials. But that has an obvious weakness: at least naively one would expect that the cognitive processes during truncated trials would be different from those in complete trials. > > So, on the information you provide, I'd venture to say this is a weakness in the paper you're reviewing. > -- -------------------------------------------------- Dr. S.F.W. Neggers Division of Brain Research Rudolf Magnus Institute for Neuroscience Utrecht University Medical Center Visiting : Heidelberglaan 100, 3584 CX Utrecht Room B.01.1.03 Mail : Huispost B01.206, P.O. Box 85500 3508 GA Utrecht, the Netherlands Tel : +31 (0)88 7559609 Fax : +31 (0)88 7555443 E-mail : [log in to unmask] Web : http://www.neuromri.nl/people/bas-neggers : http://www.neuralnavigator.com (CEO) -------------------------------------------------- ------------------------------------------------------------------------------ De informatie opgenomen in dit bericht kan vertrouwelijk zijn en is uitsluitend bestemd voor de geadresseerde. Indien u dit bericht onterecht ontvangt, wordt u verzocht de inhoud niet te gebruiken en de afzender direct te informeren door het bericht te retourneren. Het Universitair Medisch Centrum Utrecht is een publiekrechtelijke rechtspersoon in de zin van de W.H.W. (Wet Hoger Onderwijs en Wetenschappelijk Onderzoek) en staat geregistreerd bij de Kamer van Koophandel voor Midden-Nederland onder nr. 30244197. Denk s.v.p aan het milieu voor u deze e-mail afdrukt. ------------------------------------------------------------------------------ This message may contain confidential information and is intended exclusively for the addressee. If you receive this message unintentionally, please do not use the contents but notify the sender immediately by return e-mail. University Medical Center Utrecht is a legal person by public law and is registered at the Chamber of Commerce for Midden-Nederland under no. 30244197. Please consider the environment before printing this e-mail.