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Hi again > > > The effect you point out lies behind the reason for the differences > in the copes - if the true BOLD responses to the different duration > trials are in fact more similar than is assumed by the HRF linear > convolution model, then the shorter duration trial regressors will > need much larger betas to fit the responses. This does make > comparisons of trial types that have brief and variable durations > trickier to interpret - the effect could be due to greater neural > activity or a non-linearity of the BOLD response to brief stimuli. Eugene is being subtle :) What he means to say is that you simply cannot expect to compare a 1s trial to a 3s trial. There are two reasons for this: i) The linearity assumptions of the hrf are not good in this range. ii) Your modelling assumptions of neural activity are not good. For example, what if there were a region that exhibited phasic activity to the onset of a trial (instead of continued activity throughout the trial)? The neural activity would be identical in each case. Presumably the BOLD response would be identical, but your betas will be 3x bigger for the short than the long trial - it will look like a massive difference in activity, even though the underlying activity is identical. T > As Tim mentioned, this is not as much of an issue if trials are > longer than around 5 secs. > > Eugene > > > > > Thanks again, > Todd > > > > > On Wed, Feb 17, 2010 at 4:59 PM, Tim Behrens > <[log in to unmask]> wrote: > > Hi > > On 17 Feb 2010, at 20:53, Todd Thompson wrote: > > > >> If the cope is reflecting the "scaling factor" by which you have to > >> multiply the regressor to re-construct the raw signal data, > shouldn't > >> these copes be equal in order to reconstruct the 2.5% signal > change? > > > > > > > > No - The model goes like this > > Y=Xb > > > > so if X(n) is 8 times smaller than X(n-1), then b(n) has to be 8 > times > > bigger to make the same Y (and Var(b(n)) has to be 64x bigger!!) > > > > In fact X(n) will not be 8 times smaller, because hrf convolution > doesn't > > carry on adding height after a certain number of seconds (roughly > 5, I would > > guess off the top of my head, but it is a guess - it would be easy > to figure > > it out in matlab), but it could easily be 4 or 5 times smaller - > hence you > > have vastly different copes and varcopes for the same %signal > change. > > > > T > > >