Dear Users, Concatenation or higher level stats? We would value the input of any users who have experience of 'combining' fMRI data across multiple runs for more robust averaging. The issue arises when trials of an experient are acquired in different data blocks due to technical limitations of scan scquisition protocols or often to reduce the strain on participants in a single session. I have searched many available resources. There are well documented routines for (1) analysinng the sessions individually and then compring them with a higher level analysis or (2) demeaning the two timeseries and combining them into a single one .. then following the standard analysis individual subject routine. Comments would be appreciated (especially from anyone has first hand experience of doing this). Thanks Andre'

Hi, Andre, To continue with this issue. I think from a user point of view, it would be better to follow option (2) because you then do analyses as usual treating all the data for each subjects as you would if you did not have memory limitation. If people have complicated designs, as it is normally the case for people who need to scan for a long time, the data analysis becomes even more complicated, and the room for errors increases. I think I was wrong to suggest earlier that FSL would automatically deal with this problem. If I understand it correctly, the problem is that for different scans, the arbitrary intensity values are going to be very different, right?. So the mean intensity for one scan can be, say, 1000, while for another scan of the same individual, it could be 4000. Doing grand-mean scaling does not seem right here, which is what FSL automatically does. But if you de-mean each scan (i.e., subtract its raw mean intensity, which results in a mean intensity of 0 for each scan), there should be no problem then combining the scans as if they were all coming from the same one, as the mean intensity would be the same for all. The 'avwmaths' commands in FSL will compute this very easily (assuming I am getting this right). Apparently, people did this routinely at the Waismann Institute, so I actually never heard of someone having to analyze the data separately for each scan. Maybe some softwares do this pre-processing by default. My experience with Voxbo was such that, even when we had 8 or 10 different scans per person (an hour of scanning), we did not have to worry about it in the analysis. Silvia On 8 Jul 2008, at 16:20, Tim Andrews wrote: Silvia Gennari Department of Psychology University of York York, YO10 5DD United Kingdom -- ynic-users mailing list ynic-users(a)ynic.york.ac.uk <mailto:ynic-users@ynic.york.ac.uk> https://www.ynic.york.ac.uk/mailman/listinfo/ynic-users