 |
 |
Hello, I'm hoping someone can help me understand a bit about the differences between selective averaging and the PSTH functions in SPM. I've been comparing the time courses of activations in a single voxel using 3 methods: 1) PSTH using the standard FIR basis set (window = 32 seconds, bin = 2.5 seconds which is my TR) with adjustment for the other conditions in the session. I've been using Darren's get_psth.m from the list. 2) The same without adjustment. 3) Selective averaging on the filtered time course from one voxel, binning the data into 2.5 second bins from 0 to 32 seconds. My confusion arises from the fact that these 3 methods are giving me very different results. The FIR-adjusted looks most similar to a canonical HRF, but I would expect the FIR-no-adjustment and the Selective Averaging to at least look similar to each other, but most of the time they don't. They also don't look much like an HRF, even in visual areas which should be strongly driven by my task. Is this plausible? and which method should I believe? A few details of my experiment: it is a 2x2 factorial design, and on every trial subjects saw a 2.5 second video, then a blank screen for 700msec before the next video. Trial order was pseudorandomised. I've been looking at PSTHs from single subjects in single voxels which showed significant effects in a standard SPM2 single subject analysis. I would be happy to email plots or code if that would be useful. Thank you for your help, Antonia.