Dear SPMers,<div><br></div><div>I have been trying to perform a simple analysis with a fixation vs movement visual task with ASL. My TR is 2.5s, and due to the control-tag nature of the ASL scans I say in my model that the interscan interval is 5s. I am using the standard double-gamma canonical HRF.  My design matrix (image below), however, seems odd to me as the fixation task (starting at scan &quot;0&quot;) only starts to show non-zero values in X(3,1), i.e. at 3*interscan interval = 15 seconds! Given the canonical HRF peak at 6-8s or so, this seems a bit off. The results confirm this: if I change the onsets so as to subtract 2 scans from each onset, the results are as expect - as it is, I see an empty glass brain. Using the derivatives hasn&#39;t helped either.</div>
<div><br></div><div>The details of the blocks are: Fixation = [ 0     6    12    18    24    30    36    42    48    54    60    66    72    78    84]; Activation = [3     9    15    21    27    33    39    45    51    57    63    69    75    81] (all block durations are 3 perfusion scans, i.e. 6 &quot;normal&quot; scans). Other parameters were left as default.</div>
<div><br></div><div>Any help will be much appreciated.</div><div><br></div><div>Thank you!</div><div><br></div><div>Joao</div><div><br></div><div><img src="/cgi-bin/wa-jisc.exe?A3=ind1302&L=SPM&E=base64&P=1757231&B=--f46d0447a291d72b0e04d62bde9c&T=image%2Fpng;%20name=%22image.png%22&N=image.png" alt="Inline images 1"><br></div>