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Methods

We start by generating an artificial stimulus. Inter-stimulus intervals (ISI) between single-events were drawn at random from a Poisson distribution with mean 8 seconds. ISIs were discarded if less than 4 seconds or greater than 16 seconds. Events were then randomly assigned as being stimulus or rest with equal probability. We then generated two artificial datasets, using two different HRFs. The first HRF is shown in figure 4(a) and has a post stimulus undershoot. The second HRF, shown in figure 3(a), is the same except it has no post stimulus undershoot. The artificial stimulus was then convolved (at 0.5 second resolution) with the two different HRFs to give the two different artificial signals shown in figures 4(b) and 3(b). The signals were then added to 100 voxels in a $ 10\times10$ square section of a slice of a null/rest dataset, to generate two artificial datasets. The null/rest dataset data was obtained with the subject performing no specific task using echo planar images (EPI) acquired using a 3 Tesla system with TR=3 seconds, time to echo (TE) = 30ms, in-plane resolution 4mm and slice thickness 7mm. The first 4 scans were discarded to leave $ N=196$ scans and the data was motion corrected using MCFLIRT (31) and high-pass filtered as described in (43). We can then use the full model (including the best noise model) in two forms on the two datasets. One with the ARD prior on the undershoot size parameter, $ c_2$, and one with a non-informative prior.
Figure 3: (a) the HRF used to generate the artificial signal without undershoot ($ m_1=0$, $ m_2=8$, $ m_3=8$, $ m_4=0$, $ c_1=0$, $ c_2=0$) (b) artificial signal used in the artificial data generated from convolving the stimulus with the HRF in (a).
\begin{figure}
\centering
\begin{tabular}{cc}
\psfig{file=fig3a.ps,width=0.2...
...ile=fig3b.ps,width=0.23\textwidth}\\
(a)&(b)\\
\end{tabular}
\end{figure}
Figure 4: (a) the HRF used to generate the artificial signal with undershoot ($ m_1=0$, $ m_2=8$, $ m_3=8$, $ m_4=12$, $ c_1=0$, $ c_2=0.3$), (b) artificial signal used in the artificial data generated from convolving the stimulus with the HRF in (a).
\begin{figure}
\centering
\begin{tabular}{cc}
\psfig{file=fig4a.ps,width=0.2...
...e=fig4b.ps,width=0.23\textwidth}\\
(a)&(b)\\
\end{tabular}
\end{figure}

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Next: Results Up: HRF Inference Evaluation Previous: HRF Inference Evaluation