It is sometimes useful to project an HRF to examine the exact time course of the BOLD response, to a neuronal event (Bagshaw et al. 2005,Stefanovic et al. 2005, Lu et al. 2006).Our group often uses a Fourier basis method to calculate the HRF (Benar et al. 2002), in which a number of Fourier basis functions are applied to a time windowto determine the shape of the HRF response. Many software packages, such as AFNI and SPM include the ability to examine the BOLD time course associated with an event.

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Example of some HRFs projected using our method, from a time scale of -18 seconds prior to the event to 48 seconds after the event.
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These HRFs have a high signal to noise ratio, as demonstrated by their clear shape.

Activations (positive changes in the BOLD response) are generally better understood than deactivations (negative changes, or decreases, in the BOLD response). We have often found that the HRFs associated with deactivations are often not as clear as those associated with activations.