¥ h(t ) = 0 for any t not on the TR grid
¥ This limitation means that, using stick functions as our
basis set, we can only model stimuli
that are ÒlockedÓ to the TR grid
H That is, stimuli/activations donÕt occur
at fully general times, but only occur at integer
multiples of TR
¥ For example, suppose an activation is at t =1.7×TR
H We need to model the response at later
times, such as 2×TR, 3×TR,
etc., so need to model h(t ) at times such as t=(2-1.7)×TR=0.3×TR, t=1.3×TR, etc., after the stimulus
¥ But the stick function model doesnÕt allow for such intermediate times
¥ or, can allow Dt for sticks to be a fraction of TR for data
¥ e.g., Dt =
TR/2 , which implies twice as many bq parameters to cover the same time interval
(time interval needed is set by hemodynamics)
¥then would allow stimuli that
occur on TR-grid or halfway in-between