fmri.stimulus {fmri} | R Documentation |
Create the expected BOLD response for a given task indicator function.
fmri.stimulus(scans = 1, onsets = c(1), durations = c(1), rt = 3, times= NULL, mean = TRUE, a1 = 6, a2 = 12, b1 = 0.9, b2 = 0.9, cc = 0.35)
scans |
number of scans |
onsets |
vector of onset times (in scans) |
durations |
vector of duration of ON stimulus in scans or
seconds (if !is.null(times) ) |
rt |
time between scans in seconds (TR) |
times |
onset times in seconds. If present onsets
arguments is ignored. |
mean |
logical. if TRUE the mean is substracted from the resulting vector |
a1 |
parameter of the hemodynamic response function (see details) |
a2 |
parameter of the hemodynamic response function (see details) |
b1 |
parameter of the hemodynamic response function (see details) |
b2 |
parameter of the hemodynamic response function (see details) |
cc |
parameter of the hemodynamic response function (see details) |
The functions calculates the expected BOLD response for the task indicator function given by the argument as a convolution with the hemodynamic response function. The latter is modelled by the difference between two gamma functions as given in the reference (with the defaults for a1, a2, b1, b2, cc given therein):
(x/d1)^a1 * exp(-(x - d1)/b1) - c * (x/d2)^a2 * exp(-(x - d2)/b2)
The parameters of this function can be changed through the arguments
a1
, a2
, b1
, b2
, cc
.
The dimension of the function value is set to c(scans,1)
.
If !is.null(times)
durations are specified in seconds.
If mean
is TRUE (default) the resulting vector is
corrected to have zero mean.
Vector with dimension c(scans, 1)
.
Karsten Tabelow tabelow@wias-berlin.de
Worsley, K.J., Liao, C., Aston, J., Petre, V., Duncan, G.H., Morales, F., Evans, A.C. (2002). A general statistical analysis for fMRI data. NeuroImage, 15:1-15.
# Example 1 hrf <- fmri.stimulus(107, c(18, 48, 78), 15, 2) z <- fmri.design(hrf,2) par(mfrow=c(2,2)) for (i in 1:4) plot(z[,i],type="l")