fScan Reference Manual, Chapter 2 (CmdLineOptions): fithrf
Fithrf -- Fit a hemodynamic response function to time series data
usage: -fithrf flag nparams latency pctsigmin
FITHRF fits a biphasic hemodynamic response function to each voxel in a time series data set. This command assumes that the input data set contains a single response time course, typically generated by averaging multiple individual responses (see the MOVIE command).
Command arguments
NPARAMS - Number of fitting parameters, Minimum=4 (default= 7,18; if FLAG&4, NPARAMS=9; if FLAG&32, NPARAMS=8)
LATENCY - (default= 0)
PCTSIGMIN - (default= 80%)
The fitting algorithm
The hemodynamic response is estimated by fitting 7 time course parameters (see below):
delay,rise,peak,width,fall,undershoot,recovery
Of these, all are time values except PEAK and UNDERSHOOT, which are amplitude parameters.
Each input time course is fitted independently by estimating the 7 parameters using the following steps:
identify maximum, minimum, and mean of time course
identify time points where the curve crosses the mean value
starting at the peak, find preceding mean-crossing
linearly extrapolate slope of curve at rising mean-crossing to find time at base of rise and at peak of rise
define delay as time at base of rise and rise as time at onset of peak
repeat for the falling phase (find the falling mean-crossing and extrapolate its slope to find the end of the peak and the end of the fall)
calculate the time width of the peak (end of peak minus onset of peak)
calculate the amplitude of the peak
calculate the amplitude of falling undershoot (pre-rise level - post-fall level)
calculate recovery time from undershoot minimum to pre-rise baseline amplitude
Once the fitting parameters are have been estimated the fit is checked by using those parameters to generate an idealized hemodynamic response curve and then calculating the correlation between the original time course and the idealized fit. The calculated correlation is reported as goodness-of-fit on a 100 point scale (i.e., the correlation coefficient multiplied by 100).
Time course statistics
The first step in the fitting process is to generate a standardized set of statistical parameters for each time course. If you select the FLAG 64 option, those statistical parameters will be returned as output instead of the standard HRF fitting parameters.
The time course statistics returned with that option are:
vmean - mean valuee
pctsignal - (vmax-vmin)/vmean X 100
vmin - minimum value
vmax - maximum value
imin - time index of minimum value
imax - time index of maximum value
crossings - number of times the time course crosses the mean value
v0min - minimum non-zero value
v0max - maximum non-zero value
HRF time course fitting parameters
Primary fitting parameters:
delay - time from beginning of time course to start of response
rise - time from beginning of rise to reach peak
peak - amplitude of peak (peak-baseline)
width - time from onset of peak to end of peak
fall - time from end of peak to minimum
undershoot - amplitude of undershoot (baseline-minimum)
recovery - time from undershoot minimum to return to baseline
Additional HRF parameters not usually fitted automatically:
Goodness of fit parameter: