The **Stimulus** variable choice is used to select the correlation function for segmentation.
Selection of a ``**Stimulus**'' generates a graphical plot of the stimulus function
currently in use (provided that an image calculator graph
window to be available). The parameters *period/2*, *offset*, *iphase* and *deadtime*
are used to control the caracteristic shape and timing of the time curve.
**SQR** is a normalised square-wave with duration
and initial phase set in the **Stim Params** dialog box. **ROI** allows the user
to define a function from the image sequence. The user marks the region on
the image visible on the **Sequence Tool Tv**, typically the first image in the sequence. Clicking on the **ROI** button causes the program to build the
correlation function using the image data within this region. On each frame
in the sequence the average of the grey-level data within the ROI is
calculated and used as the function sample. Once complete the mean of the
function is computed and then subtracted from each sample and finally the
function is normalised such that its sums to 1.

**period/2** specifies the half-wave length of the square wave function **SQR**. The **offset** variable specifies the range of the phase search to be performed during the correlation. A value of 0 specifies in-phase detection only, i.e. no phase search. A value of 1 indicates that shifts of -1, 0 or +1 are allowable and similarly for other values. The correlation function is always performed over the minimum amount of data. **iphase** specifies the initial phase of the square-wave either +ve or -ve. The actual value is not important as the square-wave function is both mean subtracted and normalized.