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nirsLAB User’s Manual
actually want to apply a frequency filter! One reason is that, as a practical formality, nirsLAB will not
allow you to perform a Hemodynamic States computation (Chap. 4) until you first open and close the
Apply Frequency filter GUI. We have incorporated this feature into nirsLAB in order to ensure that the
absence of any frequency filtering is a deliberate choice and not a simple oversight.
The other user-definable filter properties (Figure 3-38) are:

‘Roll off width:’. There is a theoretical possibility that, after you apply a frequency filter, you will
notice a phenomenon called “ringing”4 in your data time series. If it does occur, you can reduce
its magnitude by entering larger values into the ‘Roll off width:’ field, and then re-applying the
filter. We expect, however, that the default values will give you satisfactory results in most cases,
especially if you remove discontinuities and spike artifacts (if any) before you apply the
frequency filter.

‘Low cutoff freq. (Hz):’. The threshold value that sets the time scale for long-term fluctuations
(e.g., drifts) that you want to remove from the data. With the default value of 0.01 Hz, any
fluctuation that has 100 or more seconds per cycle will be deleted. Note that this number field
will be grayed out if you select the ‘Low pass’ filter type, and the label will change to ‘Cutoff freq.
(Hz):’ if you select the ‘High pass’ filter type.

‘High cutoff freq. (Hz):’. The threshold value that sets the time scale for short-term fluctuations
(e.g., noise) that you want to remove from the data. With the default value of 0.2 Hz, any
fluctuation that has 5 or fewer seconds per cycle will be deleted. Note that this number field will
be grayed out if you select the ‘High pass’ filter type, and the label will change to ‘Cutoff freq.
(Hz):’ if you select the ‘Low pass’ filter type.
After you specify the filter type, cutoff frequency or frequencies, and (optionally) roll-off width, click on
the Filter button shown in Figure 3-38 to apply the filter. (If the number of measurement time frames is
large, a noticeable amount of time may be required for the frequency-filtering computation. A pop-up
window containing a progress bar will pop up on your screen, so you will know that the computation is
ongoing and nirsLAB has not stalled or crashed.) With the default ‘raw & filtered data’ display-options
menu item selected, the plot field will show the both the original time series (in gray) and the frequencyfiltered one (in the original color). Alternatively, you can view only the original data or only the filtered
data, by selecting the appropriate item from the pull-down menu.
If you want to change one or more filter parameters, you can simply specify the new values and click on
Filter again. It is not necessary for you to first “undo” the effect of the previous filter computation(s).
However, you may, if you want, restore the original data by selecting ‘No filter’ from the ‘Filter type:’
pull-down menu and clicking on the Filter button.
Examples of frequency filtering, for each of the three filter types, are shown in Figures 3-39 (band pass),
3-40 (low pass) and 3-41 (high pass). To facilitate viewing of the time series, the ‘Markers: off’ display
option is selected.
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