  In a previous report (Raju et al.,2023) we concluded that, if the goal was to
preserve events such as saccades, microsaccades, and smooth pursuit in
eye-tracking recordings, data with sine wave frequencies less than 100 Hz
(-3db) were the signal and data above 100 Hz were noise. We compare 5 filters
in their ability to preserve signal and remove noise. Specifically, we compared
the proprietary STD and EXTRA heuristic filters provided by our EyeLink 1000
(SR-Research, Ottawa, Canada), a Savitzky-Golay (SG) filter, an infinite
impulse response (IIR) filter (low-pass Butterworth), and a finite impulse
filter (FIR). For each of the non-heuristic filters, we systematically searched
for optimal parameters. Both the IIR and the FIR filters were zero-phase
filters. Mean frequency response profiles and amplitude spectra for all 5
filters are provided. In addition, we examined the effect of our filters on a
noisy recording. Our FIR filter had the sharpest roll-off of any filter.
Therefore, it maintained the signal and removed noise more effectively than any
other filter. On this basis, we recommend the use of our FIR filter. Several
reports have shown that filtering increased the temporal autocorrelation of a
signal. To address this, the present filters were also evaluated in terms of
autocorrelation (specifically the first 3 lags). Of all our filters, the STD
filter introduced the least amount of autocorrelation.
