From MNH to the Friedman--Komogortsev Method---which changes improve eye-movement classification?

Abstract

The Friedman-Komogortsev Method (FKM) is a rule-based classifier of fixations, saccades, and post-saccadic oscillations in 1000-Hz recordings, developed from the modified Nyström-Holmqvist algorithm (MNH) and used in several recent studies. FKM changes MNH in many ways at once: it centres MNH's Savitzky-Golay filter, adds a position-based test for whether an apparent post-saccadic oscillation continues the saccade, adds rejection tests, and changes several thresholds. We asked which of these changes alter agreement with human classification. Both algorithms were scored against twenty 26-s GazeBase reading recordings hand-classified by three trained raters, and each change was reverted in turn. FKM agreed better than MNH as MNH was used: saccade Cohen's kappa rose on all 20 recordings (median +0.034), and saccade boundaries moved from 2-3 ms late to the raters' sample. One change produced this: centring the filter, which removes a three-sample delay. Reinstating the delay returned FKM's saccade kappa to MNH's value, and centring MNH's filter alone made MNH match FKM on saccades and exceed it on post-saccadic events. The position-based test changed how saccades with several velocity peaks were divided, splitting 8% of rater-labelled "wavy" saccades into a saccade and a spurious oscillation against 39% for delay-corrected MNH. FKM's higher post-saccadic velocity floor traded recall for precision. The remaining changes had no measurable effect on these recordings. FKM's improvement over MNH is therefore a filter-alignment correction that MNH users can apply directly, plus a segmentation rule for multi-peak saccades.