Fig. 1.

Method of analysis of SIFSs A–E and vector representation of SIFS patterns F. Blue traces, horizontal eye position; red, vertical. Magenta and light blue rectangles reflect results of interactive analysis; left edges mark onset times of SIFSs, right edges capture start of the postsaccadic steady-state position; heights indicate net displacement. Dashed horizontal lines mark approximate fixation point position. Pattern types formed by successive SIFSs are indicated by letters between the constituent SIFSs; S, stair case; U, unpaired back-and-forth; P, paired back-and-forth pattern forming 'classical' SWJs; P', paired back-and-forth patterns that do not constitute SWJs. A PSP patient with large amplitudes and correspondingly high PBF frequency. B ALS patient with small SIFS amplitudes and, hence, low PBF frequency (postsaccadic oscillation is neurally generated and not an artefact). C Control subject; note grossly overshooting 'corrections' after small centrifugal SIFSs resulting in bursts of three SIFSs. D PSP patient; left SIFS out of series of three consecutive SIFSs (intervals cut out) demonstrates the possibility that SIFSs have virtually zero amplitude by our definition. E PSP patient; amplitude determination in cases of SIFS variants with dynamic overshoot, followed by a slow decay to the final steady-state position. F Vector representation of individual SIFSs and of patterns. Vectors S_i with amplitudes and orientations (A_i, α_i) represent leading (i = 1) and trailing SIFS (i = 2) of pattern, respectively. Pattern vectors S_P with amplitude and orientation (A_P, α_P) equal vectorial average of S₁ and S₂ (staircase pattern) or of S₁ and -S₂ (back and forth), respectively