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J. Tian, X. Shan, M. F. Walker, D. S. Zee; Vertical Saccades in Monkey Do Not Obey Listing's Law. Invest. Ophthalmol. Vis. Sci. 2008;49(13):130.
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For Listing’s Law (LL) to hold during an eye movement, the eye velocity axis must vary by half the amount of a change in eye position (Tweed & Vilis, 1990). This is known as the half-angle rule, which is used to evaluate the validity of LL for different types of eye movements. Here we examined the validity of half-angle rule for vertical and horizontal saccades and pursuit in normal monkeys.
In two rhesus monkeys, we used the 3-D scleral search coil method to record vertical and horizontal 20º saccades (between ±10°) and sinusoidal pursuit (37.5º/s, 0.3 Hz, ±20°). Monkeys sat in a primate chair with the head immobilized and viewed a target 66 cm away. Vertical eye movements were elicited at one of three horizontal positions and horizontal eye movements at one of three vertical positions (0°, ±20°). Data were grouped by direction and position of the eyes and expressed in Listing’s coordinates. For saccades, we calculated the average axis (from saccade onset to peak velocity) by performing a linear regression of the torsional component to the vertical or horizontal component of angular eye velocity. For pursuit, an average velocity axis was determined. Then the slope of the linear regression of these velocity axes against the measured horizontal or vertical positions was taken as the tilt-angle slope (TAS).
Results (combining data for both directions) were similar in the two monkeys (Figure). The overall mean (two animals combined) TAS was 0.30 for vertical saccades and 0.47 for horizontal. For pursuit, the overall mean TAS was 0.52 for vertical and 0.55 for horizontal.
The TAS was close to the prediction of half-angle rule for both horizontal and vertical pursuit, and for horizontal but not vertical saccades. This is consistent with the previous finding that the thickness of Listing’s (another measure for the validity of LL) during pursuit is smaller than during saccades. This violation of half-angle rule during vertical saccades should be considered when evaluating LL and using data based upon saccades made in different directions.
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