Influence of orientation of exiting wire of search coil annulus on torsion after saccades.

PURPOSE Research has shown that the orientation of the exiting wire from a scleral search coil (nasal in the original coil, inferior in the modified coil) influences the pattern of torsion associated with blinks, presumably due to the upper eyelid touching the wire. The present study was conducted to investigate whether coil wire orientation also influences the pattern of torsion during fixation after saccades and consequently influences parameters that determine Listing's plane and Donders' law. METHODS Three-axis, binocular eye rotations were recorded in three healthy human subjects looking at an array of small light-emitting diodes at a 124-cm distance. They made predominantly vertical or predominantly horizontal saccades (5-40 degrees, amplitude) at three (0 +/- 20 degrees) azimuths, or elevations. RESULTS With the modified coil, the horizontal component of Listing's primary position was similar for horizontal (mean temporal deviation, 2.9 degrees) and vertical (3.3 degrees) saccades. With the original coil, the horizontal component of primary position was more temporal after horizontal (12.3 degrees) than vertical (6.1 degrees) saccades. The vertical component of primary position was similar with both coils after horizontal and vertical saccades. Listing's plane was thinner with the modified coil after horizontal (standard deviation of torsion from a plane of 0.71 degrees vs. 1.31 degrees) and vertical (0.84 degrees vs.1.67 degrees) saccades. The standard deviation of torsion at a given eye position (Donders' law) was smaller with the modified coil. CONCLUSIONS Measures of torsion after horizontal or vertical saccades depend on whether the coil wire exits nasally or inferiorly and, with the original coil, whether the saccades are horizontal or vertical. Lid contact may account for these potential artifacts. They must be appreciated in the interpretation of three-dimensional search coil recordings, using the scleral annulus.