Resets of torsional eye position errors in different lighting conditions.

To investigate a resetting mechanism of torsional eye position errors, spontaneous scanning eye movements and visually guided eye movements in different lighting conditions were recorded three dimensionally. Two monkeys (Macaca fuscata, TS, MI) were engaged in this experiment. A dual scleral search coil method was used for three-dimensional (3-D) eye movement recordings. In complete darkness, the thickness of Listing's plane at the onset of spontaneous saccades (0.51/0.37 degrees (TS/MI)) and that at the end of spontaneous fixation periods (0.47/0.34 degrees (TS/MI)) were significantly (P<0.001) smaller than that at the end of spontaneous saccades (0.59/0.45 degrees (TS/MI)) and that at the onset of spontaneous fixation periods (0.58/0.44 degrees (TS/MI)). Such differences in the thickness of Listing's plane were not observed in the light (P>0.10). Amplitude of torsional drift during post-saccadic fixation period was correlated with the amount of torsional position error at the end of saccade (P<0.001). The slope of regressed line in the dark or dim light (-0.40 to -0.50/-0.32 to -0.33 (TS/MI)) was steeper than that in the light (-0.04/-0.03 to -0.09 (TS/MI)). A resetting mechanism for torsional eye position errors during post-saccadic fixation periods is active in the dark but inactive in the light.