Rotational and translational optokinetic nystagmus have different kinematics

Abstract

We studied the dependence of ocular torsion on eye position during horizontal optokinetic nystagmus (OKN) elicited by random-dot translational motion (tOKN) and prolonged rotation in the light (rOKN). For slow and quick phases, we fit the eye-velocity axis to vertical eye position to determine the tilt angle slope (TAS). The TAS for tOKN was 0.48 for both slow and quick phases, close to what is found during translational motion of the head. The TAS for rOKN was less for both slow (0.11) and quick phases (0.26), close to what is found during rotational motion of the head. Our findings are consistent with the notion that translational and rotational optic flow are processed differently by the brain and that they produce different 3-D eye movement commands that are comparable to the different commands generated in response to vestibular signals when the head is actually translating or rotating.

Figure 1

Angular eye orientation in head-fixed coordinates as a function of time for epochs of leftward rOKN and tOKN in the same animal. Time and position scales are as shown. Note that the torsion traces have been magnified relative to the horizontal and vertical traces to facilitate comparison of torsion in the two conditions. Here, and elsewhere, the right-hand rule is followed: positive directions are leftward, downward, and clockwise, from the perspective of the animal. For the first three traces, eye positions are shown relative to the standard coordinates of the coil frame. The fourth trace of each panel shows torsion in Listing’s coordinates. Note that there is a smaller range of torsion (in Listing’s coordinates) for tOKN, suggesting that the eye remains closer to Listing’s plane than it does during rOKN.

Figure 2

Eye velocity axis as a function of orbital position for rOKN and tOKN (M2). Each line represents the mean axis (torsional vs. horizontal eye velocity) for all slow phases within a vertical position window of 5°, centered on the indicated position. The shaded areas show the 95% confidence intervals. Axes are shown in the coordinates of the coil frame (not in Listing’s coordinates), in order to allow direct comparison with the axis of the visual stimulus.

Figure 3

Examples of tilt-angle slope (TAS) calculations for (A) rOKN, and (B) tOKN, for rightward slow-phases in M1. For each slow phase, the tilt angle (arctangent of the ratio of median torsional to median horizontal eye velocity) is plotted as a function of median vertical eye position. The dashed line shows the result of a least-squares linear regression, the slope of which is the tilt-angle slope. (C) and (D): Calculation of TAS for quick phases. There is a smaller number of quick phases than slow phases, because quick phases that had peak vertical velocities exceeding 15% of the peak horizontal velocity were excluded from the analysis. For TAS calculations, data were expressed in Listing’s coordinates.

Figure 4

Mean tilt-angle slopes (TAS) for each condition in all animals. As indicated in the legend, each symbol and line represents the TAS (mean of both eyes and both directions) for an individual monkey. The superimposed bars indicate the mean of the values from the three animals. TAS for slow-phases were different (p < 0.012, one-way ANOVA), but the corresponding difference for quick phases did not reach significance (p=0.057).