Three-dimensional binocular kinematics of torsional vestibular nystagmus during convergence on head-fixed targets in humans

When a human subject is oscillated about the nasooccipital axis and fixes u pon targets along the horizontal head-fixed meridian, angular eye velocity includes a vertical component that increases with the horizontal eccentrici ty of the line-of-sight. This vertical eye movement component is necessary to prevent retinal slip. We asked whether fixation on a near head-fixed tar get during the same torsional vestibular stimulation would lead to differen ces of vertical eye movements between the right and the left eye, as the di rections of the two lines-of-sight are not parallel during convergence. Hea lthy human subjects (n = 6) were oscillated (0.3 Hz, +/-30 degrees) about t he nasooccipital axis on a three-dimensional motor-driven turntable. Binocu lar movements were recorded using the dual search coil technique. A head-fi xed laser dot was presented 1.4 m (far head-fixed target) or 0.25 m (near h ead-fixed target) in front of the right eye. We found highly significant (P < 0.01) correlations (R binocular = 0.8, monocular = 0.59) between the conv ergence angle and the difference of the vertical eye velocity between the t wo eyes. The slope of the fitted linear regression between the two paramete rs (s = 0.45) was close to the theoretical slope necessary to prevent verti cal retinal slippage (predicted s = 0.5). Covering the left eye did not sig nificantly change the slope (s = 0.52). In addition, there was a marked gai n reduction (<similar to>35%) of the torsional vestibuloocular reflex (VOR) between viewing the far and the near targets, confirming earlier results b y others. There was no difference in torsional gain reduction between the t wo eyes. Lenses of +3 dpt positioned in front of both eyes to decrease the amount of accommodation did not further change the gain of the torsional VO R. In conclusion, ocular convergence on a near head-fixed target during tor sional vestibular stimulation leads to deviations in vertical angular veloc ity between the two eyes necessary to prevent vertical double vision. The v ertical deviation velocity is mainly linked to the amount of convergence, s ince it also occurs during monocular viewing of the near head-fixed target. This suggests that convergence during vestibular stimulation automatically leads to an alignment of binocular rotation axes with the visual axes inde pendent of retinal slip.