Vergence eye movements under natural viewing conditions.

PURPOSE To determine whether there are any fundamental differences in vergence dynamics under different viewing conditions, both in instrument space and free space. METHODS Symmetric vergence responses were measured for a variety of conditions under the traditional instrument space as well as the more natural free space viewing environment. Vergence eye movements were recorded objectively in three subjects using the infrared reflection technique. Within each environment, four conditions were tested: aperiodic self-initiated voluntary gaze shifts between two simultaneously viewed targets; periodic (0.33 Hz) voluntary gaze shifts between the same two targets; gaze shifts in total darkness to the near target following initial far target fixation in the light; and gaze shifts in total darkness to the two remembered target positions. In addition, an experiment was performed in instrument space using randomized step changes of target disparity, in which the responses served as the standard for comparison. For all conditions, target disparities ranged from 0.5 degrees to 10 degrees. The peak velocity of each vergence response was calculated and plotted versus its amplitude. RESULTS It was found that the data for all conditions tested fell within the standard "main sequence" cluster, indicating similarity in dynamics and thus similarity in the motoneuronal controller signal. Also, the data from investigators who claimed differences in dynamics were also typically found to fall within the normal cluster. CONCLUSIONS This indicates that the vergence motoneuronal controller signal produced the same dynamics for a particular response amplitude, independent of both viewing environment and test condition.