Dynamic asymmetries in convergence eye movements under natural viewing conditions.

PURPOSE To clarify differences in the dynamics of convergence eye movements in response to symmetrical stimuli between the dominant eye and the nondominant eye under natural viewing conditions. METHODS This study was conducted in 11 normal volunteers (age range, 22-30 years). The subject was seated on a chair, and the head was fixed using a bite-block. The dominant eye was determined with the "hole-in-the-card" test. Horizontal positions of the eyes were recorded using an infrared reflection device. Recordings of right eye movements were subtracted from left eye movements to obtain the vergence response. The fixation target was moved toward the center of both eyes from 50 to 20 cm in front of the face at a constant velocity in five steps (20, 30, 40, 50, and 60 cm/s). The target was moved by a pulse motor controlled by a microcomputer. The dynamic properties of each eye movement and the vergence components were analyzed. RESULTS At velocities < or =40 cm/s, convergence responses consisted of two components: the fusion-initiating component and the fusion-sustaining component. The fusion-initiating component drove the initial faster dynamic portion of the response, and the fusion-sustaining component maintained the latter slower dynamic portion of the response. The mean peak velocity of the fusion-initiating component was significantly greater in the dominant eye than in the nondominant eye. The mean peak latency of the catch-up component was significantly shorter in the dominant eye than in the nondominant eye. CONCLUSIONS The present study indicated that the trajectory of convergence eye movement under natural viewing conditions consists of two components, the fusion-initiating component and the fusion-sustaining component, and that the convergence response to symmetric stimuli was asymmetric between the dominant eye and the nondominant eye during the fusion-initiating phase. The neural control system of convergence eye movements preferentially drives the dominant eye during the fusion-initiating phase of the response.