Dynamic response of the vertical vergence system

Purpose. Earlier reports suggested that the vertical vergence system has a narrow disparity range and a gain which fails to compensate for disparity even at low frequencies. This contrasts with the small fixation disparities reported in the clinical literature for disparate stimuli of up to several degrees. The displays used in earlier reports either suffered from correspondence ambiguity, were not described adequately, or did not provide a strong stimulus for vertical fusional movements. We investigated the frequency response of vertical vergence for several amplitudes using a well-defined textured stimulus which filled a large binocular field. Methods. The magnetic scleral search coil technique (Robinson, 1963) was used to record the movements of both eyes in two subjects. Dichoptic stimuli were projected on the screens mounted on opposite sides of the coil frame and were viewed through mirrors in a Wheatstone stereoscope configuration. The stimulus was a textured display which subtended 65° visual angle and consisted of various geometric shapes which provided a strong unambiguous fusional stimulus. The two images were oscillated in counterphase at frequencies of 0.05, 0.1, 1.0, 2.0 and 4.0 Hz for amplitudes of 0.25, 0.5, 1.0, 2.0 and 4.0° of peak disparity. Results. Vergence gain was nearly unity and phase lag was small for low frequency disparity modulation. At high frequencies phase lag increased and gain fell indicating a low pass response. The response was nonlinear showing decreased gain at high frequencies. At the highest frequencies and amplitudes, diplopia occurred as the response deteriorated. Conclusions. When using a stimulus designed to optimize a vertical vergence response, a compensatory low frequency vertical vergence response was obtained. This work demonstrates a low pass response of the vertical vergence system with a nonlinear dependence on stimulus amplitude.