Automated cover testing for binocular misalignment in awake monkeys using spectacle-mounted liquid crystal shutters

Purpose: In humans, pathologic misalignment of the eyes (strabismus, or heterotropia) is readily detected using a clinical technique in which the visual axis of each eye is blocked briefly by a hand held occluder (i.e., the cover test, AKA the single cover test, or unilateral cover test). This manual technique is impractical for use in non-human primates. This paper reports the electronic hardware and computer software we have devised and adapted to perform an automated cover test in awake macaque monkeys (Macaca mulatta or rhesus, and Macaca nemistrina or pigtail) to facilitate the study of strabismus in this species. Methods: Liquid crystal shutters (Displaytech LV2500OP-OEM) are mounted in aluminum spectacle frames which are positioned in front of the monkey's eyes by attaching the frame holder to an acrylic skull cap. The shutters are electrically actuated alternately to an 'open' (transparent) or 'closed' (opaque) state, occluding each eye in turn. The actuation signal (±5 volts) is generated by a single logic chip. The chip is driven by commercial data control and acquisition software (Spike 2, Cambridge Electronic Design) running on a Macintosh computer. Eye position and movement during this cover tests is monitored and recorded by the computer using surgically implanted scleral coils. Monkeys are rewarded for fixating small, moveable targets so that binocular misalignment can be measured with high precision at different or cardinal positions of horizontal and vertical gaze. Results: The shutters produced occlusion of each eye as effective as that of an opaque plastic occluder used in previous experiments that required monocular viewing. Heterotropias were detected and recorded in monkeys and closely resembled those observed in human patients. It was also possible to detect heterophorias by actuating the shutters alternately. Conclusions: The shutters operated reliably, with negligible delay and transition times, providing automated, repeatable cover tests that are, in fact, in many respects superior to the standard clinical manual technique. In addition, their application to humans, using infrared electro-oculography instead of scleral coils to monitor eye position and movement, holds promise for future applications.