Effect of Artificial Scotomas on Open-loop Disparity Vergence Eye Movements

Vergence eye movements serve to align both eyes on the same target for single binocular vision. When a target moves in the midsagittal plane, symmetrical vergence eye movements occur; with targets located to one side of the midsagittal plane, asymmetrical vergence movements are induced.1–5 Maxwell and Schor’s6 research has shown evidence supporting a single binocular control mechanism for vergence while others endorse the role of a strong monocular component.3–5 Enright suggested that the asymmetrical vergence occurred because each eye independently responds to its own stimulus.3 To better understand control mechanisms underlying vergence eye movements, we have investigated open-loop disparity vergence responses and monocular responses in the vergence responses to asymmetrical disparity stimuli with monocular artificial scotomas. Open-loop disparity vergence responses (DVR) are automatic and can be elicited in monkeys and human subjects at ultra-short latency, 55 ms for monkeys and 85 ms for human.7–11 It has been shown that DVR emanate from binocularly processed visual signals mediated by the medial superior temporal area of the cortex (MST)12 We are particularly interested in the open-loop DVR because they provide a direct measure about the relationship between motor responses and sensory input of disparity. The disparity can be sensed directly by disparity-selective neurons in an early stage of the visual system.13–15 Another advantage is open-loop responses leave little chance for other factors, such as attention, to influence them during this period of time. When a longer time was allowed to obtain a full fusion of disparate stimuli, vergence is heavily influenced by visual feedback. In the present study, we report how a monocular artificial scotoma (MAS) influences the amplitude, latency, and symmetry of the DVR. The artificial scotoma is similar to a clinical condition of suppressive scotoma in patients with childhood strabismus16. Suppressive scotoma are common in strabismic subjects. The effect of artificial scotoma impact on vergence provides the potential for clinical application in suppressive scotoma measurements.