Study of fusional convergence using eye tracking: preliminary results on subjects with normal binocular vision

Purpose: In the field of ophthalmology, vision researchers use eye tracking to study oculomotor behavior, cognitive visual function and vision deficiencies. Fusional vergence is a disjunctive movement of the eyes that is made in order to obtain single vision with two eyes. Fusional vergence disorders are often associated with the most common forms of strabismus. The aim of the study was to characterize fusional convergence response in subjects with normal binocular viewing by using an eye tracking system. Methods: Base-out horizontal prisms of 4 and 6 diopters were used to elicit fusional convergence of the right eye in 11 subjects with normal binocular vision. Eye movements were continuously recorded using the Viewpoint infrared video eye tracker (Arrington Research, Scottsdale, AZ, USA) and custom software was developed to automatically analyze off-line the saccadic response (gaze angle and angular velocity). The Fusional Convergence Amplitude (FCA) was calculated by subtracting the baseline position of the right eye (i.e. before the introduction of the prism) from the maximum saccadic excursion of the right eye during the disconjugate eye movements in the vergence phase (Figure 1.a). Results: The typical eye-movement response in subjects achieving the fusional convergence was a preliminary saccadic movement of both eyes (version phase) followed by a convergence movement with the eyes moving towards each other to achieve single vision (vergence phase) (Figure 1). Eye tracking recordings showed that 2 patients were not able to achieve fusion with neither 4 nor 6 diopters prisms. Fusion was achieved in 8 subjects when using the 4 diopters prism and only in 6 subjects when using 6 diopters prism. One patient was excluded from the analysis because the saccadic movements were highly affected by blink artifacts. The FCA varied from 2.2°±0.2 to 3.0°±0.2 when increasing prism diopters from 4 to 6 (Figure 2). Conclusions: The fusional convergence can be accurately studied and evaluated using an eye tracking system. Preliminary results on subjects with normal vision showed that when increasing the prism diopters the fusional convergence ability decreases