Influence of textured backgrounds on fusional vergence: preliminary results using an eye tracker

Purpose: The vergences ensure and maintain binocular fixation and binocular vision. The amplitude of vergences depends on many factors, one of which is the amount of fusible material on the binocular field of vision. In casual seeing the whole surface of the two retinas is exposed to similar stimuli and the conditions for motor fusion should be optimal. The retinal periphery is a powerful factor in production of fusional movements and makes an important contribution to stabilization of relative position of the eyes. The purpose of the study was to evaluate whether the presence of a textured background in the visual stimulus may influence the convergence fusional response. Methods: Eleven subjects with normal binocular vision underwent fusional disparity stimuli: a base-out prism of 6 diopters was placed in front of the right eye to elicit disparity. Eye movements were continuously recorded using the Viewpoint infrared video eye tracker (Arrington Research, Scottsdale, AZ, USA) and a custom software was developed to automatically analyze off-line the saccadic response (gaze angle). Each subject was examined twice, once by presenting the visual stimulus (black dot) on a white background, then by presenting the same stimulus on a textured background (Figure 1). The order of presentation of the two tests was randomly chosen. Results: Eye tracking recordings showed a typical pattern for the fusional behavior: a preliminary saccadic movement of both eyes (version phase) and a following convergence movement with the eyes moving towards each other to achieve single vision (vergence phase) (Figure 2.a). When presenting the stimulus on the white background only 6 subjects (54%) achieved fusional convergence. This number increased up to 9 subjects (82%) when presenting the same stimulus on the textured background (Figure 2.b). Conclusions: The fusional vergence response can be accurately studied and evaluated using an eye tracker. In subjects with normal binocular vision the presence of a textured background seems to facilitate the fusional convergence ability