The Effects of Distorted Optic Flow in Multifocal Glasses on Self-Motion Perception

Progressive addition lenses (PALs) are ophthalmic lenses mainly used to correct presbyopia. They improve far and near vision in different areas of the lens. This inevitably creates local distortions of the spatial field of view. Novice wearers of PALs often report unnatural motion percepts during self-motion. To better understand these effects, we analyzed the impact of the distorted field of view in PALs on the estimation of heading, eye rotation and scene layout in an established computational model of self-motion perception. We simulate distorted optic flow fields for a particular PAL design and a typical self-motion scenario, a straight walk over a flat plane while fixating a point on the ground. Analyzing these flow fields in the model showed changes in heading mainly along the vertical axis. A VR-based psychophysical experiment confirmed this prediction as participants perceived upwards deviations from the true heading when gaze crosses the peripheral lens areas. We then conducted a systematic investigation of distorted optic flow on model estimates of rotation and depth in the scene. We took the PAL distorted flow fields, presented them to the model as if they were normal flow fields and estimated the most likely heading, rotation and scene layout, i.e., those self-motion and scene parameters that provided the best fit. We found that PAL distortions induced systematic rotational components, including torsions that exceed what is physiological plausible. Moreover, uneven surfaces often yielded better fits for the distorted flow than the flat planes used to simulate the flow fields. This suggests that PAL distortions may cause illusionary self-motion percepts due to their distorting influence on optic flow. The strength of these percepts depends on the gaze direction through the lenses and might be remedied by a careful gaze strategy.