Postural stability and optic flow sensitivity following sight restoration from congenital bilateral cataracts

Vision is crucial for maintaining balance and facilitating locomotion. Optic flow, for example, provides key self-motion cues for navigation. Congenital blindness typically leads to increased postural sway and impaired navigation. Here, we investigated postural stability and optic flow sensitivity in individuals surgically treated for congenital dense bilateral cataracts years after birth. Experiment 1 assessed whether cataract-treated participants rely on vision to stabilize their stance with eyes open compared to closed. In contrast to controls, cataract-treated participants only marginally decreased their sway with open eyes, indicating a reduced ability to use vision for stabilization. Interestingly, they improved over time following surgery, suggesting partial learning in utilizing visual input to enhance stability. Experiment 2 assessed whether different radial and translational optic flow patterns elicit distinct effects on body sway, which would indicate illusory sensorimotor perceptions. We included a group of typically sighted controls and a group of controls with experimentally reduced visual acuity. While cataract-treated participants exhibited greater sway than controls, their sway was less influenced by specific optic flow patterns. Overall, the study showed that cataract-treated individuals exhibit only partial learning in utilizing vision for stabilization after surgery. Moreover, optic flow evokes less pronounced illusory self-motion perception compared to typically sighted individuals.

Methods
In Experiment 1, cataract-treated participants and sighted controls stood on a foam pad placed on a Nintendo Wii Balance Board. Their static balance was recorded under two conditions: with eyes open and closed. In the closed eyes condition, participants were blindfolded to ensure that even if they briefly opened their eyes, they would still be unable to see anything. The order of the two conditions was counterbalanced across participants, with each condition lasting 40 s. Data was recorded with a sampling rate of 30 Hz. Body sway was assessed by determining the area of the 95% confidence ellipse for each participant and condition.

In Experiment 2, we tested static balance in a group of cataract-treated participants, a group of age-matched sighted controls tested in normal visual conditions, and a second group of age-matched sighted controls tested with visual blur. Participants were tested during the presentation of a static baseline (static), during which a static field of 300 dots was displayed for 40 seconds. Subsequently, four conditions were presented in random order: expanding optic flow, contracting optic flow, translational motion (either left-to-right or right-to-left, counterbalanced across participants), and static (repeating the initial static field of dots). For conditions involving motion, optic flow patterns were presented for the initial 20 seconds, followed by 20 seconds of static dots. During this static phase, the final frame of the optic flow phase was displayed statically for 20 seconds. This solution allowed us to explore whether participants exhibited more sway during the presentation of optic flow patterns or as an after-effect once the motion stopped abruptly. Since there was no significant difference in sway between the first and second halves of each condition, we analyzed the entire 40 seconds together. We fitted the 95% confidence ellipse to the Centre of Pressure (CoP) data for each participant and condition. Then, we extracted the following standard parameters: the ellipse area, the standard deviations (SD) along the anteroposterior and mediolateral directions (AP-SD and ML-SD, respectively), and computed the ratio between the latter two.