Vertical shear disparity processing in depth for inclination perception and cyclovergence

<p dir="ltr">This thesis investigates how binocular disparity information, namely vertical shear disparity (VSD)—horizontal gradients of vertical differences between the eyes' images—is processed over depth for inclination perception (horizontal axis slant) and cyclovergence (disconjugate torsional eye movement). It extends previous studies that were limited to stimuli at the fixation plane. Inclination was measured using tactile matching, and cyclovergence was assessed using a nonius line technique. The thesis includes three experimental chapters. Chapter 3 explores the effect of VSD in a surface away from the fixation plane on inclination and cyclovergence, and whether VSD-induced inclination is attributed to cyclovergence. The results showed that VSD across a wide range of depth separations from the fixation plane (±40 arcmin, defined by horizontal disparity) strongly induced both inclination and cyclovergence, with the induced inclination explained by cyclovergence. These results suggest that VSD processing outside the fixation plane improves cyclovergence and inclination accuracy. Chapter 4 examined VSD integration in two surfaces at different depths. The results revealed that VSD is integrated over crossed disparities (-40 arcmin) for cyclovergence, but only with negative VSD, indicating greater sensitivity to negative VSD in crossed disparity. For inclination, the results over crossed disparity were mixed; one experiment showed neither VSD integration nor independent processing, while another suggested VSD integration over crossed disparity. These differences were likely due to individual differences in depth interactions. For uncrossed disparity, both experiments supported independent VSD processing at fixation for cyclovergence and inclination, suggesting lower sensitivity to uncrossed disparity. Chapter 5 investigates VSD averaging at the fixation plane. The results suggest that lower absolute VSDs are more heavily weighted for inclination and cyclovergence, suggesting greater tolerance for small negative VSD values. However, when the negative VSD is larger, equal weighting for inclination emerged.</p>