Stereopsis with normal and reversed binocular parallax using a head mounted display in normal and strabismic subjects

A virtual reality system using a head mounted display (HMD) was constructed for the purpose of finding out how human beings perceive the three-dimensional world. Four ophthalmologically normal subjects and 4 strabismic patients (2 with exotropia, 2 with esotropia) were examined. They wore the HMD and viewed some familiar objects (e.g. human face) and an unfamiliar object (e.g. tangled ropes) under normal parallax, reversed parallax, and monocular conditions. They also attempted the ring and hook test under each condition. They recognized the normal familiar objects under each condition, but some normal subjects were confused when they perceived unfamiliar objects. The normal subjects barely passed the ring and book test under the reversed parallax condition. The results of the strabismic subjects showed change under each condition. Stereoptic ability in normal and strabismic patients were considered based on the above. Also considered were some stereoptic keys, binocular parallax, monocular stereoptic keys such as shadows, shades, texture, etc., the near reflex and the fact that the objects were familiar. It was found that the strabismic patients used monocular stereoptic keys more than normal people to supplement their lack of binocular parallax perception.