Measurement Of The Expansion And Contraction Effects Of Vertical Yoked Prism.

Vertical yoked prisms (VYP) are used by some practitioners as an alternative to the classical use of base in and base out prism in the successful treatment of convergence
insufficiency and convergence excess. VYP are used as an associating device in which there are equal amounts of prism with the same base direction in front of each eye.
'l'he theory of using vertical prism is based upon the displacement of the visual world caused by the optical properties of the prism in which the perceived object is projected toward the apex of the prism. Therefore, a person wearing VYP base down would perceive the object as
being projected upward while a person wearing VYP base up would perceive the object as being projected downward. Adaptation to the expansion and contraction effects of
VYP creates a behavioral modification affecting the relationship between eye movement and body orientation. According to Kaplan , since the object is peceived to be projected
upward with VYP base down, the object will also be perceived as being larger and further away. The apparent change in size and position forces the patient to reorganize his
visual and postural systems toward a broader and more open visual perception which corrects for the convergence excess. On the other hand, the object that is perceived to be projected downward with VYP base up will also be perceived as being smaller and closer than the actual object.
This decrease in size and distance enables the person to "tighten up" and
reorganize his/her visual and postural systems to correct
for convergence insufficiency.?