Orthoptics I - Introduction to Binocular Single Vision

What is BSV

the ability to use both eyes simultaneously so that each eye contributes to a common single perception

2 types of BSV

bifoveal
monofoveal

Describe how BSV comes about

eyes individually receive dissimilar views of an object known as binocular disparity
these two images are fused and the slight disparity allows for depth perception

Why is BSV important?

allows for stereopsis
impacts balance and motor skills
more accurate/quicker cognition

What are the main requirements for BSV?

structurally normal eyes (normal development)
normal, overlapping visual fields
normal retinal correspondence
fine VA

Why would having reduced VA in one eye disrupt BSV? Why is this not always the case?

eyes not receiving images similar enough to fuse, so will be perceived individually = diplopia

individual tolerances vary between how much disparity the brain can handle

What is normal retinal correspondence?

each retinal element/node localises a specific visual direction in space - basically does the fovea project straight ahead
- nasal retina -> temporally
- temporal retina -> nasally
- superior retina -> inferiorally
- inferior retina -> superiorly

What happens when non-corresponding retinal points are stimulated?

diplopia

What is the horopter?

an imaginary line in space - all objects that stimulate corresponding retinal points lie on this line
- objects on or near the horopter will be seen as single
- the further away the point of focus, the greater the size of the horopter

What is Panums fusional space?

an area in space around the horopter where objects will still be perceived as one, providing corresponding retinal points are being stimulated

What happens if you look outside of panums fusional space?

non-corresponding retinal space to be stimulated = physiological diplopia

What is physiological diplopia?

a normal physiological occurrence when objects outside of panums fusion range are perceived

What are the two subgroups of physiological diplopia?

homonymous (uncrossed) or heteronymous (crossed)

What do we not notice physiological diplopia?

suppression

What is homonymous physiological diplopia?

uncrossed
distance object appears doubled when near object is fixation target
- image separation increases as distance of further object gets further away

What is heteronymous physiological diplopia?

crossed
near objects appear double when distance object is the fixation target
- image appears on the opposite side when you close an eye
(right eye closes, left images disappears

What is simultaneous perception?

the ability to simultaneously perceive two images, one presented to each retina
- preamble to BSV

How can simultaneous perception go wrong?

if images are disparate, binocular rivalry can occur if the two presented images are too incongruous
- some fluctuation of dominance may be noticed

What happens if one eye is presented a stimulus that the other isn't?

the eye viewing the stimulus will become dominant and the other eye ignored