BV3 - stereo distortions

what kind of magnification can afocal lenses have?

overall (spherical) → mag in every meridian

meridional (cylindrical) → mag in 90° meridian

Why is the leaf room a good demo?

rich in vertical and horizontal contours

unfamiliar to patient → no preconceived notions about its shape

How does a horizontal magnifier (axis 090) in front of one eye in the leaf room work?

magnifies β angle → must make β angle in eye smaller to compensate

How does a horizontal magnifier in front of one eye in the leaf room shift the VM circle?

closer to the eye with magnifier → shifts zero disparity forward → appears further

farther from the eye without it → shifts zero disparity backwards → appears closer

How does the leaf room appear with a horizontal magnifier in front of one eye?

larger and farther on side with magnifier

smaller and closer on side w/o magnifier

Where do the distance changes in the leaf room come from? Size changes?

distance → mag causes distortions affecting the VM circle

size → Emmerts law

What is Emmerts Law?

perceived size is proportional to perceived distance

ex: would misjudge object that is further away as larger

What do afocal magnifiers distort?

stereoscopic distances (by changing VM circle)

why is there no effect of (<4%) overall magnification on the leaf room?

vertical disparities somehow negate effects of lateral disparities

What are the characteristics of the induced effect?

1. stimulated by vertical disparities

2. explained as an overall size change → alters lateral disparity as well

3. its global → affects everything in the binocular field

4. limited to 4%

what is the effect of magification >4%?

larger disparities still drive induced effect but incomplete

remaining vertical disparities are ignored

what does vertical size differences trigger?

resizing of images

where do lines rotate with oblique magnifiers?

rotated toward magnification meridian

What are the effects seen with prism?

image shift → image deviated toward apex

Distortions:

1. asymmetric mag

2. curvature of lines: curved toward base

3. rotation of lines: rotated toward apex

4. overall mag

what distortion is asymmetric magnification (Base-Apical magnification)?

magnification increases toward apex

How does BO prism change the appearance of the leaf room?

concave shape:

center farther + larger

edges closer + smaller

How does BO prism change the VM circle?

shifts the edges of the VM circle out

How does BI prism change the appearance of the leaf room?

convex shape:

center closer + smaller

edges farther + larger

How does BI prism change the VM circle?

shifts the edges of the VM circle in

How does prism affect vertical lines?

bowed with middle toward base

How does BI prism change the appearance of vertical lines?

appears convex

How does BO prism change the appearance of vertical lines?

appears concave

What is the tilting plane apparatus used for?

quantifying stereoscopic spatial distortions → subject rotates target plane to appear fronto-parallel

what are the type of targets on the tilting plane apparatus?

1. ink splatter

2. vertical lines

3. oblique cross

What is the gold standard for measuring anisekonia?

space eikonometer

How does x 090 mag affect the space eikonometer target?

lines and cross opposite is closer

ex: R x 090 → lines of left and cross on left closer

How does x 180 mag affect the space eikonometer target?

only cross same side is closer

ex: R x 180 → cross on right is closer

How does OA mag affect the space eikononmeter?

only lines opposite side is closer

ex: ROA → lines on left closer

how does oblique magification affect the space eikonometer target?

top or bottom of cross only → direction is opposite leaf room

What eye movement does R x 045 + L x 135 mag cause in the space eikonometer?

stimulus for excyclofusion

How does R x 045 + L x 135 mag affect the space eikonometer target?

cross is closer at top

What eye movement does R x 135 + L x 045 mag cause in the space eikonometer?

stimulus for encyclofusional eye movement

How does R x 135 + L x 045 mag affect the space eikonometer target?

cross is closer on bottom

What is the purpose of cycloversions/conjugate eye movements?

compensate for head or body tilts

What is the purpose of cyclofusional/disjunctive eye movements?

compensate for inclinations of objects relative to the head

What is the stimulus for cyclofusional eye movements?

target inclination

What is unique about the space eikonometer?

1. no induced effect → distortion don't change below/above 4% mag

2. distinctive distortions for each mag (ex: R x 090 is not the same as L x 180)

3. declination (eyes eliminate visual inclination) of vertical lines in 0

why is declination of vertical lines 0 for eikonometer targets?

cyclofusional eye movements are complete ∴

oblique magnifiers distort only oblique cross