Lenses and Space
Perception
What happens with anisometropic
prescription

Example
 Plano
right eye
 +3.00 -300 left eye

Implication for space perception
 What
happens to the horopter
Aniseikonia

Defintion: Anisekonia is a difference in
magnification between the two eyes that
effects the perception of size and shapes
of objects.
Aniseikonia

Optical aniseikonia is caused by a
difference in retinal image size between
the two eyes and is caused by axial
anisometropia or refractive anisometropia
Aniseikonia

Induced aniseikonia is a retinal image size
difference caused by a special lens such
as an afocal magnifier or size lens.
Aniseikonia

Neural or essential aniseikonia is a small
non-optical aniseikonia that occurs with
the images should be optically equal but
are perceived as different.
Inducing Aniseikonia

Size lens is a thick lens with parallel front
and back surfaces that change
magnification but have no dioptric power.
They can create overall magnification or
have cylindrical power where
magnification is in only one meridian.
Magnification Formulas

Power Factor
 Mp
= 1/1-hFv
 h is the vertex distance, and Fv is the back
vertex power.
Magnification Formulas

Shape Factor
= 1/1-(t/n’) F
 t is the lens thickness, n is the index of
refraction, and F is the front surface power.
 Ms
Geometric Effect
Magnify in the horizontal meridian or axis
90.
 This creates disparities in the horizontal
plane in the eye with the magnifying lens

Geometric Effect
The patient would perceive the plane
rotated away from the eye with the
magnifying lens
 What would the horopter look like?

Rotation

The degree of rotation can be calculated
by the following formula:
 Tan
q = (M-1/M+1) (d/a)
 M is the magnification of the size lens, d is the
viewing distance, and a is ½ the interpupillary
distance.
Rotation
2% size lens axis 90 at 40 cm with a 6 cm
PD.
 Tan q = (1.02 – 1.00)/(1.02 +1.00) x 40/3
 =0.131
 7 degrees

Induced Effect
Magnify in the vertical meridian or axis 180
 Vertical disparities do not yield the
perception of depth
 Small amounts of vertical disparities lead
to diplopia due to limitation in vertical eye
movements.

Induced Effect
However, this lens produces tilt as if an
axis 90 lens was introduced on the other
eye.
 The effect breaks down with 5 to 7%
magnification.

Induced Effect
Poorly understood
 May be an indicator of eccentricity

Geometric & Induced Effects
The strengths of the two effects are close
to equal up to 2% of magnification.
 Uniform magnification of small amounts
have little or no effect on the horopter.
 Higher levels of magnification differences
produce distortions.

Tolerance of Aniseikonia
1-2% usually tolerated
 Greater than 5% will affect stereopsis
 Greater than 20% eliminate binocular
vision.

Oblique Magnification
Magnification at 45 or 135 degrees
produce rotation around the horizontal
axis.
 Inclination or declination effect
 Meridian 45 degree in the left eye and
meridian 135 degrees in the right eye
produces the upper part of the image as
being farther away and larger.

How do we adjust to lenes
Baseline
 Inducing/Adaptation
 After effect/Decay

How do we adjust to lenses?
Full adaptation
 Partial adaptation
 No adaptation

Adaptation to Size Lens
10 college students wore axis 90 or axis
180 size lens over one eye for 2 weeks.
 Each day student’s would comment on
level of adaptation
 Measured horopter and Eikenometer each
day

Adaptation to Size Lens




Students perceptual adaptation occurred around
4 days
Very little adaptation seen on the horopter or
Eikonometer
Corresponding points do not recalibrate
Implication: monocular cues to depth start to
predominate over stereo cues. Cue conflict
Linear Perspective
Monocular depth cue
 Parallel lines converging towards the
horizon

Adaptation of Induced effect
Lee and Ciuffreda
 Used size lens x 180 at 4.0%
 Modified Howard Dolman apparatus
 Found rapid initial response and then
quick adaptation
 Implications

Video

http://video.google.com/videoplay?docid=2
048269788799603527#
Effect of prisms on space
perception
Prisms shift images toward apex
 Prisms either base out or base in to
compensate for horizontal deviation

Effect of prisms on space
perception
Prisms create a curve in space perception
 The base creates a curve towards the
patient

Aniseikonia
Under treated problem but difficult to
diagnose because the tests have
problems
 Magnification difference

Aniseikonia
Anisophoria or dynamic aniseikonia
 Phoria changes in different eccentricities
 Prismatic effect of lenses

 Prentice
law
Space EikonometerBest test
 No longer available

Space EikonometerUses septum to divide targets
 Measure axis 90, 180 and declination
errors
 Uses 5 vertical lines and a cross
 See examples

What lens would cause this?
X90 size lens OD
What would cause this?
X180 size lens OD
What would cause this?
X 045 OD, x 135 OS
(135 meridian OD, 045 meridian OS)
New Anisekonia Test
Direct comparison test
 Vertical and horizontal meridians
 Underestimates

New Anisekonia Test




McCormack et al (1992 IOVS)
Compared the NAT to the space eikenometer by
inducing aniseikonia or using clinic patients with
aniseikonia
The NAT underestimated the amount of
aniseikonia when compared to space
eikonometer. (see graph)
Possible factors influencing test results: 1)
methodology, 2) angle of gaze, 3) sensory
fusional response that rescales the image (redgreen target).
Aniseikonia inspector
Direct comparison test
 Underestimates
 Poor reliability

Aniseikonia inspector
Antona et al (2007 IOVS)
 Looked at validity and reliability of the AI
by either inducing aniseikonia or
measuring clinic patients
 In induced patients showed an
underestimation that was greater in the
horizontal meridian. (see graphs)

Aniseikonia inspector
Antona et al (2007 IOVS)
 Looked at agreement between the testing
sessions and found the 95% confidence
intervals to +/- 2.0%.
 The underestimation or poor reliability
make interpreting test results difficult
 They added that fixation disparities and
hetereophoria’s may also be a factor in
both validity and reliability.

Maddox Rod Test

Do not need special instrument

Maddox rod and two penlights

Need size lenses to neutralize differences
Clinical Management

Knapp’s law: When a correcting lens is
placed at the anterior focal plane of an
axially ametropic eye the retinal image
should be the same as the emmetropic
eye.
Knapp’s Law

Theoretically, treat AXIAL anisometropes with
SPECTACLES

And treat REFRACTIVE anisometropes with
CONTACT LENSES

But Knapp’s law does not always work clinically
Knapp’s Law
Clinically may not work very well
 Awaya and Von Noorden measured
amount of aniseikonia in myopic
anisometropia.
 Put correcting lens at the anterior focal
plane did not eliminate the aniseikonia

Treating Aniseikonia
What if the patient can’t wear contacts?
 We can change the design of spectacle
lenses to reduce/eliminate the aniseikonia

Estimate of Aniseikonia
1 to 2% per diopter difference
 3 diopter difference could yield 3 to 6%
magnification differences

Treating Aniseikonia

Change the Magnification Shape Factor
 Front
surface power
 Center thickness
 Refractive index

Change the Magnification Power Factor
 Vertex
distance (difficult to do-must change
bevel placement)
Sphere F1
t
n
d
OD
+1.75
7.5 3.65 1.49 12
OS
+3.75
8.5 4.90 1.49 12
OD
+1.75
5.5 3.1
1.49 12
OS
+3.75
5.5 3.1
1.49 12
New
Vertical Horopter
Starts somewhere between the viewers
waist and feet and projects outward
intersecting the fixation point and
continuing in a straight line.
 If you fixate on a vertical placed wire both
ends will be seen as double until you tilt
the wire. Singleness criteria

Clinical Application
Study looking at screen tilt preference
 Subjects viewed different tilts and heights
 Rate level of comfort
 Preference for positive tilt in same
direction as horopter
