Lenses and Space Perception

1
Lenses and Space Perception
2
What happens with anisometropic prescription

• Example
• Plano right eye
• 3.00 -300 left eye
• Implication for space perception
• What happens to the horopter

3
Aniseikonia

• Defintion Anisekonia is a difference in
  magnification between the two eyes that effects
  the perception of size and shapes of objects.

4
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

5
Aniseikonia

• Induced aniseikonia is a retinal image size
  difference caused by a special lens such as an
  afocal magnifier or size lens.

6
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.

7
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.

8
(No Transcript)
9
Magnification Formulas

• Power Factor
• Mp 1/1-hFv
• h is the vertex distance, and Fv is the back
  vertex power.

10
Magnification Formulas

• Shape Factor
• Ms 1/1-(t/n) F
• t is the lens thickness, n is the index of
  refraction, and F is the front surface power.

11
Geometric Effect

• Magnify in the horizontal meridian or axis 90.
• This creates disparities in the horizontal plane
  in the eye with the magnifying lens

12
(No Transcript)
13
Geometric Effect

• The patient would perceive the plane rotated away
  from the eye with the magnifying lens
• What would the horopter look like?

14
(No Transcript)
15
(No Transcript)
16
Rotation

• The degree of rotation can be calculated by the
  following formula
• Tan q (M-1/M1) (d/a)
• M is the magnification of the size lens, d is the
  viewing distance, and a is ½ the interpupillary
  distance.

17
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

18
(No Transcript)
19
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.

20
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.

21
Induced Effect

• Poorly understood
• May be an indicator of eccentricity

22
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.

23
(No Transcript)
24
Tolerance of Aniseikonia

• 1-2 usually tolerated
• Greater than 5 will affect stereopsis
• Greater than 20 eliminate binocular vision.

25
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.

26
(No Transcript)
27
(No Transcript)
28
How do we adjust to lenes

• Baseline
• Inducing/Adaptation
• After effect/Decay

29
How do we adjust to lenses?

• Full adaptation
• Partial adaptation
• No adaptation

30
(No Transcript)
31
Adaptation to Size Lens

• 10 college students wore axis 90 or axis 180 size
  lens over one eye for 2 weeks.
• Each day students would comment on level of
  adaptation
• Measured horopter and Eikenometer each day

32
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

33
(No Transcript)
34
Linear Perspective

• Monocular depth cue
• Parallel lines converging towards the horizon

35
(No Transcript)
36
(No Transcript)
37
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

38
(No Transcript)
39
Video

• http//video.google.com/videoplay?docid2048269788
  799603527

40
Effect of prisms on space perception

• Prisms shift images toward apex
• Prisms either base out or base in to compensate
  for horizontal deviation

41
Effect of prisms on space perception

• Prisms create a curve in space perception
• The base creates a curve towards the patient

42
(No Transcript)
43
Aniseikonia

• Under treated problem but difficult to diagnose
  because the tests have problems
• Magnification difference

44
Aniseikonia

• Anisophoria or dynamic aniseikonia
• Phoria changes in different eccentricities
• Prismatic effect of lenses
• Prentice law

45
(No Transcript)
46
(No Transcript)
47
Space Eikonometer-

• Best test
• No longer available

48
Space Eikonometer-

• Uses septum to divide targets
• Measure axis 90, 180 and declination errors
• Uses 5 vertical lines and a cross
• See examples

49
(No Transcript)
50
What lens would cause this?
51
X90 size lens OD
52
What would cause this?
53
X180 size lens OD
54
What would cause this?
55
X 045 OD, x 135 OS(135 meridian OD, 045 meridian
OS)
56
New Anisekonia Test

• Direct comparison test
• Vertical and horizontal meridians
• Underestimates

57
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
  (red-green target).

58
(No Transcript)
59
Aniseikonia inspector

• Direct comparison test
• Underestimates
• Poor reliability

60
(No Transcript)
61
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)

62
(No Transcript)
63
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
  hetereophorias may also be a factor in both
  validity and reliability.

64
Maddox Rod Test

• Do not need special instrument
• Maddox rod and two penlights
• Need size lenses to neutralize differences

65
(No Transcript)
66
Clinical Management

• Knapps 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.

67
Knapps Law

• Theoretically, treat AXIAL anisometropes with
  SPECTACLES
• And treat REFRACTIVE anisometropes with CONTACT
  LENSES
• But Knapps law does not always work clinically

68
Knapps 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

69
(No Transcript)
70
Treating Aniseikonia

• What if the patient cant wear contacts?
• We can change the design of spectacle lenses to
  reduce/eliminate the aniseikonia

71
Estimate of Aniseikonia

• 1 to 2 per diopter difference
• 3 diopter difference could yield 3 to 6
  magnification differences

72
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)

73
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
New
OD 1.75 5.5 3.1 1.49 12
OS 3.75 5.5 3.1 1.49 12

74
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

75
(No Transcript)
76
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

77
(No Transcript)