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  Ms = 1/1 (t/n’) F  t is the lens thickness, n is the index of refraction, and F is the front surface power.

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 Eikonometer  Best test  No longer available

Space Eikonometer  Uses 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 (red green 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)

OD OS New Sphere F1 +1.75

+3.75

7.5

8.5

OD OS +1.75

+3.75

5.5

5.5

t n d 3.65 1.49 12 4.90 1.49 12 3.1

3.1

1.49 12 1.49 12

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