Chapter 6: Aniseikonia

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Chapter 6 Aniseikonia
Definition Inter-ocular difference in
magnification
Type A All meridian magnification (no
stereoscopic effects) associated with
anisometropia. Type B Meridional Magnification
(creates stereoscopic tilting plane illusion)
created by ophthalmic corrections (i)
horizontal meridional magnification (ii)
vertical meridional magnification (iii) oblique
axis meridional magnification (iv) meridional
magnification that varies across the visual
field
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Examples of B-scan ultrasound images from two
eyes of an anisometrope. Notice that the primary
differences is axial length.
Right Eye
Left Eye
All significant anisometripias are caused by
inter-ocular differences in axial length and not
optical power
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Magnification Retinal image size can be easily
estimated by tracing the undeviated nodal ray.
Remember, for a focused image, all rays
coincide at the image plane, thus any ray will
give the same image size. The nodal ray is
preferred because of ease since it is undeviated.
Nodal ray
Emmetrope
n
However, if the image is not focused, then each
ray arrives at a different retinal location (each
forms a different part of the blur circle). In
this case, we must use the Chief ray to determine
image location since it forms the center of the
blur circle.
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Using the chief ray we can see that retinal image
size is larger in uncorrected axially myopic eyes.
Axial Myope
Chief ray
Emmetrope
But what will happen when we correct the myopia?
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Correcting ametropia with a spectacle lens
Using the anterior focal point ray, the ray is
parallel to the optical axis in the eyes image
space. By placing the correcting lens in the
eyes anterior focal plane, the lenss undeviated
nodal ray is the eyes anterior focal point ray.
Thus, in the eye corrected with a spectacle lens
placed at the anterior focal plane will have the
same retinal image size as an emmetropic eye.
Axial Myope
Emmetrope
Fe, NL
Graphic Demonstration of Knapps Law retinal
image size is independent of level of ametropia
when corrected at anterior focal plane
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Correcting ametropia at corneal plane (contact
lens, refractive surgery, orthokeratology)
Corneal plane correction
Chief ray
Axial Myope
Emmetrope
This technique changes the optical power of the
first and most significant refracting surface, in
so doing, it changes the optical power of this
surface, and thus changes the eyes nodal points,
anterior focal point, principle planes etc. The
only ray that is relative stable is the chief
ray. We thus use the chief ray to assess image
quality in this case.
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Prediction based upon Knapps Law Anisometropes
corrected with SL will have same size image in
both eyes and thus no Aniseikona. This
prediction is solely based upon optics, but of
course axial ametropia is created by expanding
the vitreal chamber and thus the area of the
globe that the retina must cover.
Knapps Law prediction (correct anisometropia
with SL to avoid aniseikonia) does not work. For
example, objects appear smaller in the myopic eye
of anisometropes corrected with a SL
(minification).
Axial Myope with retinal stretching corrected
with SL
Axial Myope with retinal stretching corrected
with CL
Emmetrope
Neural image 4 PR tall
Neural image 3 PR tall
Neural image 4 PR tall
CL not SL may produce equal perceived size (no
aniseikonia) in axial anisometropia
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Part 2 Meridional Aniseikonia and the
stereoscopic tilting plane illusion
Note that RE horizontal magnification, makes
objects in the right field appear to move to the
right (temporal, uncrossed), while objects in the
left field appear to move to the left (nasal,
crossed), and thus we see a stereo tilting plane
illusion in which the right half tilts away and
the left toward.
Geometric Effect caused by an inter-ocular
difference in horizontal magnification
X90 magnifier
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The Tilting Plane Illusion is created by
spectacle magnification, which is a combination
of the power and shape factors. The power
factor is the most significant
Shape factor
Power factor
Note Fv is back vertex power, and h is back
vertex distance
Power factor is approximately 1.5 per diopter
(h0.015 meters) and varies directly with lens
power and thus refractive error. Note with CL, h
is approximately zero, and thus magnifications is
also approximately zero. Because of this,
retinal image size is larger in axial myopes
corrected with CL than with SL.
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Axis 90 lens
This figure is designed to show the impact of
meridional magnification on image location.
Notice that Horizontal meridional magnification
alters the horizontal location of images, and as
with all magnification, images farther from the
optical axis are moved more. Notice also, that a
direct consequence of the above statement, is
that lines oblique with respect to the meridian
of magnification will be tilted by this
meridional magnification.
axis
Axis 180 lens
Axis 135 lens
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1. Geometric Effect Inter-ocular difference in
horizontal magnification creates an inter-ocular
difference in horizontal position (Horizontal
Disparity) which increases linearly with
increasing eccentricity, i.e. a horizontal
gradient of HD will exist, and thus a FPP will
appear to have increasing depth with increasing
eccentricity which will appear as a tilt in this
plane.
2. Induced Effect is created by inter-ocular
differences in vertical magnification, which will
introduce vertical disparities but no HD. Since
there will be no HD induced, what is causing the
stereoscopic tilting plane illusion which is
opposite in sign to that of the Geometric Effect?

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Explanation of the induced effect
From this analysis, can you figure out why a
spherical magnifies does NOT generate a tilting
plane illusion?
The inter-ocular difference in vertical
magnification produced by a monocular axis 180
lens replicates the retinal images experienced
during asymmetric convergence, and thus a zero HD
plane will not appear normal to the egocentric
visual direction, it will appear tilted.
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Geometric Effect, x90
Remember these cases
Perceived tilt in FPP
FPP
The plane that will appear fronto parallel AFPP
X90, min
X90, mag
X90,mag
Induced Effect, x180
X180 min
X180 mag
X180 mag
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Declination Error Stereoscopic tilt illusion
generated by inter-ocular differences in oblique
axis magnification
Right Eye
Left Eye
Notice that a vertical line viewed through an
oblique axis meridional magnifier will appear
tilted such that if the top of the line appears
moved to the right, the bottom will appear moved
to the left. If this effect is mirror symmetric
in the right and left eyes (oblique astigmatism
usually is), then the change in visual direction
will be left/right opposite in the two eyes. In
the above example, the new visual directions of
the top of the line will to the right in the
right eye and to the left in the left eye
(uncrossed HD), whereas the bottom of the line
will be moved to the left in the right eye and to
the right in the left eye (crossed HD)
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Summary
Inter-ocular differences in oblique axis
magnification creates a stereoscopic tilting
plane illusion in which the FPP appears to tilt
around a horizontal axis axis
Inter-ocular differences in axis 90 and 180
magnification create a stereoscopic tilting plane
illusion in which the FPP appears to tilt around
a vertical axis.
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How to prevent tilting plane illusion in patients
with inter-ocular difference in astigmatism?
1. Eliminate spectacle magnification by using CL
or corneal-based correction
2. Fortunately, this depth illusion seems to
disappear over time. The visual system slowly
adapts to the new magnification and the tilt
slowly disappears over days and week.
Perceived Tilt (deg)
zero tilt
1
2
3
0
Lenses on
Time (weeks)
Lenses off
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Aniseikonia that varies across the visual
field Lenses created approximately uniform
magnification effects across the entire field,
but prisms create a field varying magnification
effect, which causes a tilting plane illusion
that varies across the field. Instead of the
Fronto Parallel Plane being uniformly tilted, it
appears curved.
Notice that objects in the right field are seen
through the prism apex in left eye, and through
prism base in right eye. Because prisms displace
images more through apex than base, this is
equivalent to seeing the right field with an axis
90 magnifier in front of the left eye, and,
conversely, seeing the left field with an axis 90
magnifier in front of the right eye. Thus the
right field will tilt forwards on the right, and
the left field forwards on the left (review
previous notes on this). Thus, the FPP will
appear curved and concave toward the patient.
FPP
Perceived Curvature of FPP
BO
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Field curvature through BI prisms
The opposite curvature will happen with BI
prisms, because in this case, the right field
will appear magnified through the right eye.
Can you see why in this case the field will
appear curved and convex to the patient?
Perceived Curvature of FPP
FPP
BI