VS117 - Coop#3
J. Szeto
Dr. Schor
2-3-98
The handout serves as a parallel study guide to help us organize the material for chapter 16-18. Please refer to the reader for the corresponding figures in chapter 4 and 16.
Objective eye movement measurement techniques
Electro Oculography (EOG)
Most clinically utilized technique
Utilizes electrical property of eye: retina generates a negative voltage
causing the cornea to assume a positive voltage to establishes an equilibrium
of voltage potentials. By putting a pair of electrodes on skin next to
nasal and temporal canthi of the eye, the electrosensors pick up the positive
voltage from front of the eye. If we subtract one electrode out from the
other (e.g., subtract the left from right) and if eyes move to the left,
we will get a negative voltage. If the eyes move to the right we will get
a positive voltage. If we put electrodes above and below the eye and subtract
the lower from the upper, we get a positive voltage when the eyes look up,and
we will get a negative voltage when eyes look down, .
Used a lot with infants and people who have difficulty communicating
Limbal tracker (IR reflection)
one application is to record eye movement when reading
shines light all over patientís eye and the white sclera will reflect
more light than the dark iris.
When eyes are looking away from sensor, sclera reflects lots of light.
When eyes move away from primary position, one sensor will see more of the
sclera than the other which will see more iris. The sensor that sees more
sclera will have more activity. Optometrist like it very much since it
is noninvasive and inexpensive
Video based systems (Pupil trackers)
measures the change in the location of the pupil center as the eyes move
around.
Can be useful for remote sensing of eye position in navigation tasks e.g.
pilots, young and old drivers, also used in ads, industry to determine features
that attract attention.
Search coil
put circular annular CL with coil on patientís eyes
patient sits in a large magnetic field
coil is moved as patient rotates the eyes and this generates voltage
the voltage tells us the direction of ocular gaze with respect to the magnetic
field and room.
Very uncomfortable and invasive technique
a lot of neurologists use it
Most of these electronic devices are mainly used to study dynamic eye
movement,
They are not used for strabismus. Static tests like the cover test is
used for strabismus. The objective tests are used to evaluate dynamic problems
in conjunction with neurological problems, as well as reading
Chapter 16 Accommodation and Presbyopia
Accommodation
Ability of the eye to vary its dioptric power to focus images over a range
of viewing distances on retina.
It makes retina conjugate to the target we are looking at in space, called
conjugate point of optical system
We want to shift the conjugate point at the retina to different distances
in space
Different possible mechanisms of accommodation:
Non-optical solution
1. Pinhole pupil
limit the light that enters the eye, called paraxial bundles
advantage-very clear image
disadvantage-cuts out a lot of light
nature invented the vertical slit pupils, e.g., cats, reptiles to gain more
light.
When we lose our ability to accommodate, we use the pupil to focus. As
we age our pupils get smaller to help us form a clear image. Our pupils
begin to constrict with accommodation when we are around 20. Prior to that
it only constricts with light level. That ís why older people need
a lot of light to read
Optical solutions
1. Change axial length
Dynamic variations - move image further back and change the axial length
of the eye
e.g., accordion eyes of lamprey eels-shorten the axial length of the eye
by flattening the front of the eye to look at something far away, and lengthens
the axial length for things up close.
Static variations- eyes with variable axial lengths
e.g., horse has a ramp retina. The upper long axial portion of the retina
sees the lower part of the visual field. The short upper axial length sees
the upper part of the visual field.
The process of the eyes grow to normal refractive error (emmetropizaation)
occurs by changing axial length in response to blur.
At birth, infants have hyperopia (2-3D). Blur of retinal image created
by hyperopia simulates the growth of the axial length of the eye, eventually
the length of the eye matches the power of the eye and far targets are imaged
on the retinal without accommodation. As we start accommodating more accurately,
blur is decreased and the eye stops growing.
Emmetropization sometimes gets out of control when we read a lot. In a
condition called lag of accommodation (eye accommodates less than the stimulus
producing a hyperopic error), the extra blur stimulates increasing growth
of axial length and leads to near-sightedness.
We don't want to fully correct a child with very high hyperopia with spectacles,
we want to leave a little hyperopic blur there to stimulate the axial length
growth of eye. However, it is OK to correct myopia because it has already
beyond the stage of blur that stimulate the growth of eye.
2. Change the power of the cornea
myope squint their eyes to see better by making their pupils smaller and
their corneas flatter
Dynamically, eel and owl compress the eye to make the axial length shorter
and flatten the cornea to reduce the power when viewing distant targets.
Statically, some animals can have different corneal powers because they
have cylindrical eyes. One meridian of the cornea is flat to allow them
to see far away, while the other meridian is bulged to see near. They don't
see the whole but part of the image clear.
3. Change the power of the lens
Dynamically, e.g., amphibious birds can accommodate 40-50D by pinching the
front surface of their lenses. They have to get very close to things they
want to see. When they dive under water, their corneal power is reduced
so they increase the power of their lens to make up the lost power of the
cornea. In contrast, we accommodate by flattening the lens to focus target
far away by dilating the ciliary body and stretching the lens capsule.
Statically, e.g., anableps sit on the surface of the water to catch insects.
They have two different powers on the lens due to two different optical
axis. When they look under water, there is no effective corneal power and
the lens provides the majority of the power. With air phase, cornea refracts
light and the lens power is weaker. This is called ìdouble optical
systemî or ìphysiological eye bifocalî.
4. Change the position of the lens (translation) within the eye
move the lens close or far away from the cornea to alter the power. The
power increases as the cornea and lens move closer together.
e.g., eels push the cornea back towards the lens to increase the power
5. Change internal structure of the lens (index of refraction)
lens has layers, it continue to increase in size as we grow, and eventually
it becomes opaque and has to be removed surgically. There are layers in
which index of refraction (n) is constant, called isoindical surfaces.
Lens has different n values for each layer. As the light travels through
the lens, the difference in n refracts the light. This results in a higher
lens power than if you just added the power of the front and back surface
of the lens.
As you age, the lens grows and becomes more curved. However, the power
of the lens remains constant. This is called the lens paradox. Explanation:
The increase in lens power due the increased curvature is offset by the
decrease in the difference in the index of refraction (n) which becomes
more uniform with age..
2 optical zones in Gullstrand schematic eye-nucleus of the lens has n=1.406,
cortex of the lens has n=1.386. The real lens has many opical indices which
are highest at the nucleus and lowest in the cortex of the lens.
History of accommodation
Scheiner in 17th century was the first one to show that the eye could accommodate.
He used a double pupil that caused targets to have double images when they
were out of focus. In the figure below, a distant target (house) is seen
as single image in the unaccommodated eye. The near target (pencil) is
seen as double since it is focused behind the eye and results in two images
on the retina (A). When we accommodate and focus on the near target, the
distant target is formed in front of the retina and we see two images of
the distant target on retina (B).
We can put a red filter on the top pinhole and a green filter on the bottom
pinhole. When the eye is unaccommodated, upper hole is formed on the upper
retina, and lower hole is formed on the lower retina. If the eye is accommodating,
upper hole is on lower retina and lower hole is on upper retina, so green
light is on the top and red light on the bottom.
A lot of optometric devices measure the power of the eye based on Scheiner's
principle. We determine the focus error from measured separation of 2 retinal
images and get the direction for out of focus by order of upper and lower
pupil.
Helmholtz developed relaxation theory of increasing accommodation that
involved a passive mechanical lenticular mechanism (relaxation of the lens)
during an active muscular contraction (CB) mechanism.
Anatomy of accommodation
The lens zonules (suspensory ligiments) are attached at the lens,
ciliary processes and posteriorly at the ora serrata. When ciliary muscle
contracts, the attached point of the zonules moves forward, anterior zonule
relaxes and stretches posterior zonule causing the eyes to become accommodated
(more power). When the CB dilates, the lens is stretched and its power
is reduced.
Passive mechanism: Fincham described the capsule of the lens as an
elastic structure with memory; it keeps returning to its original shape.
The body of the lens is plastic and it tends to stay whatever shape the
capsule places on it. When the lens is stretched, it gets flattened by
increasing the diameter and decreasing the anterior and posterior thickness
to keep the same volume.
When the stretched force is released, the lens changes curvature most in
the flattened region of the anterior surface. When the eyes are accommodating,
the least curved region becomes as curved as the posterior pole.
When we are accommodating, the posterior surface of the lens stays
the same, mainly anterior becomes more curved and hence the diameter of
the lens decreases.
When we accommodate, 3rd Purkinje image becomes a little smaller because
the front surface of the lens becomes more curved. The first and the 4th
of the Purkinje images stay the same sizes.
When we accommodate, all the internal isoindical surfaces change so the
power of the lens changes through out.
Non-uniform curvature of the lens and cornea produce spherical aberration
of the eye (variation of power of the eye at different eccentric centers
of the pupil) . When we look far away, the power of the lens is slightly
greater away from the center of pupil than along center of the pupil producing
positive spherical aberration ( a myopia of about 1.5 D).
If pupils get dilated, we get more power and also get a little myopia.
At night, our pupils dilate and manifest the spherical aberration. When
we accommodate (pupils get smaller), the power of the center of the lens
increases and hence the spherical aberration is reduced. When we accommodate
3D, the lens surface is nearly spherical (no aberration).
If accommodation is greater 3D, we get a negative spherical aberration,
in which lens is more powerful in the center and less powerful in the peripher
(a hyperopic error).
Take home message: when pupilís size changes, the power of the eye can be changed.