Test 2 Flashcards
Physiological diplopia
If the binocular disparity of an objects image in the two eyes exceeds the limits of panums, the object will be perceived as double.
Ex: brock sting
Binocular confusion
When two DIFFERENT objects share the same visual direction, they may be perceived simultaneously in the same place- binocular confusion.
Dissimilar images falling on corresponding points.
Suppression/rivalry
To minimize/eliminate confusion or diplopia, our brains may ignore information coming from one eyes.
Perceptual adaptation associated with strab.
Anomalous retinal correspondence
Spatial remapping of visual direction associated with deviated eye.
Sensory remapping of one (abnormally deviated) retina to correspond to the retina of the other (normally fixating) eye.
The primary visual direction of the deviated eye is associated with some other non-foveal retinal location.
Usually associated with constant strab!
ARC is usually associated with
Constant strab
What two binocular sensory adaptations for strabs often exist simultaneously?
Suppression and ARC in the form of foveal suppression and peripheral ARC
Motor fusion
Movement of the eyes so that the two foveal are pointed at the same object. Pre-req for sensory fusion.
Sensory fusion
Neural combination in the brain of the two retinal images to form one unified percept.
What is a pre-req for sensory fusion
Motor fusion. Must be able to align both foveas on target in order for brain to combine them
How can you test for motor fusion?
Hirschberg
How do you cross fuse
Adduct both eyes so that the visual axes cross in front of the physical object plane. The physical card will now be in uncrossed space relative to the horopter where the visual axes are interesting.
How do you uncross fuse?
Relax vergence so that the visual axes cross behind the physical object plane. The physical card is now in crossed space relative to the horopter where the visual axes are intersecting.
Motor vs sensory fusion
Motor: Employs vergence. Involves EOMs to bifoveally fixate a desired target.
Sensory: Neural combination of the images from the 2 eyes that occur in the brain to form one percept.
2 ways to obtain a single unitary perception with 3 eyes
Fusion or suppression of either eye
Alternation suppression theory (not accepted today)
The view of one or the other eye is always suppressed. Our perception alternates so rapidly between eyes that we are never aware of the suppression. Serial.
Fusion theory (accepted today)
Information from both eyes is processed simultaneously in parallel so that we continuously perceive similar images from both eyes as single. Parallel.
2 normal exceptions to fusion
- Binocular rivalry/suppression if targets are dissimilar.
2. Stereoscopic depth
How to find the angular width of the extent of panums?
Have observer focus on two points superimposed. Move one point back until person perceives 2 points. Then forwards.
The linear width will differ depending on distance between observer and fixation point. Does not stimulate vergence.
Panums area will be largest based on what stimuli
Large, low spatial frequency at low temporal frequencies.
AKA
larger and longer stimulus duration
Panums is targer in the periphery due to receptive fields.
What magnification difference is tolerated peripherally before person becomes aware of aniseikonia?
6-7% tolerance in periphery.
Less is tolerated at the fovea.
Why does VT for strab usually begins in the periphery with large slow moving stimuli.
Greater disparity tolerated for large, peripheral stimuli
Utrocular discrimination
The ability under bino viewing to consciously determine which aspects of the bino info come from each of the two eyes. NOT possible.
Conscious knowledge of which eye receives which image is NOT a pre-req for stereo. Good. Don’t have to think about it. Would slow down our actions.
True or false: Conscious knowledge of which eye receives which image is NOT a pre-req for stereo.
true
Fixation disparity
Residual fixation error that may occur even when the phoria is compensated and sensory fusion occurs.
Can be central or peripheral
A small constant error of vergence present when similar stimuli are simultaneously presented to the two eyes.
Displaces the entire horopter from being coincident with the fixation stimulus to coincident with the true intersection of the visual axes.
FD is measured in what units
Minutes of Arc
Why does a FD exist/develop?
Provides an error signal needed to stimulate continued compensation of the heterophoria.
n absolute perfect compensation for heterophoria CANNOT exist
Can an absolute perfect compensation for heterophoria exist?
No
Is FD a fixed number? What can influence it?
Not a fixed number.
Test distance Dissociated phoria- if pt is eso vs exo Prism adaptation Size of test stimuli Presence of central vs peripheral fusion locks as well as their size and location of the pts fixation disparity.
Associated phoria
The amount of prism needed to fully compensate for FD
Even tho a FD is less than 1 pd, several pd’s may be needed to reduce the FD to zero.
Usually the neutralizing prism is in the same direction of the dissociated phoria.
2 motor responses to prism
Fast (Disparity vergence system): Latency less than 1 sec. Motor fusion’s attempt to eliminate the FD introduced by the prism. Exists only for horizontal vergence.
Slow (Vergence adaptation system): Adjustment of tonic/sustained mergence. Reduces effectiveness of prism. Better able to withstand long periods of use than fast system. Minimizes asthenopia during sustained demand.
Vergence adaptation is a good thin except in those pts who you want to Rx prism for.
Forced mergence FD curves. What are the X and Y intercepts?
Y: FD (arc min)
X: Associated phoria (prism diopters)
Slope: rate of change in FD with prism.
Type 1 FD
- How many people have this
- What shape indicates a healthy vergence adaptation?
70% distance and 60% at near
Not much adaptation. Accepts prism.
Flatter central region indicates healthy vergence adaptation. Not much change in FD with small amounts of added prism. These patients are more likely to be symptom free.
Type 2 FD
- How many people have this
- Found more often in ___ patients who adapt poorly to __ prism
25% at distance and near
Found most often in eso pts who adapt poorly to BI but adapt well to large amounts of BO. If you rx BO prism to compensate for their eso, the patient will adapt and the esophoria will remain the same.
Type 3 FD
- How many people have this
- Found more often in ___ patients who adapt poorly to __ prism
10% at near and 0% at distance
-Found more often in exo patients who adapt poorly to BO prism but adapt well to large amounts of BI. If you rx BI prism to compensate for their exo, the patient will adapt and the exophoria will remain the same.
Type 4 FD curve
-how many people have this
5% at distance and 5% at near
Adapts well to BO and BI prism.
FD curve. What does it mean if there is no flat portion?
Your patient does not adapt well to prism. This means prescribing prism may relieve their symptoms rather than inducing an even greater FD.
If you Rx prism, it will actually help.
FD what does it mean if the curve is flat?
Adaptation is occurring. These patients are likely to be symptoms free, but if you rx them prism, their response won’t change. They will still have the same phoria.
ARC (anomalous retinal correspondence)
An adaptation to strabismus of early childhood onset. A 45 yr old who develops a rt eso will not develop an ARC.
Avoids visual confusion from dissimilar images at corresponding points in the two eyes by neurologically remapping visual directions in the deviated eye.
Without ARC, it would result in visual confusion.
To diagnose ARC, you must first determine
If the fovea OD and OS have the same visual direction when both are viewing together. Must make sure there is no eccentric fixation!!!! Make sure that the target’s image fall on the fovea of each of the two eyes.
Hering Bielschowsky afterimage test used to diagnose ARC
1st: Have to make sure each eye has central fixation- not eccentric.
2nd: Tag strab eye’s fovea with a bright vertical line during monocular fixation.
3rd: Tag normal eye’s fovea with bright horizontal line during monocular fixation.
4th: Have patient binocularly fixate a point on a flat surface.
Abnormal RC: the vertical and horizontal lines are seen separate from each other. The space between them tells you where the foveas are projected in space.
Normal RC: Patient sees a perfect cross.
ARC angles
Objective angle (H) Subjective angle (S) Angle of anomaly (A) Point zero Point a
Objective angle (H)- Angle of ocular deviation measured objectively. CT, Krimsky (neutralize hirschberg with prism)
Subjective angle (S)- The angle of ocular deviation measured by some subjective test. Asking what pt sees.
Angle of anomaly (A) - the difference between the objective and subjective angles.
A= H - S
Point zero: Retinal point on the deviating eye where the true straight ahead image physically falls.
Point a: Retinal point on the deviating eye that acquires the anomalous oculocentric visual direction of straight ahead under binocular viewing.
Normal Retinal Correspondence relationship between
H, S, and A.
H= S A= 0
Harmonious anomalous retinal correspondence (HARC) relationship between H, S, and A.
Most common type
A= H
S= 0
Point zero and point a are the same point, but not on the fovea.
Since point zero of the strab eye corresponds with the fovea of the normal eye, both points project “straight ahead” and the images will appear superimposed.
Unharmonious anomalous retinal correspondence (UARC) relationship between H, S, and A.
H > S
H > A
S does not equal 0
Objective larger than subjective and angle of anomaly.
point a is located between the fovea and point zero.
The sensory visual system only partially compensates for the motor misalignment of the eyes.
Paradoxical anomalous retinal correspondence (PAC) relationship between H, S, and A.
Usually post-surgical.
Type 1:
A > H
S and H have opposite directions.
EX: See eso during CT, introduce BO and pt sees more jumping.
point a and 0 are on the same side of the fovea.
Type 2:
S > H
The retinal point in the deviating eye corresponding to the fovea of the fixating eye is on the opposite side of the true fovea than the image of the fixation point.
point a and 0 are on different sides of the fovea.
How to find the corresponding points on the retina
The eyes are dichoptically presented with super imposable stimuli.
The positions of the shape seen by the deviating eye is moved until it is super imposed with other target.
The retinal point when the two images are superimposed corresponds with the non-deviating eyes fovea.
Hess Lancaster Test
Differentiates between accommodative and non-accommodative deviations.
SUBJECTIVE measurement of the misalignment of images.
Pt wears red filter over OD, sees green light.
Pt wears green filter over OS, sees red light.
Covariation
Existence of both NRC and ARC in the same strab patient.
Due to intermittent trope. ARC exists when strab is manifested, and may vary throughout the day. with NRC present during normal motor fusion.
3 proposed mechanisms of ARC
- Sensory theory. ARC is a sensory adaptation to constant strab.
- Motor theory. ARC is a response to abnormal innervation/signals provided to the EOMs.
- Abnormal disparity vergence detection. Most recent.
ARC causes this trap.
No definitive theory. Must work on sensory and motor in VT
The geometric horopter (VM) assumes what
The corresponding points are equally spaced throughout the retina and that there are no variations mag.
Helmholtz proved that when eyes converge on a near point in a oblique direction, the horopter becomes
A twisted cubic curve
Convergence isn’t always in the horizontal plane, nor is it always symmetric.
Empirical horopter
Does not coincide with VM geometric horopter. The shape changes depending on the viewing angle.
The best steroacuity is achieved when
The observers visual axes actually intersect at the intended fixation point. AKA no fixation disparity.
Stereothreshold
How close you can bring an object to the fixation point/horopter and still detect a difference in depth.
Stereo for a depth judgement made relative to a non-horopter point is worse than when made relative to the horopter.
Difference between geometrical and empirical
Geometric: Goes through fixation and nodal point.
Empirical: Located where visual axes intersect- not always right on fixation point due to FD.
Best stereo acuity
Best stereo acuity for any location in the F is achieved when comparison is made relative to the horopter.
Objects with zero disparity stimulate sensory fusion with no stereopsis.
How do we know if a given object is on the observers horopter?
Be perceived as single
Not stimulate a reflexive motor vergence response when introduced into the field of view.
5 ways to locate/measure the horopter
Identical visual directions/nonius horopter Equidistance (AFPP) Singleness (haploid) Minimum stereoacuity threshold Zero vergence
Identical visual directions/nonius horopter (way to measure horopter)
Both eyes fixate at rod. Right eye sees upper half, left eye sees lower half.
Pt is asked if the top line is in line with bottom line. If not, that means the target rod is not on the horopter.
The rods are moved closer/further from the observer until the observer states that the upper and lower rods appear to be aligned. Subjective.
Identical visual directions/nonius horopter (way to measure horopter)
When the targets are physically lined up in a row, the person fixating on the center target will see
When the targets a physically lined up in a semi circle, the person fixating on the center target will see
A semi circle. Not aligned.
A straight line. Right and left eye images are perceived to be in the same direction.
Equidistance (AFPP)- way to measure horopter
What we do in lab.
More precise and easy to do with untrained subjects.
Way to measure horopter- Singleness Haploid
Measures the extent of panums fusional zone at the fovea and at eccentric locations. The central rod is always fixated. The test rod is moved closer until diplopia is perceived, then further until diplopia is perceived. This ends panum’s zone.
The center of the zone is the singleness horopter.
Minimum stereoacuity threshold - way to measure horopter
Takes a long time, done in research.
Fixate on central rod. Move test rod slowly until a change in depth is detected. Continue doing this at different eccentricities.
Zero vergence- way to measure horopter
Not available. Viewer wears eye movement device.
Viewer fixates centrally.
Test rod appears suddenly- if the flashed target is on the horopter, there will be no reflexive vergence eye movement.
Equation used to describe mag difference between both eyes
R= tan alpha 2/ tan alpha 1
Alpha 2= right eye
Alpha 1= left eye
R is relative uniform magnification.
For every point on the VM circle, R=
- Geometric fact. Doesn’t have to do with what person is perceiving.
For objects on the VM circle, there is no uniform difference in image mag between the 2 eyes.
Angles positive and negative
Alpha 2 is larger than alpha 1. What does this mean?
Positive angles- to the right
Negative angles- to the left
Take absolute values of the angles and the larger number is more magnified.
The larger number has more physical separation of the images.
The typical empirical horopter tends to be ___ curved than the VM circle
Less sharply
Hering-Hillebrand horopter deviation (H)
The difference in curvature between the empirical horopter and VM circle.
Reflects non-uniform relative mag across the VF
H (difference in curvature between empirical and VM circle)
H=0
H > 0
H < 0
Equal
Empirical is flatter than VM
Empirical is curvier/steeper than VM
The deviation in the curvature of the empirical horopter from the VM is due to
Non uniform neural magnification.
The perception of temporal retina is magnified relative to nasal retina to offset the physically greater magnification of nasal retina for points farther than the VM circle.
The nasal retina is physically more magnified, so your temporal retina’s perception is more mag’d to equal the image out.
When you put plus in front of the right eye, how does the horopter change?
Horopter will tilt towards eye with more mag.
The analytical plot
Measured for right eye only.
For every point on the observer’s horopter, calculate R and plot it.
The slope is H- difference in curvature of the empirical and VM
Typical value for H (slope) is +0.1 to +0.2, indicating that the empirical horopter is flatter than the VM.
R0; value of the angle at fixation. Difference indicates what
Indicates the overall differences of magnification between the two eyes at the fixation point.
This causes a perceived rotation of space- the horopter moves towards the eye having the more mag.
R0= 1
R0 > 1
R0 < 1
R0 = 1 indicates no rotation of perceived space/mag
R0 > 1 indicates OD magnification
R0 < 1 indicates OS magnification
Difference between H and R0 and what are their values.
H is related to horopter CURVATURE- difference between nasal vs temporal retina of each eye individually.
H=0 Equal
H > 0 Empirical flatter than VM. Nasal retina perceived as smaller angle than temporal.
H < 0 Empirical more curved than VM. Nasal retina perceived as larger angle than temporal. Uncommon.
Non-uniform- magnitude of magnification changes.
R0 is overall differences in magnification between the two eyes at fixation. ROTATION. Ratio.
R0 = 1 indicates no rotation of perceived space/mag
R0 > 1 indicates OD magnification
R0 < 1 indicates OS magnification
Nasal packing
The physical area subtended on the right nasal retina is perceived as subtending a smaller area than the same physical space subtended by the left temporal retina.
Temporal field is packed into a smaller perceptual space on nasal retina.
You will perceive temporal field as being smaller. So you must make that angle bigger.
Monocular asymmetries
Neural elements closer together in temporal retina.
Abathic distance
The distance from the viewer at which the empirical horopter is flat. Usually 6m.
Fixation distance= interpupillary distance/ H AKA the deviation between empirical and Vm.
How does the H deviation change at different fixation differences?
It doesn’t. Net result stays the same.
AFPP vs OFPP
AFPP: Horopter is curved. We perceive it as flat. Empirical.
OFPP: Horopter is in physically straight line. We perceive as curved.
Geometric/theoretical/VM Vertical horopter vs empirical
Geometric: At near fixation distances, a vertical line running through the point of fiction.
Empirical is tilted approx 20-30 degrees with the top farther than the bottom.
How does the vertical horopter change as you bring something closer to you?
As you bring something closer to you, the vertical horopter tilts more, making it more similar to the horizontal horopter of a crawling infant.
The horopter with exo or eso tropia
Exo: Excessive horopter concavity/more steep.
Eso: Flom notch. Distortion of horopter within the objective angle of deviation. in children, as similar targets are moved towards corresponding points in the two eyes, they will be perceived as moving toward and then jump past the point of correspondence.
What does it mean that the horopter is the reverse of perception.
A physically flat plane is seen as < so I make the horopter like > to perceive the plane as flat.
The empirical horopter is usually seen as flatter than the geometric/theoretical. This flattening can be explained by
Nasal packing. Temporal field is perceived as smaller than objects in nasal field. Even tho the two objects physically subtend the same angle on the retina.
Two examples of uniform magnification
Ophthalmic and size lenses (Shape factor, no power factor)
Refractive or axial anisometropia
Two examples of non uniform magnification
Prism - image bowed towards the base. Greater mag at the apex of prism.
Lateral aniseikonia- difference in image mag between the two eyes.
Size lens
Thick lens with parallel front and back surfaces. No refractive power/change in focus, only magnification.
Overall magnifier- magnifies entire image equally in all directions.
Meridional magnifier- Magnification is perpendicular to cylindrical axes. Ex; axis 90 magnifies horizontally.
Neural vs optical aniseikonic
Neural/essential: Non optical. Retinal images have the same size but are perceived different.
Optical: A difference between the two eyes in size or shape of the retinal images of an object.
Optical uniform: Can be refractive or axial (length)
Optical nonuniform: Lateral aniseikonia due to prism or spec lenses.
Sources of oblique mag
Cylindrical refractive lenses at oblique axes
Cyclorotary eye movements
Upward divergent anisokonia
Axes are tilted up and away, mag are tilted up and in.
Upper portions appear enlarged and tilted away.
Downward divergent aniseikonia
Axes are tilted down and away,
Upper portions appear smaller and tilted towards observer.
1D of Anisometropia = __% anisekonia.
1.4
Knapps law for refractive anisometropes
CLs
Knapps law for axial anisometropes
Place second principal plane of correcting lens to same axis of the am tropic eye.
