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.