Visual Direction II Flashcards
Eccentric Fixation Eccentric Viewing Binocular Visual Direction & Corresponding Retinal Points (Veith-Muller Circle) - Ocular Dominance -Binocular Disparity -Diplopia and Confusion -Anomalous Retinal Correspondence
Define Eccentric Fixation
Some abnormalities (like strabismus, amblyopia) of the visual system, a point other than the fovea may be used to determine visual direction
Which disorders can cause eccentric fixation?
- Common consequence of strabismus
- Contributes of vision loss in amblyopia
- Possibly consequence of sensory visual distortions that are most pronounced at the fovea of strabismic eyes
True or False: A pt who fixates with an extrafoveal retinal locus will always have eccentric fixation.
False. A pt who fixates w/ an extrafoveal retinal locus does not necessarily have eccentric fixation
Ex. In low vision pt whose foveal vision is compromised by disease processes an extrafoveal point may be used (trained) for fixating eye to identify objects & reading. this is called eccentric viewing. The pt picks a retinal locus with better resolution than the diseased fovea to serve as a new fixation point.
Name the differences between eccentric fixation vs. eccentric viewing
Eccentric Fixation
- A point other than the fovea may be used to determine visual direction
- Typically develops in childhood on concert with strabismus and amblyopia
- Visuoscopy is used to diagnose. If the foveal reflex does not appear in the center of ophthalmoscopic target (when pt is fixating), eccentric fixation is present
Eccentric Viewing
- Require conscious effort on part of pt (low vision) to utilize the undamaged part of the retina
- Typically occurs after loss of vision from injury or disease & is a normal adaptation to the visual loss rather than the abnormality
- Pt still uses the destroyed fovea as a zero direction reference (the principal visual direction has not changed)
The Hering window experiment
Binocular Visual Direction
Hole in hand demonstration
Binocular visual Direction, Demonstration of egocentric visual direction
Define corresponding retinal points
pair of points, one in each eye that, when stimulated simultaneously or rapidly in succession, are perceived to lie in identical directions
Ocular Dominance
- Monovision CL
- Dominant (preferred) hand vs. Dominant eye
- Preferred eye vs. Dominant eye
- Taken into account when prescribing spectacle or CL corrections to avoid binocular vision problems
Veith-Muller Circle
predicted theoretical location of points in space whose images will be formed on corresponding points in the retina
_____ and _____ are basic building blocks of our pereception of distance & depth
Visual direction & corresponding points
Define corresponding points
can be thought of as a reference point or zero point for stereopsis
- images from corresponding points are perceived as arising from targets at the same distance
The perception of depth arises from stimulation of noncorresponding retinal points
Define Disparity
Images of a single object that do not stimulate corresponding retinal points
If observed monocularly these images would be perceived as lying in different visual directions
The difference in visual direction (of the images) between corresponding points in the 2 eyes is called binocular disparity
Disparity is denoted by n = defined as a difference in binocular subtense angles a1 and aR or n=a1-aR
Define vertical disparity
Vertical differences in the positions of similar images
Define horizontal disparity
Horizontal differences in positions of similar images
allows perception of stereoscopic depth
can be classified as crossed or uncrossed in relation to the fixation point
Images w/ crossed disparity are perceived nearer, wheras those with uncrossed disparity appear to be more distant than the fixation point (within VM circle)
Corresponding retinal points have ____ disparity
Zero, n= a1 - aR = 0
In crossed disparity, angle n, the difference betwen the binocular subtense angles aL and a<span>R </span>is _______.
Positive aL is larger than aR
In uncrossed disparity, angle n, the difference between binocular subtense angles aL and aR is ____
negative such that aR is larger than aL
Diplopia results when….?
binocular disparity is too large
Small disparities
visual system is able to combine them into a single percept
The upper limit of this disparities that can produce the single percept is determined by the _______.
Panum’s area
Small differences in perceived horizontal direction between two eyes give rise to a percept of _______
3-D depth or stereopsis, this principal is used to create stereograms
True or False: Vertical disparities do not directly give rise to percept of depth as do horizontal disparities, but they can still affect our percept of depth directly
False. Indirectly
Diplopia
Images on widely separated corresponding points (that’s beyond Panum’s area), will not be fused as single
This phenomenon is called physiological diplopia because it is NORMAL, pathological diplopia - BV disorders
Object whose image fromed on corresponding retinal points have ____ visual direction
Same
Objects whose images are formed widely separated noncorresponding retinal points are perceived as arising from ____ visual directions
different
The nature of diplopia depends on…?
the placement of noncorresponding retinal images
What happens when a distant object is fixated bifoveally?
a nearer object in front of it will be imaged on the temporal retina of each eye on noncorresponding points
if these points are far enough apart (to be fused), diplopia results
This is called crossed diplopia
Binocular confusion
Result of dissimilar images formed on corresponding points, in which 2 different objects are seen as being in the same visual direction (or location)
converse of diplopia
occurs when image is formed on one locus in one eye but a nonidentical image is fomred on its corresponding point in the fellow eye
Result = 2 different objects are superimposed in the same location, predominantly the foveal area
Diplopia & confusion are both the result of…?
misalignment of the eyes
Ex. Strabismus (eye turn) is a major consequence of diplopia and binocular confusion
Can be very devastating to visual function of the patient - hard to navigate in a confused/diplopic visual world - to reach out a object if another object is also seen at the same location
Diplopia of the fixation points occur because..?
fixation target is not stimulating the corresponding points
confusion occurs because…?
the two foveas do not point to the same direction
Most strabismic pt learn to eliminate diplopia or confusion by..?
suppression (switching off the input from the strabismic eye)
Physiological diplopia can be useful in evaluating _____
Binocular vision
Presence or absence can provide important insight
Not only tell us about pt binocular vision (presence/absence) but also about binocular fixation
Brock String technique
takes advantage of physiological diplopia to make a pt aware of binocular percept and degree of convergence
Anomalous Retinal Correspondence (ARC)
Pt might use a point other than the fovea to fixate under binocular conditions
Some strab pt may learn to remove diplopic image not by suppressing, but by “remapping” the corresponding points
The extrafoveal point in the strabismic eye and the fovea of the nonstrabismic eye now serves as the zero reference
Now, there is no diplopia or confusion in the strab pt b/c all the points in the VF are again corresponding
The mechanism underlying ARC are cortical
stereopsis is always affected
Clinical assessment: involves determination of suppression or ARC as the VT to treat the two differ
This can disrupt stereopsis
Anomalous Retinal Correspondence (ARC) clinical assessment
Sometimes during the course of therapy some pt may exhibit both normal and abnormal correspondence at the same time
this may result in transient binocular triplopia (binocular condition) or monocular diplopia
Goal of VT tx for strab pt is to break down anomalous correspondence and reestablish normal correspondence
