Visual Direction II

Question 1

Define Eccentric Fixation

Answer 1

Some abnormalities (like strabismus, amblyopia) of the visual system, a point other than the fovea may be used to determine visual direction

Question 2

Which disorders can cause eccentric fixation?

Answer 2

• 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

Question 3

True or False: A pt who fixates with an extrafoveal retinal locus will always have eccentric fixation.

Answer 3

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.

Question 4

Name the differences between eccentric fixation vs. eccentric viewing

Answer 4

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)

Question 5

The Hering window experiment

Answer 5

Binocular Visual Direction

Question 6

Hole in hand demonstration

Answer 6

Binocular visual Direction, Demonstration of egocentric visual direction

Question 7

Define corresponding retinal points

Answer 7

pair of points, one in each eye that, when stimulated simultaneously or rapidly in succession, are perceived to lie in identical directions

Question 8

Ocular Dominance

Answer 8

• 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

Question 9

Veith-Muller Circle

Answer 9

predicted theoretical location of points in space whose images will be formed on corresponding points in the retina

Question 10

_____ and _____ are basic building blocks of our pereception of distance & depth

Answer 10

Visual direction & corresponding points

Question 11

Define corresponding points

Answer 11

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

Question 12

Define Disparity

Answer 12

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 a_R or n=a₁-a_R

Question 13

Define vertical disparity

Answer 13

Vertical differences in the positions of similar images

Question 14

Define horizontal disparity

Answer 14

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)

Question 15

Corresponding retinal points have ____ disparity

Answer 15

Zero, n= a₁ - a_R = 0

Question 16

In crossed disparity, angle n, the difference betwen the binocular subtense angles a_L and a_{<span>R </span>}is _______.

Answer 16

Positive a_L is larger than a_R

Question 17

In uncrossed disparity, angle n, the difference between binocular subtense angles aL and aR is ____

Answer 17

negative such that a_R is larger than a_L

Question 18

Diplopia results when….?

Answer 18

binocular disparity is too large

Question 19

Small disparities

Answer 19

visual system is able to combine them into a single percept

Question 20

The upper limit of this disparities that can produce the single percept is determined by the _______.

Answer 20

Panum’s area

Question 21

Small differences in perceived horizontal direction between two eyes give rise to a percept of _______

Answer 21

3-D depth or stereopsis, this principal is used to create stereograms

Question 22

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

Answer 22

False. Indirectly

Question 23

Diplopia

Answer 23

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

Question 24

Object whose image fromed on corresponding retinal points have ____ visual direction

Answer 24

Same

Question 25

Objects whose images are formed widely separated noncorresponding retinal points are perceived as arising from ____ visual directions

Answer 25

different

Question 26

The nature of diplopia depends on…?

Answer 26

the placement of noncorresponding retinal images

Question 27

What happens when a distant object is fixated bifoveally?

Answer 27

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

Question 28

Binocular confusion

Answer 28

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

Question 30

Diplopia & confusion are both the result of…?

Answer 30

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

Question 31

Diplopia of the fixation points occur because..?

Answer 31

fixation target is not stimulating the corresponding points

Question 32

confusion occurs because…?

Answer 32

the two foveas do not point to the same direction

Question 33

Most strabismic pt learn to eliminate diplopia or confusion by..?

Answer 33

suppression (switching off the input from the strabismic eye)

Question 34

Physiological diplopia can be useful in evaluating _____

Answer 34

Binocular vision

Presence or absence can provide important insight

Not only tell us about pt binocular vision (presence/absence) but also about binocular fixation

Question 35

Brock String technique

Answer 35

takes advantage of physiological diplopia to make a pt aware of binocular percept and degree of convergence

Question 36

Anomalous Retinal Correspondence (ARC)

Answer 36

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

Question 37

Anomalous Retinal Correspondence (ARC) clinical assessment

Answer 37

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