Alternative to Fusion

Question 1

Define Binocular fusion

Answer 1

2 eyes, 2 images = SINGLE PERCEPT

Question 2

Define motor fusion

Answer 2

Eye mvmt based

Question 3

Define sensory fusion

Answer 3

Function of visual cortex which requires a similarity of 2 monocular images, otherwise diplopia, suppression of confusion

Question 4

What are the 4 types of Binocular fusion

Answer 4

• Binocular confusion
• Binocular suppression
• Binocular Rivalry
• Binocular Luster

Question 5

Describe Binocular confusion

Answer 5

• Binocular confusion - occurs when very different images are formed simultaneously on corresponding retinal points in the 2 eyes
  
  • Results in incorrect percept of different objects simultaneously occupying same location
• As the visual system cannot fuse grossly dissimilar images into a single unified percept - it must use other mechanisms to handle the differing monocular images

Question 6

Describe binocular suppression

Answer 6

• Occurs when visual system - ignores (turn off) all or part of image to one eye to avoid conflicting information between 2 eyes
• Arises from binocular interaction that suppresses some monocular information from binocular percept
• Reduced sensitivity (increased thresholds), prolonged reactiontimes occur in suppressed region
• Suppresion may not occur instantly with diplopia & confusion. May take up to 75-150ms to begin

Question 7

Which eye is suppressed?

Answer 7

A. Similar target/orientation - binocular fusion

B. Dissimilar target/orientation - suppress one eye (here left)

Question 8

What’s hapening in this image?

Answer 8

• Combined binocular percept
• Local suppression of some of left eyes information
• Local suppression occurs only under binocular conditions
• Binocular suppression at times may seem abnormal but at times, inhibitory binocular processes (suppression) can be normal and helpful
  
  • Ex. Avoding physiological diplopia near fixation

Question 9

What is physiological suppression

Answer 9

• Normal binocular suppression to avoid diplopia is called physiological suppression
• Example
  
  • Fixate near target while observing distant target
  • If objects are further from or closer than object of regard and beyond Panum’s area, then suppression
• Clinical Application
  
  • ​Pathological suppression is response to (adapatation to) more prolonged diplopia
  • Starbismus (who suppresses) . have dominant (fixating eye) and nondominant eye (Some can alternate - alternating strabismus

Question 10

Which eye will the binocular suppression occur?

Answer 10

• In general, if the 2 eyes are equal in their contribution to the visual system ,
  
  • The eye presented with a weaker or less salient image will be suppressed
• An image that is
  
  • Dimmer
  • Lower contrast
  • Blurred
  • Stationary
  • In retinal periphery
  • Will be more likely to be suppressed than bright, high contrast, sharply focused, moving image at the fovea

Question 11

Describe Binocular rivalry

Answer 11

• When dissimilar contours are presented to corresponding retinal areas fusion becomes impossible and retinal rivarly may be observed
• Simultaneous exciting of corresponding retinal areas by dissimilar objects does not permit fusion and leads to confusion
• In order to remove this confusion, image from one of the eyes is suppressed
  
  • This constant foveal suppression of one eye with cessation of rivalry leads to complete sensory dominance of the other eye, which is a mjaor obstacle to binocular vision
  • Return of retinal rivalry is a requisite for re-establisment of binocular vision

Question 12

Binocular Rivalry is referred to as _______

Answer 12

• Altnernating perception
• Happens when ocular images are equal in salience (clarity)
• The brain cant choose between the images and so you get rivalry
  
  • The images flip back and forth in position
  • At any instant in time, one image will be visible and the image of the othe reye wont be visible
• This is true physiological suppression and then the eyes swap
  
  • The eye that was suppressed is now seeing and the seeing eye is now suppressed
  • Pt rarely notice the rivalry because we dont know how often it happens and it is also in peripheral vision

Question 13

Intermittent/alternating suppression due to _____ binocular inputs

Answer 13

• Dissimilar
• May be suppression of brightness, color, and/or contour (line)
• Characteristics of suppression depend on size of target
  
  • Exclusive dominance - small targets
  • Mosaic dominance (local rivalry) - large targets
  • If 2 (left & right) oblique lines are fused, the percept will be of continously changing patchwork of oblique lines - as different regions of the figure are supposed
• These independent areas are called spatial zones of binocular rivalry
  
  • ​similar in size to cortical hypercolumn receptive field

Question 14

T/F Binocular rivalry is noticed mostly in the periphery

Answer 14

• Binocular rivalry is rarely noticed in periphery
• Patient reports of frequent rivalry (confusion) with the object they are attending to are abnormal and suggest the presence of ocular misalignment. Confusion is the clinical synonym for binocular rivalry
• Stimuli of similar visual strength (A) may be perceived alternating 50% of the time
• A visually stronger stimulus (e.g. higher contrast B) may be perceived for more than 50% of the time

Question 15

Describe Da Vinci Stereopsis?

Answer 15

• Binocular rivalry is not limited to artificial conditions
• Da Vinci stereopsis
  
  • ​Distant object partially occluded by nearer object
  • Near object and part of distant object fall on corresponding points
  • Lack of matching points in each eye’s view is clue to relative depth

Question 16

What is the difference between dichoptic masking vs monocular suppression?

Answer 16

• Binocularly rivalry is similar to other inhibitory processes
  
  • Dichoptic masking
    
    • ​target to be detected presented to one eye
    • Mask that reduces visibility of the target presented to the other eye
    • However, behavior of Binocular Rivalry differs from that of Dichoptic Masking; both could be combined - so not same phenomenon
  • Monocular suppression
    
    • ​Seen in saccadic eye mvmts
    • However, similarities do not prove shared mechanisms
  • Although rivalry is though to be a higher level phenomenon, the exact locus is unclear

Question 17

Diplopia vs Binocular Confusion vs Rivarly?

Answer 17

Question 18

Describe the Determinants of Fusion, Suppression and Rivalry

Answer 18

• What determines which particular mechanism of combining information (fuse, suppress or undergo rivarly from the two eyes used)
• The most important determinant is similarity of targets in each eye
  *

Question 19

Small rivalrous contours tend to be ______ and large areas of counters tend to undergo ____.

Answer 19

• suppressed, rivalry

Question 20

Rivalry and suppression likely share some underlying _______

Answer 20

cortical mechanism

• Ex. strong stimulus in one eye will lead to more complete suppression
• The stronger the target of one eye, the longer that target will “win out” in rivalry
• Also, contour within dominant images in one eye can suppress adjacent background in the other eye, replacing it with background of the dominant eye

Question 21

When does binocular luster occur?

Answer 21

• Occurs when objects of different contrast polarities are fused
  
  • objects of similar shape/orientation of contours to each eye
  • Different luminance, color, or opposite sign contrast
• Occurs when corresponding retinal points receive the same counter info, but different luminance or color
• The phenomenon of luster is a specalized form of rivalry
• Luminance luster (luminance based rivalry) can be like looking at chrome surface

Question 22

T/F The rivalry of binocular luster does preclude some level of stereopsis

Answer 22

F. Does NOT

Ex. Stereopsis in stereogram composed of opposite contrast images

Question 23

What is an example of binocular luster clinical application?

Answer 23

• Worth four-dot test
  
  • ​A pt wears red-green (anaglyphic) glasses) during vision therapy
  • Pt views red and green targets
  • Presence of luster suggest fusion, otherwise suppression

Question 24

What is the mechanism of suppression and rivalry?

Answer 24

• Inhibitory interactions
  
  • Dominant eye neurons inhibit activity of nondominant eye neurons
  • possibly within lateral geniculate nucleus (LGN)
• Feedback loops - attentional gating by feedback from cortex to LGN
• Lateral connections - intra-cortical pathways
• Exact mechanism is not known, but several have been proposed
  
  • Dominant eye inhibits nondominant eye
  • Feedback loops which modulate attention may serve to shut off information from the suppressed eye
  • Connections between cortical cells (intracortical pathways may modulate suppression
• Investigators determined which visual functions are or are not interrupted by binocular rivalry (ex. tilt and spatial frequency after effects)

Question 25

Does adaption B or C impact characteristics of rivalry for A

Answer 25

* Yes, and suggests rivalry must be attributed to cortical mechanisms (ex. after site spatial frequency and orientation processing)

Question 26

What are the two theories of rivalry?

Answer 26

* **Helmholt's theory:** is that rivalry is an alternation of attention in one, then the other eye * **Hering's theory:** eye with the greater degree of contours present in its image will be dominant * its possible - neither theories are completely correct * **Recent models of rivalry → include** * **​**Reciprocal lateral inhibition between monocular cortical cells just before their convergence on binocular cells (or) * **Comparator neuron →** That monitors the output of a cell whose output is switched alternately to gate either the left or right eye's input * Pathways of suppression are still a mystery, but there is insight into which stimuli might be suppressed * Strength of suppression is related to duration * **Dominant stimulus** - perceived stimulus * **Suppressed stimulus** - stimulus that is not perceived * **Stronger stimulus is dominant and will be seen longer** * **Suppression of a blurred image by a sharp image on the fellow is why Monovision CL work** * **Monovision CL** - correction with CL for near eye and distant one (dominant eye) * Distance (near) vision blurred and sharp near (distance) target, is perceived * Also, moving stimuli tend to be dominant, and may break suppression.

Question 27

Worth four-dot test is used for what? What is it composed of?

Answer 27

* Measuring suppression * **_Worth four-dot_** * **​**One white dot (bottom) * One red dot (top) * Two green dots (right and left) * Pt wearing * **_Worth four-dot test_** * Fusion - rivalry * Fusion - luster * Exotropia - crossed * Esotropia - uncrossed * Can be used to grade size of suppression * Suppression at standard 13 inch working distance suggests large area of suppression * Suppression only at a distance suggests a smaller area of suppression * Supression helps strabismus deal with navigation real world without diplopia and confusion but with loss of BV

Question 28

Suppression Scotoma

Answer 28

* Binocular suppression scotoma * E.g in esotropia extends from fovea (F) to eccentric fixation point (E) * More extensive in exotropia * Anisometropes might suppress image of more blurred eye * The size of the area of suppression is proportional to the size of angle deviation of the strabismus * Size of the suppression scotoma is related to the degree of stereopsis regardless of etiology, if suppression is very deep & constant → amblyopia * **Prism base out test** - suppression revealed using prisms

Question 29

Phoria testing (Measuring suppression)

Answer 29

* When performing prism dissociated phoria testing → **diplopia** should be noted * **No diplopia** - suggests suppression

Question 30

Vergence Ranges (Measuring Suppression)

Answer 30

* Divergence & Convergence * Can be determined by increasing base-in and base-out prisms * Normal BV → will continue to fuse the target with vergence eye mvmts * Pt with suppression → will see a single image moving in the direction of the prism apex (before the non suppressing eye) * The perceived direction of motion → tells us which eye is suppressing * i.e. the image in front of the good eye may appear to move laterally (as prism is adjusted)

Question 31

Vectographic Visual Acuity Slides (Measuring Suppression)

Answer 31

* Eye charts presents only some letters visible only to right eye * Others visible only to left eye * If letters are omitted that eye is suppressed

Question 32

Bagolini Lenses (Measuring Suppression)

Answer 32

* Bagolini lenses * Similar to maddox rod - less dissociating * Striated plano lenses (45 and 135 in another eye) * Pt sees a streak from each eye (Normal X) * **Strabismus** -streaks do not cross at the central points * **Suppression** - only one streak is seen * **​****Depth/degree of suppression** can be graded by combining bagolini lenses and neutral-density filters

Question 33

Breaking Suppression

Answer 33

* **If the goal is to train a strabismic pt to use the amblyopic eye** * Patch the dominant eye for short periods of time * Use neural density filter over dominant eye * Use a blurred or frost lens over dominant eye * Use image motion in front of the suppressing eye to make its image more salient

Question 34

What is binocular suppression?

Answer 34

* ignore or turn off all or part of image to one eye to avoid conflicting information between two eyes * Dominance means stronger stimulus will suppress weaker stimulus more thoroughly * Arises from binocular interaction that suppresses some monocular information from binocular percept * Reduced sensitivity (Increased thresholds) in suppressed region

Question 35

Binocular Rivalry vs Binocular luster

Answer 35

* **Binocular rivalry** - Intermittent alternating suppression in local areas (exclusives vs mosaic); dominance means stronger stimulus is seen longer * **Binocular luster** - a specialized form of binocular rivalry arising from similar contours OD/OS, but different colors or luminance * Precise neural mechanisms responsible for suppression are not yet fully known * Some suggest inhibitory interactions between * the dominant eye & nondominant eye * Between the laminae of the LGN * Between the cortex and the LGN and * Between cortical areas

Question 36

Clinical technique to measure suppresion

Answer 36

* Worth four dot test * Suppression scotoma (4 prism BO test) * Phoria testing * Vergence Ranges * Vectographic visual acuity slides * Bagolini lenses