Alternative to Fusion Flashcards
1
Q
Define Binocular fusion
A
2 eyes, 2 images = SINGLE PERCEPT
2
Q
Define motor fusion
A
Eye mvmt based
3
Q
Define sensory fusion
A
Function of visual cortex which requires a similarity of 2 monocular images, otherwise diplopia, suppression of confusion
4
Q
What are the 4 types of Binocular fusion
A
- Binocular confusion
- Binocular suppression
- Binocular Rivalry
- Binocular Luster
5
Q
Describe Binocular confusion
A
-
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
6
Q
Describe binocular suppression
A
- 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
7
Q
Which eye is suppressed?
A
A. Similar target/orientation - binocular fusion
B. Dissimilar target/orientation - suppress one eye (here left)
8
Q
What’s hapening in this image?
A
- 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
9
Q
What is physiological suppression
A
- 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
10
Q
Which eye will the binocular suppression occur?
A
- 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
11
Q
Describe Binocular rivalry
A
- 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
12
Q
Binocular Rivalry is referred to as _______
A
- 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
13
Q
Intermittent/alternating suppression due to _____ binocular inputs
A
- 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
14
Q
T/F Binocular rivalry is noticed mostly in the periphery
A
- 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
15
Q
Describe Da Vinci Stereopsis?
A
- 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
16
Q
What is the difference between dichoptic masking vs monocular suppression?
A
- 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
-
Dichoptic masking
17
Q
Diplopia vs Binocular Confusion vs Rivarly?
A
18
Q
Describe the Determinants of Fusion, Suppression and Rivalry
A
- 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
*
19
Q
Small rivalrous contours tend to be ______ and large areas of counters tend to undergo ____.
A
- suppressed, rivalry
20
Q
Rivalry and suppression likely share some underlying _______
A
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
21
Q
When does binocular luster occur?
A
- 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
22
Q
T/F The rivalry of binocular luster does preclude some level of stereopsis
A
F. Does NOT
Ex. Stereopsis in stereogram composed of opposite contrast images
23
Q
What is an example of binocular luster clinical application?
A
-
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
24
Q
What is the mechanism of suppression and rivalry?
A
-
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)
25
Q
Does adaption B or C impact characteristics of rivalry for A
A
- Yes, and suggests rivalry must be attributed to cortical mechanisms (ex. after site spatial frequency and orientation processing)
26
Q
What are the two theories of rivalry?
A
- 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.
27
Q
Worth four-dot test is used for what? What is it composed of?
A
- 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
28
Q
Suppression Scotoma
A
- 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
29
Q
Phoria testing (Measuring suppression)
A
- When performing prism dissociated phoria testing → diplopia should be noted
- No diplopia - suggests suppression
30
Q
Vergence Ranges (Measuring Suppression)
A
- 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)
31
Q
Vectographic Visual Acuity Slides (Measuring Suppression)
A
- 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
32
Q
Bagolini Lenses (Measuring Suppression)
A
- 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
33
Q
Breaking Suppression
A
- 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
34
Q
What is binocular suppression?
A
- 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
35
Q
Binocular Rivalry vs Binocular luster
A
- 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
- Some suggest inhibitory interactions between
36
Q
Clinical technique to measure suppresion
A
- Worth four dot test
- Suppression scotoma (4 prism BO test)
- Phoria testing
- Vergence Ranges
- Vectographic visual acuity slides
- Bagolini lenses
