Fusion & Correspondence Flashcards
What is the hierarchic arrangement of the BV system?
- Photoreceptors
- Retina (local or global)
- Optic nerve, track (LGN), radiations
- Visual cortex
- So a diagnosis does not simply arise from detecting an abnormality
- Clinician must find areas of abnormal function and the cause of the abnormal functions
- An artifical concept, matches the neurologic arrangement of the visual system
- 3 independent, but not exclusive, components
- Sensory, Integrative, & motor system
Describe the pyramid of binocular vision
- Sensory neurons carry signals from the periphery to CN
- Motor neurons carry signals from the CNS to periphery
- Sensory component serves as the found (fundamental part) followed by integrative & then motor process
- Deficits in the sensory process may cause deficiencies in either or both the remaing systems
- The least fundamental part of the system is the motor process is at the top of the pyramid
- Sensory process - includes anatomic, physiologic and psychologic activities involved in the collection & transmission of visual info to the cortex
- Integrative process - includes those activities that are involved in the fusion of the two cortical images to form a single binocular percept of visual space
- Motor process - includes those activities necessary to properly align eyes at various distances and directions of gaze
- The pyramid guide is a good guide to approaching BV issues
- some anomalies have both sensory & motor aspects (ex. amblyopia , strab)

What are some anomalies of sensory processes?
- Because anomalies of the sensory process may caues deficiencies in either or both of the remaining systems, they are extraordinarily important as they can be a barrier to good BV
- ametropia - refractive condition where the far point is not at infinity (myopia, hyperopia, astig)
- Amblyopia - reduced VA not correctable w/ best refraction
- Eccentric fixation - amblyope does not use central foveal area for fixation under monocular conditions
- Accommodative dysfunction - poor amplitude, decreased facility, spasm
- Disease - ex ptosis, keratconus, media opacities, retinal or visual pathway disease
What are some anomalies of Integrative Processes
- When the images from 2 eyes are too different, various process are used to eliminate the resultant diplopia & visual confusion. Although these anomalies process impede normal BV, they are beneficial restoring a single clear image (of one eye)
- Anomalies of the integrative process should be manipulated with caution becaue the consequence of their elimination are not easily reversed
- Suppression - lack or inability of perception of normally visible objects in all or part of the field of vision in one eye, attributed to cortical inhibition
- Anomalous retinal correspondence - fovea of 2 eyes are not aligned for a common vision direction
- Horror fusionalis - inability to obtain fusion or superimposition of haploscopically presented targets
- Aniseikonia - relative difference in image size between OD and OS
What are some anomalies of motor processes?
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Motor anomalies are at the apex of the pyramid of BV and are last to be treated
- Ex. vergence disorders should be approached after treating sensory and integrative problems
- Motor anomalies are common and have a significant effect on both visual comfort and performance
- Vergence dysfunction - ex. esophoria, exophoria, vertical phoria, convergence insufficiency; among most commonly diagnosed binocular anomalies
- Strabismus - crossing of the eyes arising congenitally or due to trauma, surgery, tumor, etc
- Nystagmus - rhythmic oscillation of the eyes, beyond normal fixational or endgaze mvmts
Describe binocular fusion
- Binocular fusion - process by which 2 images, one from each eye, give rise to a unified percept of one single object
- separating 2 eyes does not tell us about the process or limits to fusing 2 images
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Classifying Binocular fusion
- 3 degrees/grades of fusion used clinically
- All 3 degrees of fusion are necessary for deriving the full benefit of binocular vision
- Grade I = simultaneous perception
- Grade II = Flat fusion
- Grade III = Fusion w/ stereopsis (highest level of BV function)
- Any of these degrees of fusion may be affected individually or in combination by motor and sensory binocular visual disorders
Simultaneous Pereception
- Simultaneous view of 2 disparate, dichoptically - viewed DISSIMILAR images
- Dichoptic - viewing a separate and independent field by each eye
- Ex. A = OS, B = OD
- Diplopia and/or confusion
- can be tested with major amblyoscope
- Grade I

Super Imposition
- Superimposition of 2 disparate, dichoptically - viewed dissimilar images (A = OS, B = OD)
- No TRUE fusion
- No diplopia or confusion
- No frame or object to serve as fusion lock
- Grade I

Flat fusion (fusion without depth)
- Represents 2 dichoptically-viewed images combined into a single percept
- Note: the images to each eye have some similar detail and non-similar detail
- involves binocular summatin, binocular correspondence and fusion without depth (2-D)
- Note the fusion lock
- true fusion but NO stereopsis

Fusion with Stereopsis
- Worth’s highest level of binocular vision (Grade III)
- Stimulation of non-corresponding points that are fairly close together. This small retinal disparity gives rise to stereopsis
- Binocular 3D depth percetion derived from similar, dichoptically-viewed images to OD and OS

Define fusion
- a process of cohesively merging the 2 images (one from each eye)
- Fusion occurs when a point in one eye and corresponding region in the other eye are stimulated
- Two types
- Motor fusion
- Sensory fusion
- Fusion, whether sensory or motor, is always a central process i.e. it takes place in the visual cortex
- Combining the information from the 2 eyes involves some combination of motor and sensory fusion
Define motor fusion
- Motor fusion denotes the use of vergence eye mvmts reflex to position the eyes so that corresponding points are superimposed. (it is the ability to align eyes in such a manner that snesory fusion can be maintained)
- Convergence or divegence
- Ex. In order to fuse the tip of the pencil, your eyes have to converge toward the pencil until the tip of imaged on respective foveas
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Stimulus - retinal disparity outside panum’s area and the eyes moving in opposite direction (vergence)
- Unlike sensory fusion, motor fusion is the exclusive function of the extrafoveal retinal periphery
What are some clinical applications for motor fusion?
- Most strabismus pt unable to achieve motor fusion becaue of their inability to place images of the object on the fovea of each eye
- Impairments of BV, caused by
- Visual stress
- Pathology
- Trauma
- Drugs
- Fatigue
- Will make the task of bringing together similar features in 2 retinal images difficult
Sensory Fusion
- Sensory fusion - neurophysical and physiological process by which the visual cortex
- SF present even without convergence or divergence (motor fusion)
- e.g. slit lamp with parallel optics
- Requires somewhat strict similarity between 2 monocular images
- For sensory fusion to occur, the images not only must be located on corresponding retinal areas but also must be sufficiently similar in size, brightness and sharpness to permit sensory
- significant dissimilarities between the 2 images will disrupt sensory function
- For sensory fusion to occur, the images must be
- located on corresponding retinal areas
- Be presented simultaneously
- Be similar in size, brightness, and sharpness
- Significant dissimilarities between the 2 images will disrupt sensory fusion
What are some clinical applications for sensory fusion?
- Dissimilarities between images - will disrupt sensory fusion
- Ex. Anisometropia, visual differences induced by amblyopia, absence of motor fusion
- The visual system then retorts to either suppression or anomalous correspondence to form percept of unified world
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suppression - represents absence of sensory fusion
- E.g strab pt who suppresses has inadequate motor & sensory fusion
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anomalous correspondence - has sensory fusion but inadquate motor fusion
- E.g strab pt who compensates by anomalous correspondence
What are the 2 theories of fusion?
- Alternation or suppression theory - only one of the monocular images reaches consciousness at a time (alternating right and left eye views)
- Fusion theory - we can attend to similar images in both eyes at the same time
Describe the, I. Alternation or suppression theory
- States fusion is the result of rapid succession of alternating left and right monocular views, which mutually inhibit eachother
- Binocular rivalry - perception alternates between different images presented to each eye
- Dissimilar images seen independently by right & left eyes (dichoptic view)
- Percept is of alternating, but not fused, viwes of right & left images
- Theory - proven to be false (through experiments)
- E.g, rxn time experiment - where flash of light superimposed on only one or other eye’s image (sine wave grating) -> had short reaction time irrespective of the eye stimulated
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Does not occur - under natural viweing conditions with similar images in both eyes (as we can experience stereopsis)
- Assuming if it was true - then we would perceive apparent motion between similar but desperate images - as we alternatively supress one eye or the other
- Can occur in artificial viewing conditions (experiment) in phenomenon -> Binocular rivalry, but takes a long time to switch attention from one eyes image to the other
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Suppression theory
- Suggests higher - level access to info from only one eye at a time
- Irregular pattern suggests local rivalry*
Fusion Theory
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Fusion theory
- Postulates that similar images in the right and left eye can be processed simultaneoulsy
- processing is not successive, as in the alternating theory
- Proved to be true under natural viewing conditions
- Doesn’t mean that it holds true under ALL viewing conditions
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Two minor exceptions of fusion theory
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Binocular rivalry does occur with dissimilar images to eahc eye
- Fusion = rule with similar monocular images
- Rivalry = rule with dissimilar images
- Even though we combine the 2 eyes information, the visual system still has some access to uncombined monocular information so that it can detect small differences between the 2 eyes image ( that gives us binocular disparity/stereopsis)
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Binocular rivalry does occur with dissimilar images to eahc eye
Summarize the alternate vs. fusion theory

Limits of fusion - Panum’s Fusional Space/Area
- Panum’s Fusional Area (PFA) - The range over which fusion can occur (or) the region/area on one retina such that any point in it will with a single point (corresponding area/region not corresponding point) on the other retina
- Horopter - line or locus of points in space whose images fall on corresponding retinal areas of RT/LT eyes
- Panum’s fusional space - region in visual space over which we perceive single vision
- Narrow band around the horopter within which the objects stimulation disparate retinal elements transmit the impression of single vision
- Thus the range of horizontal disparities around the horopter within which the stimulus will continue to be perceived as single is known as panum’s fusional space

Panum’s Fusional Area - The limits of fusion
- Fusion occurs when a point in one eye and corresponding region in the other eye are stimulated
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Panum’s area shows that fusion - is not synonymous with identical visual direction
- images within panum’s area can be fused and seen as single yet stil have slightly different visual directions in 2 eyes
- Panum’s area is useful to binocular fusion as it allows for some precision drift in eye mvmts without introduction of diplopia
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Ex.
- Microdrifts & tremors - that occur during fixation are uncorrelated betwen the 2 eyes, but panum’s area is large enough to allow fusion of displaced images
- Panum’s area reduces the adverse effects of fixation disparity, (a small monocular misalignment of the eyes in which fusion & single fusion are achievable)
- Images in panum’s area , are not exact corresponding point, can still be fused
- In order for an object to be perceived as single
- its retinal image in the 2 eyes do not have identical visual direction
- they just need to fall within panum’s fusional area
- However - differences in visual direction can produce a percept of depth
Panum’s Limiting Case (PLC)
- PLC - minimum condition for stereopsis. when fused, one line looks closer than the other
- The horizontal extend of Panum’s area can be measured using PLC
- PLC - minimum conditions for the perception of stereopsis consisting of three lines, one for one eye and two for the other. (Called limiting case because 3 targets is the minimum number needed to see stereopsis and removal of any one abolishes depth)
- Wheatstone - Panum limiting case - Sir charles Wheatstone (1938)
- Measuring Panum’s limiting case
- one target to one eye
- two targets to the other eye
- 3rd target elicits perception of depth (or diplopia)
- Can test PLC vertically and horizontally

angular extent of panum’s fusional area
- Image: A = Uncrossed disparity PLC, B = Crossed disparity
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Full extent
- Uncrossed disparity
- Crossed disparity
- This full range represents the width of panum’s fusional area
- Target becomes diplopic 50% of the time at border of PFA
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Panum’s Fusional Area
- Panum’s fusional area is 3-6x larger horizontally (& elliptical) than vertically (due to reduced eye mvmts vertically)
- Panum’s fusional area is 5-20 arcmin foveally and increases in proportion to eccentricity away from the fovea
- ex. panum’s area equals 6-7% angle of eccentricity when measures > 5o from fovea
- Duration of the target exposure - alters size of PFA
- Effect of target orientatin in PFA - difficult to quantify because of cyclorotation of eyes and unequal retinal and horizontal disparity along the length of the target
- Changes in luminance or contrast have little or no effect on PFA

Clinical Relevance of PFA
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Central-Peripheral difference in the size of panum’s fusional area
- aniseikonia - image size difference OD & OS
- Greater aniseikonia tolerated peripherally than centrally
- However, a simialr size difference (aniseikonia) or smaller stimulate at the center will result in loss of binocular fusion
- As a result, central suppresion is much more likely to occur than peripheral suppression (ex. misalignment in strabs)
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VT
- One goal of VT - improve fusion in pt with BV problems
- During VT - larger targets (low spatial frequency) are used in periphery (PFA - largest) at the beginning till the pt fusional abilities improve (then small foveal stimuli is presented)
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Size of PFA - may be affeted by presence of
- strabs
- Small-angle strab - size of PFA enlarged
- Anomalous Retinal Correspondence - PFA may be abnormally large


