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Binocular Vision > Fusion & Correspondence > Flashcards

Fusion & Correspondence Flashcards

1
Q

What is the hierarchic arrangement of the BV system?

A
  • 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
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2
Q

Describe the pyramid of binocular vision

A
  • 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)
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3
Q

What are some anomalies of sensory processes?

A
  • 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
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4
Q

What are some anomalies of Integrative Processes

A
  • 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
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5
Q

What are some anomalies of motor processes?

A
  • 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
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6
Q

Describe binocular fusion

A
  • 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
  • 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
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7
Q

​Simultaneous Pereception

A
  • 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
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8
Q

Super Imposition

A
  • 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
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9
Q

Flat fusion (fusion without depth)

A
  • 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
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10
Q

Fusion with Stereopsis

A
  • 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
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11
Q

Define fusion

A
  • 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
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12
Q

Define motor fusion

A
  • 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
  • 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
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13
Q

What are some clinical applications for motor fusion?

A
  • 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
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14
Q

Sensory Fusion

A
  • 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
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15
Q

What are some clinical applications for sensory fusion?

A
  • 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
  • suppression - represents absence of sensory fusion
    • E.g strab pt who suppresses has inadequate motor & sensory fusion
  • anomalous correspondence - has sensory fusion but inadquate motor fusion
    • E.g strab pt who compensates by anomalous correspondence
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16
Q

What are the 2 theories of fusion?

A
  • 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
17
Q

Describe the, I. Alternation or suppression theory

A
  • 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
  • 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
  • Suppression theory
    • ​Suggests higher - level access to info from only one eye at a time
    • Irregular pattern suggests local rivalry*
18
Q

Fusion Theory

A
  • 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
  • Two minor exceptions of fusion theory
    • ​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)
19
Q

Summarize the alternate vs. fusion theory

20
Q

Limits of fusion - Panum’s Fusional Space/Area

A
  • 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
21
Q

Panum’s Fusional Area - The limits of fusion

A
  • Fusion occurs when a point in one eye and corresponding region in the other eye are stimulated
  • 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
  • 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
22
Q

Panum’s Limiting Case (PLC)

A
  • 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
23
Q

angular extent of panum’s fusional area

A
  • Image: A = Uncrossed disparity PLC, B = Crossed disparity
  • 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
  • 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
24
Q

Clinical Relevance of PFA

A
  • 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)
  • 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)
  • 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
25
Q

Describe monocular, dichoptic, and binocular information

A
  • Monocular - visual information available to one eye only
  • Binocular - same/similar visual information available to both eyes
  • Dichoptic (bi-ocular) - similar or dissimilar information presented independently to each eye
  • In normal BV - 2 eyes may not contribute equally - one may be favored (dominance)
    • fMRI (functional magnetic resonance imaging) confirms this, dominant eye activates large portion of striate cortex than the non-dominant eye
    • Dominant eye can differ at distance & near (DO NOT ASSUME THEM TO BE THE SAME)
  • So, once combined into a single percept, is each eye’s monocular information lost?
    • Ultrocular discrimination - the ability to identify the stimulated eye under binocular conditions.
      • Experiments: difficult to remove cues like target shift or contrast differences between 2 eyes
      • However, even when these cues are controlled - humans with normal BV unable to make utrocular discrimination
    • Monocular information lost to conscious perception
      • ​But may be used internally by the visual system without our awareness
  • Clinical Application
    • ​As monocular information is lost to conscious perception
      • We can independently present targets to each eye & still achieve single unified percept
      • This is called dichoptic or bi-ocular stimulation
    • This method allows
      • Scientist to study how the 2 eyes images are combined and when this combination fails to occur
      • Clinicians to determine which eye is suppressing its information and manipulate stimulus strengths in each eye to treat amblyopia (under binocular viewing conditions)
26
Q

Describe vergence & binocular vision

A
  • Binocular fusion has sensory and motor components
  • Inaccurate motor fusion would impair sensory fusion
  • Most eyes have heterophoria, misalignment manifest when one eye is covered or the eyes are dissociated (ex. maddox rod can be used to dissociate the eyes)
  • The heterophoria represents the fusional vergence demand
  • Clinical application: Vergence
  • Esophoria - requires NEGATIVE FUSIONAL VERGENCE
  • Exophoria - requires POSITIVE FUSIONAL VERGENCE
  • Heterophoria - succesfully compensated by FUSIONAL VERGENCE
  • Uncompensated heterophoria - STRABISMUS (eyes are not able to achieve motor fusion)
27
Q

Describe fixation disparity and how do we measure it?

A
  • Fixation disparity - small error in vergence (purposeful) prevents image of fixation target from falling on corresponding retinal points
  • That is a point that may not be fixated precisely bifoveally
  • There may be a vergence error of a few arcmin
  • Clinical application - fixation disparity
    • ​Fixation disparity - more than few minutes of arc is an indicator of BV problems
    • Larger the FD => more likely the pt will be symptomatic

Measure Fixation Disparity

  • FD: Measured clinically at distance or at nearpoint with simple targets (2 parts)
    • Some binocularly visible details that serve as a binocular fusion lock
    • 2 monocularly seen nonius lines (via polaroid filters)
  • Nonius lines - like fine lines of vernier caliper, used for precision measurements
  • Offset of lines is measured when binocular fusion induced
  • Wesson disparometer - fixation disparity card
    • Tested with polarized glasses/lenses
  • Sheedy disparometer - rotating wheel
28
Q

Describe forced vergence fixation disparity cuve (FVFDC)

A
  • FDC may be used to predict how pt will respond to prism and other stresses on the vergence system
  • i.e measure fixation disparity when prism is introduced
  • Prisms probe relationship between fixation disparity and vergence demand
  • Positive and negative fusional vergences induced
  • Type I
    • ​Curve obtained by 60% (near) and 70% (distance) of people
    • Symmetric
    • Gradual change in fixation disparity (with addition of prism) except at extremes
    • The steeper the central clope of the FD curve, the more likely the pt will be symptomatic of BV problems
  • Type II
    • ​Represent intolerance of forced divergence
    • 25% people at distance and near
    • Flat on base out side
    • Corresponds with esophoria (intolerance to divergence)
  • Type III
    • ​10% of people at near and 0% at distance
    • Flat on the base in side
    • Associated with high exophoria (intolerance to convergence)
  • Type IV
    • ​Rare-exhibits little change in fixation disparity
    • 5% at distance & near
    • Flat at base out and in
    • Associated with aniseikonia & sensory fusion problems
30
Q

Describe Anomalous Retinal Correspondence

A
  • In ARC the 2 foveas no longer correspond with eachother
  • Corresponding points are measured relative to retinal angles
  • However, correspondence is a cortical phenomenon
  • Harmonious ARC (HARC) - shift in corresponding retinal points MATCHES the angle of strabismic deviation
    • MOST COMMON
  • Unharmonious (paradoxical) (ARC) - imprecise shifting of corresponding retinal points
  • Eccentric fixation - off-foveal point in the retina of the deviating strabismic eye is used for fixation under MONOCULAR and BINOCULAR conditions
  • Is a adaption of developing visual system to misalignment
  • Alternative to suppression or binocular confusion
  • ​Cortical phenomenon assessed as a retinal misalignment
31
Q

What are some subjective test of the angle of strabismus?

A
  • Maddox rod test
    • ​illumination source viwed by one naked eye and one maddox rod occlude
    • Location of images (line & spot) correspond with strabismus
    • Here, OD = Maddox rod, OS = None
    • Maddox rod cyls horizontal
    • Exophoria or tropia - line is to left
    • Esophoria or tropia - line is to right
  • Hess-Lancaster Test (Subjective)
    • ​Red-green anaglyiphic glasses for binocular dissociation
    • Pt superimposes red and gree annuli - superimposed on screen if the pt is orthophoric
32
Q

Objective vs Subjective Angle of Strabismus

A
  • Objetive angle is measured by cover test
  • Subjective measured as noted or by amblyoscope
  • Angle of anomaly: difference between the subjective and objective strabismus angle measurement
33
Q

What are the Three Theories to Explain Mechanism of ARC

A
  • Sensory theory - sensory adaption compensates for constant angle of strabismus
  • Motor theory - egocentric direction is altered by the pattern of innervation to the oculomotor muscles (change in correspondence is registered with oculomotr system signaling both an eye mvmt and a change in retinal correspondence)
  • Abnormal disparity vergence stimulus detection - produce eye mvmts yielding poor motor fusion between non-corresponding points
    • ARC is a neurophysioligcal disturbance which CAUSES strabismus
  • Regardless of cause - given a change, visual system with sufficient plasticity will develop ARC
  • Therefore tx of strab must occur after NRC is achieved
  • This is best accomplished in childhood-when ARC is reversible
  • Tx of exophoria is generally more sucessful than tx of esophoria
34
Q

Describe binocular fusion

A

2 eyes, 2 images, SINGLE PERCEPT

35
Q

Describe Motor fusion

A

eye movement based

36
Q

Describe sensory fusion

A

functional of visual cortex

requires a similarity of 2 monocular images, otherwise diplopia, suppression or confusion

37
Q

Explain fixation disparity

A

slight vergence error prevents image of fixation target from failing on corresponding retinal points

can be used to help vergence mvmt system conpensate for heterophoria

38
Q

Explain anomalous correspondence

A

developing visual system adapts to physical misalignment by developing new association between noncorresponding points

Binocular Vision (8 decks)

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