Fusion & Correspondence

Question 1

What is the hierarchic arrangement of the BV system?

Answer 1

• 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

Question 2

Describe the pyramid of binocular vision

Answer 2

• 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)

Question 3

What are some anomalies of sensory processes?

Answer 3

• 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

Question 4

What are some anomalies of Integrative Processes

Answer 4

• 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

Question 5

What are some anomalies of motor processes?

Answer 5

• 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

Question 6

Describe binocular fusion

Answer 6

• 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

Question 7

​Simultaneous Pereception

Answer 7

• 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

Question 8

Super Imposition

Answer 8

• 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

Question 9

Flat fusion (fusion without depth)

Answer 9

• 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

Question 10

Fusion with Stereopsis

Answer 10

• 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

Question 11

Define fusion

Answer 11

• 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

Question 12

Define motor fusion

Answer 12

• 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

Question 13

What are some clinical applications for motor fusion?

Answer 13

• 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

Question 14

Sensory Fusion

Answer 14

• 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

Question 15

What are some clinical applications for sensory fusion?

Answer 15

• 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

Question 16

What are the 2 theories of fusion?

Answer 16

• 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

Question 17

Describe the, I. Alternation or suppression theory

Answer 17

• 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*

Question 18

Fusion Theory

Answer 18

• 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)

Question 19

Summarize the alternate vs. fusion theory

Answer 19

Question 20

Limits of fusion - Panum’s Fusional Space/Area

Answer 20

• 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

Question 21

Panum’s Fusional Area - The limits of fusion

Answer 21

• 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

Question 22

Panum’s Limiting Case (PLC)

Answer 22

• 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

Question 23

angular extent of panum’s fusional area

Answer 23

• 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 > 5^o 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

Question 24

Clinical Relevance of PFA

Answer 24

• 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

Question 25

Describe monocular, dichoptic, and binocular information

Answer 25

• 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)

Question 26

Describe vergence & binocular vision

Answer 26

• 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)

Question 27

Describe fixation disparity and how do we measure it?

Answer 27

• 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

Question 28

Describe forced vergence fixation disparity cuve (FVFDC)

Answer 28

• 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

Question 30

Describe Anomalous Retinal Correspondence

Answer 30

• 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

Question 31

What are some subjective test of the angle of strabismus?

Answer 31

• 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

Question 32

Objective vs Subjective Angle of Strabismus

Answer 32

• 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

Question 33

What are the Three Theories to Explain Mechanism of ARC

Answer 33

• 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

Question 34

Describe binocular fusion

Answer 34

2 eyes, 2 images, SINGLE PERCEPT

Question 35

Describe Motor fusion

Answer 35

eye movement based

Question 36

Describe sensory fusion

Answer 36

functional of visual cortex

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

Question 37

Explain fixation disparity

Answer 37

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

Question 38

Explain anomalous correspondence

Answer 38

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