Binocular Vision (145-161 is on rigid cls by accident) Flashcards
what are the 3 levels of binocular vision?
-simultaneous perception
-fusion
-stereopsis
what is the simultaneous perception of binocular vision?
where both eyes contribute towards visual perception, whether
the images from each eye are combined into a single percept or not.
what is the fusion component of binocular vision?
where two images are fused, resulting in a single percept
what is stereopsis part of binocular vision
depth of perception of the fusion
what are the advantages of binocular vision?
- Increases field of view
- Compensates for physiological blind spot
- Binocular summation
- Stereopsis
what are symptoms that may indicate binocular vision problems?
-diplopia
-asthenopic symptoms
-blurred vision
-for young children: rubbing eyes, avoiding certain types of visual tasks e.c.t
when may you need to follow up diplopia?
if its monocular as that could indicate more serious pathology
what is the smallest prism D you can see with cover test?
2D and that is barely visible
how can you measure the size of deviation in cover test?
by doing prism cover test - Keep changing prisms as you do alternating cover tests until you no longer see the eye deviation.
The base goes in the opposite direction of deviation so exophoria needs correction with base IN prism and vice versa
why are you less likely to use maddox rod and wing?
-eyes are completely dissociated
-There is a lot of accommodation happening in maddox rod
-Maddox wing is a fixed distance which may not be the patient’s reading distance
whats the link between accommodation and convergence?
increased accommodation results in increased convergence
what kind of deviation is triggered by uncorrected hyperopia?
eso deviation as Px accommodates more
what kind of deviation is triggered by uncorrected myopia
exo deviation as Px accommodates less at near
What is fixation disparity?
where there’s a small amount of misalignment of the eyes when they’re being used binocularly after one of them’s been occluded
when correcting a deviation, which way does the prism go?
base of the prism goes opposite to the direction of the deviating eye so exo deviation prism is base IN
what could each of the following scenarios mean?
-more exo at distance than near?
-more eso at distance than near?
-more exo at near than distance?
-more eso at near than at distance?
- More exo at distance than near = divergence excess?
- More eso distance than near = divergence insufficiency?
- More exo at near than distance = convergence insufficiency?
- More eso at near than distance = convergence excess?
what do you record for cover test?
-size
-recovery for phorias
-type of deviation
-laterality for tropias (alternating?)
or
-nmd
how does mallet unit to assess fixation disparity work?
when the bottom line moves towards the right eye and the top moves towards the left eye, it is an uncrossed and so eso deviation. If the bottom line moves towards the left and the top line moves towards the right, then its crossed = exo deviation. If the lines are in line with each other and the middle of the X, then no fixation disparity is present
What are some of the symptoms involved in asthenopia?
-ocular fatigue
-discomfort
-lacrimation
-headaches
what kind of patients should you not use mallet unit on?
-those with va worse than 6/12 in each eye
-suppression / amblyopia
which vergence component are you assessing in cover test?
fusional vergence
which component of BV is the highest form?
stereopsis
what is the contralateral antagonist of the left medial rectus
the right medial rectus
what is confusion?
when a patient can see two images superimposed on top of each other making it hard to work out what they’re seeing
what is impaired laevoversion?
when movement of both eyes to the left is abnormal
what is movement of both eyes to the right called?
dextroversion
what does listing’s law suggest?
the eye has a centre of rotation at which it moves
what are the 3 axes of fick?
- Y axis is the optical axis - does straight through the pupil = cyclorotation so twists in and out
- X axis lies horizontally = vertical rotation so up and down
- Z axis lies vertically = horizontal rotation so left and right
how can you tell in a CT scan if the eyes are dissociated?
If the Y axis are not pointing in the same direction
what are the 3 positions of gaze?
-Primary = straight ahead
-Secondary = up/down/left/ right
-tertiary = combo e.g. up and right
what does donder’s law suggest?
Ocular orientation required to look at a particular point in space is always the same and independent of the previous ocular position
what are agonist and antagonistic muscles?
-Agnostic muscles are eye muscles that work in a pair to move the eyes to look in the same direction
-Antagonistic muscles are muscles that oppose the agonist muscles
what is sherrington’s reciprocal law of innervation?
where one muscle of the eyeball contracts so the opposite muscle has to relax
what does herring’s law suggest?
contralateral agonists move with equal innervation.
what are the two types of binocular movements?
-vergences (oppostive, disjunctive)
-Versions (same direction, conjugate)
look at types of versions and vergences screenshot
ok
what is the most basic form of binocular visiom?
simultaneous perception - seeing an object with both eyes - may or may not be combined into one single precept
what is the main part of BV you want patients to achieve?
steriopsis - depth of perception because its the highest form of BV
where does processing of BV occur?
in the brain NOT THE RETINA
how is binocular vision processed?
- info enters from separate points in our visual field in each eye
- info gets crossed when the nerve fibres cross at the optic chiasm
- visual cortex can then process the info as one image
what does localisation help us do?
helps us understand where objects are in our visual field
what are the two types of localisation?
-Oculocentric localisation is seeing objects where they are in relation to each other
-Egocentric localisation is seeing where objects are in relation to you using your eyes and other proprioceptive cues
what are images in the secondary visual direction?
images that fall on the retina but do not fall on the fovea
what is the difference between images in the primary and secondary visual direction
primary fall on the fovea whereas secondary fall outside of the fovea elsewhere on the retina
why might we see two objects as superimposing eachother?
as they are both on the same visual direction
how does an image fall on corresponding points on the retina?
as visual angles of each point subtend to equal points on the retina on both eyes when using both eyes to look at an image
what does an image on corresponding points on the retina of the right and left eye mean for the image?
the two images on each retina get perceived as a single image
what is retinal disparity?
where images between the right and left eye change when you look at objects that appear to be behind each other when looking at them binocularly
what is physiological diplopia?
where there’s two images on the same visual axis. the one behind gets seen as diplopic by the cyclopian eye
how do we get over physiological dipolopia?
We do not perceive images as diplopic because our brains suppress the image (non-dominant eye usually suppressed)
what is a horopter?
an imaginary plane for a fixation point where all points along this plane fall on corresponding retinal points so all objects on the horopter are perceived as single
what is panum’s area?
a region behind and in front of the horopter where objects that are close but do not fall on the same corresponding points but the brain can still fuse the images and compare them to build information about the depth
objects perceived as single but with depth
what do objects that dont lie on the horopter or in panum’s area appear as?
double
when can BV problems arise?
-at birth or within first 6 months = CONGENITAL/ INFANTILE
=pre school/ early school = EARLY CHILDHOOD
-any other time due to e.g. trauma, cranial nerve palsy or decompensation = ACQUIRED IN LIFE
give 9 causes of BV problems
-neurogenic
-myogenic
-refractive/accommodative
-developmental
-congenital
-systematic disease
-environmental
-latrogenic
-idiopathic
give 12 types of BV problems
-heterotropia
-heterophoria
-ocular motility
-accommodation problems
-fusional vergence problems
-amblyopia/ suppression
-abnormal retinal correspondence
-aniseikonia
-fixation disparity
-microtropia
-nystagmus
What is the motor aspect of BV?
the ability of the muscles to move the eyes accurately - motor dysfunction results in loss of fixation disparity due to deficiency in the brain
what is the sensory aspect of BV?
how the brain uses signals sent from the retina of each eye combined to produce a visual precept which includes stereopsis
what tests can you do to assess motor function in BV?
-cover test
-ocular motility
-NPC
-maddox rod/wing/double
-vergence facility and jump convergence
-fusional reserves
-Hirschberg &
Krimsky
-prism reflex test
-fixatio disparity
what tests can you do to assess sensory function of BV?
-check VA
-fixation disparity
-stereopsis tests
-suppression tests
what are glasses with a line on them?
bifocals
what could cause monocular diplopia?
keratoconus
how much phoria is normal at near and distance for most people?
Normal phoria at near for most people is 3 prism diopters exophoria and for distance is 1 diopter exophoria. This is just a guide though as some people have much higher phorias and manage well as well some some people having even smaller phorias and not managing well
what does it mean if near vision is worse binocularly compared to monocularly
they most probably have a BV issue
what is a convergence excess deviation?
a deviation that is greater at near fixation target than at distance
patients that ae found to be hyperopic with no prescription what is it possible to also find?
a large esophoria
when may you only measure AoA monocularly?
when you find the NPC to be low because you expect the AoA to be closer to the eye than NPC
what is the AC/A ratio?
(accommodative convergence/ accommodation) the accommodative convergence per dioptre of convergence for horizontal movements only (vertical movements do not affect accommodation)
what might suggest a patient has a high AC/A ratio?
If you have a patient that is much more exo or eso at near than distance on cover test
What are the 4 components of accommodation?
-tonic
-reflex
-vergence
-proximal
what is tonic accommodation?
*Resting state i.e. there is no stimulus.
* Usually between 0.00D and 2.00D, mean 1.00D in young
adults
what is reflex accommodation?
- The automatic response of accommodation to maintain a
clear retinal image. - Stimulus is small amounts of retinal blur: <2.00 D.
what is vergence accommodation?
- Stimulus is convergence.
- Only present when both eyes open
- Contributes to increase in amplitude of accom. when binoc
cf. monoc
what is proximal accommodation?
Stimulus is the perception of the proximity of the fixation
target
what is the NPC?
the nearest point which can be converged upon and is generally considered adequate if it’s closer than 10cm
what is the definition of convergence?
equal adduction of each eye to a point closer than infinity
when is convergence symmetrical and asymmetrical?
if the eyes are adducting on the saggital plane then convergence is symmetrical and if not then it’s asymmetrical
what combo of movements allows for asymmetrical convergence?
a combination of symmetrical vergence and version movements
Why is AC/A good to measure?
-predicts the change in vergence that a change in accommodation will elicit
-a high AC/A ratio is associated with certain types of esotropia
How do you calculate the AC/A ratio gradient?
1.find the patients phoria with and without their prescription
2. find the difference
3. divide this by their Rx
difference in phoria/ difference in accommodative demand
how do patients overcome phorias?
with fusional vergence and as it cannot work at max all the time, they need to keep some in reserve and poor fusional reserves can cause BV problems
what happens when there are negative fusional reserves and you start to increase prism more than needed
-when a larger prism is introduced, Limit of fusional reserves reached; eyes start to accommodate to increase convergence and maintain BSV; accommodation makes target blurred but BSV maintained
-when an even larger prism is introduced, Eye can no longer overcome deviation (fusional reserves exhausted) and BSV no longer possible
what do base in (negative) fusional reserves assess?
the ability of the eye to rotate outwards (from having been turned in due to esophoria) to achieve fusion when removing the cover in cover test
what do fusional reserves assess?
the range over which you can fuse images
look at plotting fusional reserves graph
check bv2 powerpoint slides 49-58
How can you correct inadequate fusional reserves?
prescribe a prism based on fusional reserves
What is blurred vision a common symptom of?
heterophoria
what is point zero?
the point on the retina of the strabismic eye where the intended target falls
How is physiological diplopia different to pathological diploia?
as physiological is normal and gets suppressed by the brain whereas pathological diplopia is abnormal as it’s caused by a misalignment of the visual axis and so will be present over the whole field of view and can result in confusion.
detail a scenario that can cause confusion in the cyclopean eye and draw a diagram if you can
say the right eye has esotropia:
-image on the right eye that would have fallen on the fovea (fixation target) will fall on the nasal retina so will be seen as a diplopic image on the temporal field of the cyclopean eye
- an image on your left hand side will then fall on the temporal retina of the left eye and on the fovea on the right deviating eye so will be seen on the nasal field and on the fovea of the cyclopean eye
there are now two images on then fovea hence confusion
what happens in the ‘plastic’ period of the visual system?
where the visual system develops over the first few years of life and changes mostly during the first 4-5 years. any irregularities like diplopia mean the brain will try to adapt around them so the px does not actually see the diplopia or confusion.
-by age 8/9 the visual system has mostly stopped changing
what are the 4 BV adaptations?
-suppression
-abnormal retinal correspondence
-amblyopia
-eccentric fixation
look at Sensory adaptations in BV - asynchronous work for week 15 to understand how they work
who is most likely to adapt to BV problems?
children as they have ‘plastic’ vision
what are the two types of adaptation
binocular = where the adaptation is only present when both eyes are open and if the good eye is shut then the bad eye will function normally
monocular = when you close the dominant eye, the strabismic eye will still not function properly
what type of adaptation is supression?
binocular
how does suppression as an adaptation to diplopia work?
- Suppression of point zero so that image is removed from cyclopean eye
- Suppression scotoma so the foveal image on the deviating eye is also removed from the cyclopean eye
what is supression?
a cortical process which selectively removes part of what is detected to produce a single image
what is abnormal retinal correspondence (ARC)?
an alternative to suppression where the visual directions in the deviating eye re-align as they shift in the strabismic eye to match corresponding points in the fixating eye
what are the positives of ARC?
-allows for fusion of diplopic images -allows for some binocular processing like some stereopsis but this will always be more poor than in normal BV but still measurable
what are the negatives of ARC?
- Depending on the size of the deviation, there may still be suppression of fovea and point zero in deviating eye
-Effects are less for small deviations
which fall in Panum’s are
-effects are mainly peripheral
how does ARC as an adaptation to diplopia work?
- Suppression of point 0
- Suppression scotoma
- Visual directions are adjusted in the brain so that point zero corresponds with the fovea in the fixating eye
what are the two types of arc adaptations?
harmonious = When adjustment of the visual direction matches the angle of deviation
this is objective angle as it’s what you measure on prism cover test
unharmonious (rare) = happens when the px with established ARC has a change in the angle of strabismus like after a surgery or a trauma
subjective angle deviation
What is amblyopia?
reduction in vision that persists despite fully corrected refractive error and no apparent pathology and generally there is a difference in VA of 2 lines between the 2 eyes
what kind of adaptation is amblyopia?
monocular
what people can bilateral amblyopia occur in?
patients who have high uncorrected astigmatism at a young age as this has caused a reduction in retinal image quality
give 3 common and 3 less common causes of amblyopia
common:
* Anisometropia
* Refractive error (e.g. uncorrected
astigmatism)
* Strabismus
less common:
* Stimulus deprivation (e.g. congenital /
early cataract)
* Early eye disease (e.g. macular lesions)
* Nutritional / toxic
* Idiopathic / congenital
what is eccentric fixation?
where the fovea of the strabismic eye does not take up fixation when the dominant eye is occluded so an eccentric area of the retina takes up fixation
what are the two types of eccentric fixation?
-EF with identity = where the eccentric area of the retina may coincide with point zero
-EF without identity = where the eccentric area lies somewhere between the fovea and point zero
when doing cover test on right esotropia eccentric fixation with identity, what do you see?
During cover test RE is turned in, but when the left eye is covered, it
does not move out to take up fixation
when doing cover test, what do you see with right esotropia, eccentric fixation without identity?
During cover test RE is turned in, but when the left eye is covered, it only moves some of the way out to take up fixation
even though EF is rare, when may it only usually develop?
in stable, small,
early onset esotropia
why do you need to be careful if your patient has EF?
as if you try and correct this and you get it wrong, then you may cause intractable diplopia (diplopia that cannot be resolved) so you need to respect their EF
What are the broad causes of BV problems?
-refractive
-anatomical
-neurological
-health
what are refractive causes of BV?
-accommodation/ convergence relationship
-astigmatism
-anisometropia
what lifestyle factor can cause BV problems?
sudden change in ocular use like studying/ starting a new job
how can hyperopia and myopia cause BV problems?
-hyperopia = eso deviations for distance and near
-myopia = increased exo deviations at near
how can astigmatism cause BV problems?
-if uncorrected in both eyes can cause bilateral amblyopia
-if uncorrected in one eye, can cause unilateral amblyopia
how can anisometropia cause BV problems?
-uncorrected means one image is always blurred so causes high risk of amblyopia
-aniseikonia = different sized retinal images in each eye so difficult to fuse
give some anatomical problems that can cause BV issues
*incorrect attachment of EOM - can cause under/over rotation or the angle of rotation can be affected
* Abnormal cranial / orbital
development
* Trauma e.g. fracturing inferior floor of the orbit
give some neurological causes of BV issues
*EOM is over or under stimulated causing an incompetency
*If the muscle is innervated by the wrong cranial nerve causing abnormal movements
give some systemic conditions that can impact BV
*MS causing demyelination so the neurons act abnormally
*Stroke causing blood supply to cranial nerves to be disrupted
*Fatigue/ stress = more vulnerable to decompensation of heterophorias
*TED
*intracranial lesions
*diabetes
*myasthenia gravis
give 2 examples of ocular pathology that can cause BV problems
-retinoblastoma
-macular disease (aniseikonia)
what lifestyle changes can you suggest to a patient to correct BV problems
- Reduce time / increase breaks on VDU
- Improve lighting
- Change working distance
what kind of BV issue are orthoptic exercises particular good for?
exo deviations, less so for eso and vertical
what is a drawback of using orthoptic exercises as a treatment for BV problems?
requires commitment by the patient to be effective
how can spherical manipulation help fix BV problems?
to reduce/ correct eso/ exo deviation
what patients is spherical manipulation effective for?
patients with:
* A high enough AC/A ratio
* Sufficient accommodation
how can you use spherical manipulation to correct eso/ exo deviations?
-If you have a patient who is very eso at near, you can try giving them extra plus at near which causes them to have to accommodate less so they would converge less and hence help their esophoria at near. This would not work for distance however as you blur their vision doing it.
-For a patient whose very exo is near, you can do the opposite by giving them extra minus only his works for both distance and near.
how do you split prism for vertical and horizontal?
-Vertical, you need base up in one eye base down in the other eye
-In horizontal, you need the same base direction in both eyes e.g. RE BO, LE BO
who is best for eye patching?
orthoptists as they can see the PX more often to monitor them.
give an example of a patient who would find prism useful
one with decompensating heterophorias
when should you start splitting prism?
when it’s above 3dp
what is the point of eyepatching in early age?
forces use of amblyopic eye during the plastic period
what are the drawbacks to patching?
-Needs good cooperation from patient and their carers
-Need regular follow-up & experienced optometrist
-Over patching can result in “reverse amblyopia”
when would you refer adults on to HES when they present with BV problems?
-if they have recent onset diplopia
-if they have other BV issues that can’t be resolved in practise
look at slide 27 of managing patient case 4 block 15 lectures
ok
what is the point in assessing opposite fusional reserves?
as it allows you to determine the recovery which allows for fusion to be achieved
what is percival’s criterion?
in fusional reserves graph, states that the demand point (fused) should lie in the middle third of the zone of single binocular vision
-if this is not the case, prism is required to put the demand point in this third given by the formula:
P = 1/3G - 2/3L
* where P is the prism, G is the greater of the fusional
reserve blur points and L is the lesser of the blur points.
A positive P means prism is required.
what is the formula for percival’s criterion?
P = 1/3G - 2/3L
* where P is the prism, G is the greater of the fusional reserve blur points and L is the lesser of the blur points. A positive P means prism is required.
what does percival’s criterion ignore?
the magnitude of the phoria
what is the 1:1 rule?
the base in recovery (negative fusional reserve) should be at least as large as the eso deviation and if not then prism is needed to correct it
what is the 1:1 rule equation
P = (esophoria - base-in recovery)/2 and if P is positive then prism is required
what does sheard’s criterion state?
the fusional reserve should be at least twice the demand
what is a decompensating heterophoria?
any latent binocular misalignment that becomes symptomatic
what is 0.25DC = in Ks?
0.05mm
what is the formula for residual astigmatism on a rigid lens?
ocular astigmatism - corneal astigmatism
where is the astigmatism from in each of the scenarios?
-if cornea is spherical and refraction is spherical
-if cornea is toric and refraction is spherical
-if cornea is spherical and refraction is astimatic?
-if the cornea is toric and the refraction is astigmatic
-none
-mixed
-lenticular
-corneal
what are the 4 rigid toric lens designs used?
1) Front surface toric
2) Back surface toric
3) Bitoric
4) Sphere centre, toric periphery (rarely fit)
when do you fit front surface toric?
-when cornea is nearly spherical so astigmatism has to be lenticular
how can you stabilise front surface toric lenses?
-Uses prism ballast stabalisation (when adding a lot of prism use a high Dk material as it increases thickness of middle lens
-Can use truncation for stabalisation (less comfy due to increased lower lid interaction or lid laxity) as you cant use prism ballast for monocular toric correction as it introduces differential vertical prism.
what are the rigid lens options for toric corneas in chronological order?
1) Altering BOZR, depending on amount of corneal astigmatism
* Minimises edge clearance in steeper corneal meridian
2) Reduce TD
* Minimises exaggeration between 2 different meridians to reduce edge clearance in steeper corneal meridian
3) Aspheric rigid lens
* Narrower edge lift to reduce edge clearance in steeper corneal meridian
4) Spherical centre, toric periphery rigid lens
* Considered when peripheral cornea is more astigmatic and reducing TD proved ineffective
5) Toric lens
* Variety of different options – back surface toric, front surface toric, bitoric
what are the signs that make a patient a good rigid toric lens candidate?
- > 2.50D corneal astigmatism (difference in K’s of 0.5mm)
- > 0.75D residual astigmatism (ocular
minus corneal) - Poor fit or comfort with spherical rigid lens
- Excessive 3 & 9 o’clock staining
- Increase in toricity towards cornea
periphery - Unable to wear soft torics (poor comfort, vision, etc)
what are the fitting steps for front surface toric? What is the order format?
1) Fit a spherical rigid lens until alignment achieved
2) Toric over-refraction
3) Rotational stability? LARS/CAAS
Order format: BOZR / TD / sph / cyl x axis
when do you fit back surface toric?
on a toric cornea
do back surface toric lenses need stabilisation?
no as they are made to fit the shape of the cornea
what are the steps to chronologically fitting back surface toric lenses and what is the order format?
Fitting methods:
1) Order empirically from spec Rx, BVD, and Ks
2) Toric fitting set (rarely available)
3) Fit spherical lens on flattest K and use the over-refraction to calculate
amount of toricity needed
Order format: BOZR1 and BOZR2 (underneath) / TD / Sphere power
when do you fit bitoric lenses
when the cornea is toric
what are the steps to chronologically bitoric lenses and what is the order format?
1) Calculate ocular Rx (accounting for BVD)
2) Order two BOZRs at flattest and steepest K
3) Specify the power at each meridian
(…or phone the lens manufacturer for advice)
Order format: BOZR1BOZR2 / TD / Power @ flatter meridian
Power @ steeper meridian
what may cause poor vision in rigid torics?
- Toric rotation or instability?
- BOZD too small? Flare?
what may cause poor comfort in rigid toric lenses?
- Deposits/damage? Polish lens surfaces?
- Inappropriate edge lift?
- Check corneal integrity/staining
how much bigger than max pupil size should bozd be?
1.5mm larger than max pupil size
how can you monocularly determine how far or close from you an object is?
-Texture gradient
-Overlapping (interposition)
-Ariel perspective
-Hugh attenuation
-Light shading depending on the direction of light
-Retinal image size
-Accommodation
-Motion paralax (one part of a scene (background) doesn’t look like it’s moving much whereas objects in the foreground seem to be moving more)
what is the fronto parallel plane?
Plane at the fixation distance that is parallel to a line passing through the entrance pupils of the two eyes
what is the vieth muller circle?
Theoretical circle in space in front of an
observer containing points that will fall on
corresponding retinal locations (nodal points) in the two eyes when a point on the
circle is fixated.
Was initially thought that all the points on the circle will fall corresponding points in the retina.
the horopter is a bit more elliptical instead
how does the ellipse of the horopter change at different distances?
the further the distance, the flatter it gets to the point it starts to bend up instead of down at 6m
the shorter the distance the more narrow/ bendy it gets
how is the brain able to differentiate between images that are all in the same line of sight e.g. seeing 3 pencils directly in front of you?
lets say theres a red pencil, a green one and a blue one in order from closest to you to futhest away from you. You can differentiate between them as:
1. red pencil tip falls on horopter
2. Tip of blue pencil falls behind horopter but in Panum’s area
3. tip of green falls in front of horopter but still in panum’s area
4. the images do not correspond but as they fall in panum’s area they are fused
look at slides 21 and 22 on bv 3 in block 4 week 14
okay
what is normal for stereopsis
possible to measure up to 60’’ (arc seconds) but under experimental conditions with trained subjects it’s 5’’
what is normal for stereopsis in infants
-under 3 months they will have none and then as they get to 18 months the stereopsis rapidly develops
what are the factors that affect stereopsis?
-Blur reduces stereopsis
-Contrast reduces stereopsis
-Motion in depth = if targets are moving back and forth then stereopsis will reduce
-Retinal eccentricity as the further the target from the fovea, the less the stereopsis
If the target is presented for less time then stereopsis reduces
-The further the distance of the target from the eyes, the lower the stereopsis
what is another word for binocular rivalry and what is it
retinal rivalry
-where in physiological diplopia, the brain has competing images which it must present as single so where there is a dominant stimulus, a supressed one must be seen
-in small stimuli usually one or other complete stimulus is seen at any one time in exclusive dominance
-in large stimuli the perception comprises of parts of each stimulus in mosiaic dominance where bits of image from either eye are compressed together as the brain has suppressed parts of an image that are less relevant and these parts are constantly shifting or alternating hence mosaic rivalry
how is retinal disparity created?
when objects in front or behind the horopter stimulate non-corresponding points
understand binocular rivalry
ok
which part of a scene in front of you is prioritised in binocular rivalry?
-more detail over blur
-high contrast over low contrast
-brighter over dimmer
-edges or contour over plain
-moving over still
-flashed/short duration over constant
what is the definition of anisometropia?
a condition in which the refractive state of a pair of eyes differs and therefore one eye needs a different lens correction from the other.
what could correction of anisometropia induce?
aniseikonia and, when the eyes deviate from the optical axes of the lenses, anisophoria.
what may uncorrected anisometropia of low amounts cause? what about large amounts?
-eyestrain or diplopia
-large amounts rarely cause symptoms as one of the retinal images is typically suppressed or there is amblyopia
what are symptoms of aniseikonia?
-visual discomfort
-visual distortion of space
-sometimes difficulty in achieving binocular vision as e.g. in spectacle corrected unilateral aphakia
when does aniseikonia become clinically signficant? give the numbers of how much is normal and not
when the shape/ image size difference is enough to be a barrier to sensory function
* < 0.75% difference: generally insignificant
* 0.75% to 2.5% difference: significant, but
generally will still attempt to fuse, therefore
aesthenopic symptoms
* 2.5% to 4%: either attempts to fuse or
suppresses: maybe symptomatic
* > 4% difference: tends to suppress at the
expense of binocularity
give 3 causes of aniseikonia
-retinal
-neural
-optical
how could you test for anisikonia?
separating the retinal images of a large
target (e.g. a test chart) with prisms and
comparing them; placing size lenses in front
of one eye until the images appear equal
will give an indication of the amount of
aniseikonia.
give examples of how aniseikonia can have a retinal cause
*Pathology affecting the macular (e.g. macular oedema) causing micropsia / macropsia / metamorphopsia
* Changes spacing between photoreceptors
how can aniseikonia have a neural cause?
as images that fall on the retina are equally sized, but they are interpreted as different in the brain (e.g. due to different spacing of photoreceptors)
why cant you always correct anisikonia with specatcle lenses?
because if there is a big difference in prescription between two eyes it can cause the patient to see a sudden change in image size. Contact lenses can be a solution to this as they dont cause as much difference in retinal image size due to BVD
why can aniseikonia be optical?
due to differences in axial length and/or refractive components of each eye
what is refractive vs axial anisometropia?
-refractive is where the anisometropia is due to differences in corneal curvature, lens curvature or power and axial length is the same in both eyes
-axial is where anisometropia is due to difference in axial length between the eyes and other refractive components are equal
why does aniseikonia affect BV?
as BV processing requires 2 similar sized images and above 4% difference means BV is not possible but below 4% BV can still be difficult and cause patients to have visual symptoms
why can some patients with aniseikonia see images tilted?
As the outer parts of the image fall within Panum’s area will be fused but perceived in
depth: one end will appear closer and the
other further away from the person
