AOV - MST1

What is the principle sense of direction

direction signalled by the fovea (straight ahead)

above objects fall on INFERIOR retina

below objects fall on SUPERIOR retina

define binocular vision 

vision that results from the combined input from the two eyes 

advantages of binocular vision (4)

1. stereopsis: greatly enhances depth perception 

2. larger visual field (150 v 180 - coverage of blind spot ) 

3. spare eye 

4. binocular summation (2 eyes better than one)  

Disadvantages of binocular vision 

Neurophysiology required to move both eyes and combine both input is more complex and therefore prone to problems. 

Common cause of eyestrain, headaches etc. Refractive imbalance, incorrect alignment can cause dissimilar images in both eyes. Need for monovision contact lenses

misconceptions about bino vision 

1. stereopsis is the only advantage 

2. without binocular vision you have no depth perception (monocular cues like overlap, colour) 

3. stereopsis is equally useful at distance and near (only useful at short distances!! up to 6m) 

what does the foveal line represent? 

the principle visual direction

What is the monocular sense of direction? what is it organised around?

It is oculocentric 

It is organised around the fovea (principle visual direction)

Define oculocentric direction

Visual direction relative to the eye

determined by where the image falls on the retina

What gives rise to our oculocentric direction

Retinal local sign

What would displacement of the retina effect?

would impact occulocentric direction and give rise to perceived distortions

what will superimposed images on the retina give

same visual direction (but perhaps different distances)

what is the principle visual direction demonstrated by

1. Phosphenes

2. after images (despite eye movement sense of direction remains)

Can occulocentric direction account for our monocular sense of direction?

No

define egocentric direction

Visual direction relative to the observer (the head/body). Determined relative to a point called the egocentre

why are both oculocentric and egocentric sense of direction necessary

• Oculocentric direction alone is insufficient because retinal position changes whenever the eye moves. However, objects do not appear to move when our eyes move.

• Likewise, when we follow a moving object with our `fovea, we can still tell the object is moving even tho the retinal position is constant.

• egocentric direction compensates for this by incorporating information about eye and head movements, allowing the brain to determine the object's true position in space

Define the cyclopean eye

The cyclopean eye is a hypothetical single eye located midway between the two eyes that represents how the brain combines visual direction signals from both eyes into one unified binocular sense of visual direction.

define corresponding points

pairs of points on the left and right retina that have the same angular distance from the fovea (one temporal one nasal). In this case, we see the image as being in the same place.

• they signal the same oculocentric direction in each eye

define the horopter (will need to draw this in an exam)

The locus of points in visual space that stimulate corresponding retinal points. If the object falls on the horopter, it will give rise to corresponding points (aka seen as a single object)

WILL NEED TO DRAW THIS IN AN EXAM. IT IS CRITICAL TO UNDESTAND THAT A RAY GOES THROUGH THE NODAL POINT (about 1/3 down the eye)

describe disparate points and corresponding points. where must corresponding points be?

Every retinal element (in the region of binocular overlap) has a corresponding point in the other eye. One is in the temporal retina and the other in the nasal retina.

All other points are non-corresponding or disparate points. Stimulating disparate points give rise to different visual direction signals (so produces diplopia).

define stereopsis

the perception of depth resulting from binocular disparity - the slight differences between the images seen by each eye

how does stereopsis work

Each eye views an object from different angle, this creates small differences in retinal image position (binocular disparities)

Brain uses these disparities as depth cues.

Disparity is processed in the visual cortex to calculate distance

• doesn’t work past 6m due to decreasing disparity

what is the perception of these 3 retinal relationships

Retinal relationship

Fovea + fovea

Corresponding points

Disparate points

Perception

single object straight ahead

single object off-centre

double vision (diplopia)

what do points imaged on the temporal retina of both eyes give rise to?

Crossed diplopea.

object is closer than fixation —> image on temporal retina of both eyes.

left eye image appears on right, right eye image appears on left.

Remember - corresponding points will give rise to one nasal and one temporal!

what do points imaged on the nasal retina of both eyes give rise to?

Uncrossed diplopea.

object is farther than fixation —> image on nasal retina of both eyes.

left eye image appears on left, right eye image appears on right.

Remember - corresponding points will give rise to one nasal and one temporal!

what did doctor Herring observe

Fly, tree and house appeared in the same visual direction.

Because the tree and house were beyond the fixation point, they produced uncrossed diplopia, so it appeared that each of the two images shared the same visual direction

This image shows a Brock string. Assuming bifoveal fixation of the near bead on the Brock string, select the drawing that best depicts what the cyclopean eye would see. (one purple and yellow bead to begin with)

C

Brain receives two slightly different images of the world from each eye, why do we usually only see one version of the world?

• physiological suppression!

  • The brain suppresses conflicting signals from the two eyes (mutual inhibition between cells)

The following observations indicate that oculocentric visual direction is not sufficient to explain our monocular sense of visual direction?

A

B and C don’t involve a change in oculocentric direction 🙂

Which of the following is NOT an advantage of binocular vision:

a) larger field of view

b) superior depth perception under all viewing conditions

c) greater sensitivity because of binocular summation

d) a remaining eye if one is injured

B

don’t get superior depth perception under all viewing conditions!

Define and distinguish between versions and vergences as binocular eye movements

Vergence = the two eyes move in opposite directions.

• Convergence → eyes rotate toward each other (for near)

• Divergence → eyes rotate away from each other (for far)

Versions = the two eyes move in the same direction.

• not as common to measure

• only qualitative observation

Measuring versions (1)

perimeter using a moving target

• fine target end point: subject unable to resolve because not on fovea

• gross target end point: observe cessation of eye movement

Ways to measure convergence (4)

1. Push up method - objective

2. Push up method - subjective

3. Prism method

4. Synoptophore method

push up = eye on pen and bringing in

How to measure linear angle of convergence

Linear angle of convergence
d = b + 27 mm

Note: 27 mm comes from back vertex distance (~12 mm) + cornea to centre of rotation (~15 mm)

Then calculate convergence (see image)

what can the angle of convergence be measured in

Degrees

Centrads (1/100 radian)

Prism diopters

Metre angles

what is a prism diopter and what is the symbol? How to calculate with convergence?

one prism diopter is the angle formed by a displacement of 1 cm at a distance of 1 m

symbol: triangle

what is a metre angle (ma)

the convergence for an object 1m away

conv (ma) = 1/d (m)

Calculate prism dioptres, and metre angles from this information

what distance defines convergence insufficiency

when d is larger than 10 cm

describe what a prism does

• prism bends light towards the base

• object appears to shift toward apex

• eye turns toward the apex to maintain fixation

what will base out prism induce? Where is the retinal image in this scenario?

convergence. displaces the image to the temporal retina

• can keep increasing base out prism power until the limit of convergence is reached (diplopia)

is our visual system better at diverging or converging?

converging.

limit of divergence about 8 prism diopters. limit of convergence is 20-30 prism diopters.

what will a base in prism induce? Where is the retinal image in this scenario?

divergence (outward, exo). displaces the image to the nasal retina.

what are fusional reserves

Fusional reserves are the maximum amount of vergence (convergence or divergence) a person can generate beyond their resting alignment to maintain single, fused vision.

• this what we have been testing

what is the synoptophore method? What can it measure?

Synoptophore: a binocular instrument that presents separate images to each eye to measure ocular alignment, fusion, and vergence ranges by adjusting tube positions.

Convergence

Divergence

Supravergence (vertical changes in vergence)

Cyclovergence (rotational vergence)

what kind of binocular eye movement is primarily involved when a hunter follows a flock of ducks flying left to right at a distance of 100 m

version

vergence (prob a bit of accommodation as well)

What is the functional binocular position?

The fixation object is imaged at both foveae.

Requires vergence eye movements

How is the functional binocular position maintained?

• anatomical forces (controls where eyes are pointing)

• innervation of extraocular muscles

• accommodation (closely linked to convergence)

• attention

• awareness of nearness

• fusion reflux

what is the anatomic position of rest?

position when all innervation to extraocular muscles is removed (aka u dead)

17 prism diopters beyond parallel (diverged)

what is tonic vergence/ what it called?

position with normal innervation, but no stimulus (aka dark).

moves eyes to the physiological position of rest

define fusional vergence (disparity vergence). What is it called and what is it triggered by.

the spontaneous movement (reflex) of the eyes from the passive position to the active position to avoid diplopia.

the active position is maintained by tonic vergence together with fusion and fixation reflexes.

describe orthophoria. what this mean in terms of active and passive positions of the eyes

Ideal condition where the ocular motor apparatus is in perfect equilibrium so that both eyes retain their required positional relationship even when fusion is removed by dissociation of the eyes (one is covered)

the active and passive positions coincide the eyes are said to be in a state of orthophoria.

define initial vergence

movement by the eyes when they move from the physiological position of rest to the functional position of rest (passive position)

triggered by the fixation reflex

what is the functional position of rest. what is it triggered by

Passive position = where the eyes would point if fusion were removed (e.g., with cover test)

triggered by fixation reflex

define heterophoria. What is active and passive positions.

Present when a deviation of the visual axes is held latent by the power of the fusion reflex.

when the active and passive positions are different

** heterophoria = fusion reflex is doing work to keep eyes aligned

what is the fusion reflex

a neurological reflex that moves the eyes to eliminate disparity.

in heterophoria, the fusion reflex has to work to keep eye in the active position (aligned)

heterophoria represents strain on fusion reflex

what is the angle of phoria? What does it mean when the angle of phoria is negative? what angle would the fusional vergence mean?

the angle the eyes move.

If angle of phoria is -ve (the eyes in the passive position are diverged in respect to the active position) —> exophoria (+ve fusional vergence)

If angle of phoria is +ve (the eyes in the passive position are converged in respect to the active position) —> esophoria (-ve fusional vergence)

Aetiology of heterophoria (origins)

globe size and shape

muscle/ligament abnormalities

innervation factors (accommodative-convergence synkinesis, binocular reflexes)

general fatigue, excessive use of alcohol/tobacco

lesions of ocular motor neural apparatus

what is cyclophoria

when eyes rotate about the visual axis when dissociated

• inclophoria = rotate nasally

• exclophoria = rotate temporally

How to label hyper/hypo phoria/tropia (up/down)

WE ALWAYS specify the eye UP!!

picture = left hypertropia

why is it common to be expohoric at near distances? what is this called?

Requires increased convergence (+ve fusional vergence). Common for the eye to undershoot this. Fusional reflex is working to turn the eye in more. when fusion is broken the eye turns OUT

called physiological exophoria

A person with a near exophoria is fixating a near point with the left eye while the right eye is covered (see figure). According to principles of visual direction, where would the fixated object (black dot) appear to be located?

a) Left of the true object (1)
b) In its true location (2)
c) Between the two visual axes (3)
d) On the right eye visual axis (4)
e) To the right of the right eye’s visual axis (5)

B

person won’t see the object move when the eye is covered but the eye will move (but visual information is blocked)

name for terms when:

heterophoria doesn’t lead to symptoms

heterophoria leads to symptoms

1. compensated phoria (no issue)

2. decompensated phoria (symptoms)

symptoms of decompensated heterophoria

headaches

aching eyes

diplopia

blurred vision

focusing difficulties

monocular comfort

difficulties with depth perception

sore eyes

general irritation

7 methods to measure heterophoria

1. cover test

2. maddox rod

3. maddox wing

4. von Graefe

5. prism bar

6. prentice card

7. synoptophore (this may be a duplicate)

causes of decompensated heterotropia

What 5 steps make up a routine eye exam

1. History taking (how are you today, how can I help you)

2. Routine screening (VA, PD, cover test, etc)

3. Refraction (find prescription)

4. Anterior and posterior eye exam

5. Management

What 3 categories make up the routine screening portion of the eye exam

1. Refraction (2)

• VA

• PD

2. Binocular vision (4)

• cover test

• Near point accommodation (NPA)

• Near point convergence (NPC)

• stereopsis

3. Neurological (4)

• ocular motility

• pupils - bright, dim, RAPD, DCN

• colour vision (CV) - Ishihara

• visual field (VF) - confrontation, red cap and amsler grid

is the cover test objective or subjective?

objective

don’t need the patient to respond

what is myopia

nearsightedness

• see near objects clearly but objects that are farther away appear blurry

• this occurs because the eye focuses light in front of the retina instead of directly on it

• need negative power lens

lenses in myopia, hyperopia and astigmatism

myopia and hyperopia = spherical lenses (± power) - myo need neg, hyper need pos

astigmatism = cylindrical lenses (negative power)

Difference between heterotropia (strabismus) and heterophoria

Hypertropia is a manifest deviation. Eyes are visibly misaligned even when fixating on an object

Heterophoria is when eyes are not properly aligned with each other when they are at rest.

orthophoria = eyes are aligned in the resting position

what does accommodation do to the power of the lens

increases the plus power of the lens (near)

• more convex

• eyes converge

What is NPC a measure of?

Explain:

• set-up

• which eye

• target

• distance

• procedure

• recording (units?)

• normal reading

Near point convergence

• binocular vision test - a measure of “how well you can bring your eyes inwards” - e.g. when reading\

• always test from cornea

• set-up: normal room, habitual Rx

• which eye: binocularly only

• target: 1 letter-size above best corrected vision. Clear and single?

• distance: start at habitual reading distance (~40cm)

• procedure (must do at least twice - take worst reading)

  • move inwards until sustained double vision is perceived (this is the break). Get patient to blink and try to correct.

  • then move out till clear and single image is achieved (this is the recovery)

• Record: B/R 5/7cm with habitual Rx.

  • if to the nose record as TTN

Normal value: 6/8cm. Anything over 10 cm is an abnormal break.

what is it called when accommodation decreases with age

presbyopia

How will uncorrected myopia and hyperopia impact NPA

uncorrected myope: give a falsely high NPA reading

uncorrected hypermetrope: give a falsely low NPA reading

what declines with age (routine test)

Accommodation - directly age related. lens thickens. NPA more effected

Convergence - indirectly affected (small/variable change). NPC not effected much

what might abnormal values of CT/NPC/NPA/Stereopsis indicate?

• abnormalities in binocular disfunction

• need to determine underlying cause - congenital vs acquired?

• investigate further - BV workup to assess accommodation, vergence and the interaction between both systems (more to come)

What is NPA a measure of?

Explain:

• set-up

• which eye

• target

• distance

• procedure

• recording (units?)

• normal reading

Near point accommodation

• binocular vision test - a measure of focusing ability/power

• always test from cornea

• measure in Diopters = 1/distance (in meters)

• set-up: normal room, habitual Rx

• which eye: monocularly only (always R first)

• target: 1 letter-size above best corrected vision. Clear and single?

• distance: start at habitual reading distance (~40cm)

• procedure (must do at least twice for each eye)

  • move inwards until sustained blur. Get patient to blink to try to correct. measure in m.

  • repeat for the other eye

• Record: R 10D, L 12D with habitual Rx

  • if to the nose record as TTN

Normal value: 15-(1/4 age)

Monocular accommodation (D) vs age (graph)

Accommodation = 15 - (1/4 age)

If you’re 20, you should have about 11 Diopters of accommodation

what is stereopsis tests recorded in?

seconds of arc

what is stereopsis doing exactly

determining the difference in the angles between two objects at different distances (i think). This angle is expressed in seconds of arc.

3 anatomical and physiological requirements for stereopsis

1. a large binocular over-lap of VFs

2. partial decussation of afferent visual fibres

3. coordinated conjugate eye movements

normal/abnormal stereopsis readings

measured in seconds of arc (“ arc) - smaller = better!

• ≤ 40” arc: eyes are aligned well. any ocular misalignment cannot be larger than Panum’s fusional area

• 60” arc: still within normal limits

• > 60” arc: question whether accurate bifoveal fixation is present

What 3 things do you check for in stereopsis test?

How to record?

Suppression (R/L) - are both eyes working the same?

Global (random dot) - detect the shape in the dots

Local (contour) - identify what shape, animal or circle appears closer

Randot stereotest (habitual Rx): No suppression, 40” arc, global pass,

What nerves innervate what extraoccular eye muscles

CN IV (4) = superior oblique

CN VI (6) = lateral rectus

CN III (3) = all others (superior rectus, medial rectus, inferior rectus, inferior oblique)

What test measures the eye muscle function

ocular motility

• this is a neurological screening test.

• if one of the muscles isn’t working, can get double vision cause the eye won’t be able to move to fixate (patient sometimes compensates by moving head)

Principle action(s) of the 6 main extraocular muscles

adduction = toward nose (inwards)

abduction = away from nose (temporal) - outwards

intorsion = rotate toward nose

extorsion = rotate away from nose

Think of these in terms of what happens during cover test and what muscles may cause eso/exo etc.

What is ocular motility a measure of?

Explain:

• set-up

• which eye

• target

• distance

• procedure

• recording (units?)

• normal reading

Measure: eye muscle function (or nerves i think)

• set-up: normal room lights, Rx either on or off

• which eye: binocularly only

• target: H on PD ruler

• distance: perform at 40 cm

• procedure: evaluate 9 positions of gaze (HH pattern)

  • Px to keep head still, and follow the target with eyes.

  • Px to report pain or diplopia at any time if present

  • check for overaction/underaction/restriction, any jerky eye movement.

• Record: Full/Smooth/Accurate, -ve pain, DV

what is the term for when pupil sizes are different

Anisocoria

what is the sequence of evaluating pupil sizes and responses?

1. Sizes in light then dark

2. RAPD (relative afferent pupillary defect)

3. DCN (direct, consensual and near responses)

what may an afferent defect indicate?

Afferent = eye talk to brain

lesion in the retina, optic nerve head (ONH) defect

what may an efferent nerve defect indicate?

efferent = brain talk to eye

anisocoria (unequal pupil sizes)

• lesion in the motor neuron system which carries signals from the CNS to the iris

what is a normal pupil size?

2-4 mm in bright

4-8 mm in dim

• pupils get smaller (miosis) with age

what can pupil screening test evaluate

• function and integrity of the iris, optic nerve, posterior visual pathways, 3rd CN, afferent and efferent pathways

Pupil test

Explain:

• set-up

• Rx

• target

• distance

• procedure

• recording (units?)

• normal reading

Measure: “pupil size and reactivity”

• set-up: Normal room lights to measure size first, then dim lighting to measure again. Use a light source for RAPD and direct/consensual/near testing. The sequence matters: bright → dim → RAPD (distant, light off) → DCN

• Rx off.

• target: distance: fixation light (or other non-accommodative target); near: near accommodative target (not specified)

• distance: Distance: bright, dim and RAPD. Near: DCN

• procedure:

  • Ask patient to fixate a distant non-accommodative target

  • Measure pupil size in bright

  • Measure again in dim

  • While in dim, perform swinging flashlight test for RAPD

  • Turn room lights back on

  • Assess direct, consensual, and near responses

• Record: Bright: R: 4mm, L: 4mm; Dim: R: 6 mm, L: 7mm; (-)RAPD, DCN pass or PERRLA

• PERRLA:

  • Pupils
    Equal
    Round
    Reactive to
    Light and
    Accommodation

what is the consensual response when light is shone into an eye?

What would the response be in someone with an APD (afferent pupillary defect)?

1. normal: both pupils contract due to intact and consensual light reflexes

2. APD: pupil in normal eye dilates upon illumination of the pathological eye because of a lack of stimulus for the consensual light reflex

is colour vision congenital or acquired?

can be both. Can be congenital but diseases and drugs can alter colour vision as well.

If acquired there will be a difference in severity between both eyes!

what is the most common colour vision deficiency?

R-G most common

• ishihara only tests red green!

Blue yellow less common

• Protan = red-cone-type defect

• Deutan = green-cone-type defect

• Tritan = blue-cone-type defect

Colour vision test:

Explain:

• set-up

• Rx

• which eyes

• target

• distance

• procedure

• recording (units?)

• normal reading

• set up: normal room lights

• Rx: habitual

• eyes: binocular for screening (/15), monocular for diagnostic (/17) - if more than 2 errors = FAIL, go on to monocular tests.

• distance: hold at 75 cm

• procedure:

  • 3s per viewing plate

• Recording:

  • 15/15 OU

  • R 17/17 L 10/17