Introduction To Binocular Vision Flashcards
What is binocular single vision?
‘Simultaneous use of two eyes to give one mental image’
What happens if you do not have binocular single vision (BSV)?
Basically double vision (in the case of a face it would look like four eyes on a face)
What is the advantage of having two eyes?
You have a spare eye if pathology happens to one. Wider field of view.
What are the advantages of binocular single vision (BSV - I.e using two eyes to produce one image)?
—>Binocular summation - both eyes contributing to one image means there is an enhanced visual acuity, contrast sensitivity and motion perception. —> Stereopsis (ability to perceive depth). This means you have enhanced visual motor skills. (To test this try joining the tip of a vertically held pencil to another vertically held pencil - with both eyes open one can do it easily but with one eye covered it’s incredibly difficult to measure depth).
What is stereopsis?
Depth perception
What do you need to develop normal BSV?
You need Normal retinal correspondence (NRC).
How can the impact of binocular vision in regards to stereopsis be tested ?
The Pencil tip test. With both eyes open it’s very easy to connect the tip of a pencil to another pencil somebody else is holding (vertically) whereas with only one eye open this is increasingly difficult.
What is retinal correspondence?
It means that every single point on the retina corresponds to a point in space.
What is Normal Retinal Correspondence?
A Binocular condition in which the fovea and areas on nasal and temporal retina correspond and have a common visual direction with the fovea and nasal retina areas of the other eye.
What’s the cyclopean?
It’s an imaginary eye - it basically represents how vision is seen if we only had one eye in the centre of our head (same as the image produced at the moment from the summation from both eyes)
What is the theoretical horopter?
An imaginary line in which each point has an exact corresponding retinal point thus is seen as a single image.
What is the theoretical/geometrical horopter in the horizontal plane come to be known as ?
Veith-Mülleer circle
What is Panums fusional space?
The small area around the horopter in which corresponding and almost corresponding objects are still seen singularly.
What is stereopsis caused by?
The disparity of the almost corresponding points (i.e. those close to the horopter line) (as a pose to the single points i.e. those exactly on the horopter), which are fused together into one single image. This gives that 3D depth effect.
What happens when objects fall out of Panam’s fusional area?
All objects outside of Panum’s fusional space give rise to physiological (normal) diplopia.
This type of diplopia exists when you have binocular vision
What is crossed double vision refered to as and how can we identify it?
Crossed double vision is referred to as Heteronymous double vision.
In this example if you get the patient to close one eye the image moves to the other side.
(False image is on the same side as the healthy eye).
What happens in homonymous diplopia?
When you get the patient to close one eye
(False image is on the same side as the affected eye).
What are worths grades?
- Simultaneous Perception
- Fusion - Sensory Fusion
- Motor Fusion
- Stereopsis
If a patient has more grades do they have better or worse binocular vision?
If your patient has more grades they have better binocular vision.
What is simultaneous perception?
‘Ability to perceive two images simultaneously, one from each retina.’
i.e. if you showed the right eye a picture of a Vertical line and the left eye a picture of a horizontal line, the patient would see a cross altogether.
What is sensory fusion?
‘Integration of two similar images, one formed on each retina into one image within the brain.’
Images must be similar in brightness, size and form.
What is motor fusion?
‘Maintenance of sensory fusion through a range of vergence movements’.
What is Stereopsis (as a grade)?
‘The perception of relative depth of objects by the fusion of relative disparity of images from the two eyes’.
How does normal retinal correspondance work?
Normal retinal correspondence (NRC) is a binocular condition in which both foveas work together as corresponding retinal points, with resultant images fused in the occipital cortex of the brain.
I.e Fovea of the right eye corresponds to the fovea of the left eye. The nasal elements of the right eye correspond to the temporal elements of the left eye. The temporal elements of the right eye correspond to the nasal elements of the left eye.
What are possibilities of conditions that can occur when a patient has strabismus?
- Confusion- normal retinal correspondence (NRC)
- Diplopia (NRC)
- Suppression (NRC)
- Abnormal Retinal Correspondence (ARC)
How does confusion occur in a manifest strabismus?
So imagine a patient with strabismus of the right eye (right eye esotropia is shown in the image) is trying to look at a tree,
The left eye will nicely focus on the tree however because the right eye is turned in it won’t see the tree and maybe actually sees plants. Now because of sensory fusion the brain merges those two images and the patient sees a horrible hybrid of these two images which leaves them confused.
(Remember conditons for sensory fusion are images of the same size and brightness not colour)
Is confusion is strabismus patients common?
It’s actually one of the more rarer associated conditions.
What happens in pathological diplopia associated with manifest strabismus in adults?
Imagine the patient is looking at a tree and the patient has esotropia of the right eye (i.e. right eye turns inwards). The tree would land on the fovea of the left eye but not on the fovea of the right eye - In this case the tree stimulates the left nasal retina.
As the nasal retina is stimulated it projects temporally.
Which type of patient shall have a strabismus and supression?
Children - mainly under 8
How does supression in those that have strabismus occur?
Important to note –>This happens in children under 8 and that if this didnt happen the child would be experiencing Pathologic diplopia.
The human brain is extremely clever and realises we are getting double vision and that this isn’t useful thus it literally just ignores the double vision that would be occuring - this only happens though when both eyes are open!
How would you diffrentiate supression from normal binocular vision in a child that has a slight strabismus of either the right or left eye?
If the child is supressing they will usually NOT have stereopsis.
Which type of patient develops abnormal retinal correspondance?
Children under 8
What is abnormal retinal correspondance?
If the child was to develope a constant small angle strabismus. Imagine we have the same scenerio so a child with right eye esotropia is trying to focus on a tree. The image of the tree hits the left fovea and doesnt hit the right fovea as its turned in. However, the brain is really clever and understands what’s going on and actually takes the point that the tree is hiting in the right eye to be the ‘new fovea’ or ‘psuedo-fovea’ if you will.
Thus the children behave exactly like people with normal binocular vision as essentially the brain has rewired itself.
Will children with abnormal retinal correspondance have stereopsis?
They will have a form of stereopsis yes.
In a child that has abnormal retinal correspondance due to strabismus of the right eye what will happen if you cover the left eye? Will the patient view an image on the fovea or ‘psuedo-fovea’?
When you cover the left eye in this case the child will view the tree (in the example in the picture attached) with the fovea of the right eye. This is because the fovea provides the best vision as well as the fact that with one eye you don’t have stereopsis so no point using the psuedo-fovea.
Is abnormal retinal correspondance a singular or binocular vision problem?
It is a binocular vision problem as if you cover one eye the other eye shall be using its fovea rather than pseudofovea (if it is the affected eye).
[With only one eye you don’t have stereopsis ( as you need two eyes to determine depth) thus eye shall view from where it has best vision aka the fovea].
If a patient was under 8 when they developed a manifest strabismus what will they experience?
Either Supression or Abnormal Retinal Correspondance (ARc).
If a patient was an adult when they developed manifest strabismus what will they experience?
Either Confusion or Pathologic Diplopia
Do you treat Strabismus in children?
If there are no symptoms then no, however, you may choose to correct supression for the cosmetic value via surgery.
Do we treat ARC ?
Abnormal Retinal Correspondance - we don’t want to treat this as their whole brain has wired to use the psuedo-fovea.
If a patient was an adult when they developed manifest strabismus how would we treat them?
Uusually treat with prisms. lenses or surgery.
Does eXotropia (eyes turning outwards) lead to crossed or uncrossed diplopia?
It leads to Crossed (think of an X) diplopia.
Does esotropia (eye turning inwards) lead to crossed or uncrossed diplopia?
It leads to uncrossed ( homonymous) diplopia.
Which of the following statements about Abnormal Retinal Correspondance is NOT true?
a) It avoids diplopia
b) It avoids confusion
c) It only developes after 8 years old
d) It is normally associated with a form of stereopsis.
C
How would you explain supression to an 8 year old child and their parent?
Say: Both your eyes work however they don’t work together.
In the case of an alternating strabismus you would say:
When both eyes are open she/he either uses his/her right or his/her left eye to see.
In the case of an esotropic/exotropic strabismus you would say:
He/she does not use both eyes at the same time.
Thus he/she proabably finds activities where he/she has to judge distances hard i.e. sports with balls.
