Lecture 2 Flashcards
What is physiological diplopia?
Remember:
The horopter represents the point of fixation where objects will stimulate corresponding retinal points and be seen as single.
Objects placed in front of or behind the horopter are seen as diplopic because they will stimulate non-corresponding retinal points.
What does it mean to have disparity?
- The further that objects are positioned from the horopter:
- The greater the separation between the retinal points stimulated and
- The further apart the diplopic images are perceived
- When images of an object fall on non-corresponding retinal points they are said to be disparate or have disparity
How is disparity achieved?
Disparity occurs because our two eyes view a visual image from a slightly different angle.
This is because there is distance between our two eyes (IPD = 6cm approx).
What is disparity used for?
- The amount of disparity is used to determine how far from the horopter objects fall, e.g. the greater the disparity, the further from the horopter
- Whether the disparity is crossed or uncrossed determines whether objects are perceived as in front of or behind the horopter
Describe Panum’s Fusional area
- Panum’s Fusional Area lies directly behind and in front of the horopter
- Objects in this area produce retinal images which have a small amount of disparity
- This amount of disparity is able to be fused into a single image
- This forms the basis of stereopsis or depth perception
Describe stereopsis in regards to Panum’s Fusional Area
- Objects within Panum’s Fusional Area are seen as signle
- The amount of disparity that is fused (depending on the distance from the horopter) determines the amount of perceived depth (e.g. more disparity = more depth)
- Depending on whether the fused disparity is crossed or uncrossed the image will be perceived as in front or behind.
Describe Stereopsis
- It is the ability to detect horizontal disparity in images and perceive and interpret this as depth
- The highest grade of binocular vision
- Stereopsis only occurs when disparate images within Panum’s Fusional area are fused horizontally
- Vertical disparity does not produce a stereoscopic effect
Binocular vision
VS
Binocular single vision
- The presence of any of the grades of BV indicates some binocular relationship between the eyes
- This does not necessarily mean that all grades of BV are present and functional
- In fact, a patient may have the basis for BV but, be currently experiencing diplopia due to misalignment of the eyes
- In the situation where the eyes are working together to create a single image, with normal retinal correspondence and all grades of BV present:
- This is called Binocular single vision
- Typically when we are testing BSV we are testing stereopsis
What roles do cortical cells play for processing of binocular visual information?
- In order to process BV information, cortical cells need to be specialised to respond to information from both eyes rather than each eye individually
- This is Binocular cortical cells
Describe the experiments conducted by Hibel and Wiesel 1965
Experiments by Hubel and Wiesel in 1965 found that:
- 80% of visual cortex neurones are used when they receive information from both eyes at the same time
- 10% are used by the right eye and only 10% by the left eye
- When cats were reared with normal straight eyes, the majority of the cells on one side of the visual cortex are binocurlarly drived. A smaller number are for the contralateral eye and some for the ipsilateral eye
- This gives rise to binocular vision and the potential for stereopsis
- But when a cat was reared with a strabismus, less binocular cells develop and the majority of cells are monocular
- This would mean that the cat would not develop any binocular vision and would be unable to have stereopsis
Describe the development of BV in birth
BV is not present at birth
- The visual system is not fully developed
- VA is poor
- Cortical cells have not differentiated to respond to binocular or moonocular stimulus
- Ocular coordination and motor functions are not developed
Thus, in order for normal development of BV and cortical links to be made; eyes must be aligned and the eyes must receive normal BV stimulus.
Also, during birth, babies do not have myelination in axon fibres, and no ability to fixate the eyes
Importance of development of BV
The coordination of eye movements and the reflexes involved in this are important for binocular development as they allow for a stable image which reaches both eyes simultaneously.
- The ability to foveally fixate on an object straight ahead (obvious in a child by the age of 6 weeks)
- The re-fixation reflex allows foveal fixation on a moving target and allows an infant to move their eyes from one object to another (6-8 weeks of age).
Re-fixation is the ability to move from one point of fixation to another
Tests done to check the development of BV
- Infants at the age of 8 weeks react to oncoming target by blinking and attempting to move out of the way
- This indicates some appreciation of depth but, is unlikely detection of changing image size
- Measuring the HR of babies when held over a ‘visual cludd’; saw increase after 6-8 weeks
