Stereopsis Flashcards
What is depth perception?
Depth perception means you can see the world in 3D and understand how far or near an object is from you.
What are the two forms of vision?
Monocular clues i.e. strabismus
- Have DP monocularly but not as good as binocularly
Binocular clues - stereopsis = best form
- Stereopsis is used to judge depth
What are the monocular pictorial cues?
- Relative size
- Occlusion / Interposition
- Relative height
- Shadows
- Atmospheric perspective
- Linear perspective
- Familiar size
- Texture gradient
What is relative size?
When 2 objects that are equal in size - the object that is far away will take up less space than the one nearer.
What is occlusion/interposition?
When one object is hidden, the hidden object is considered to be further away
What is relative height?
Below horizon = objects further away have higher bases
Above horizon = objects further away have lower bases
What do shadows do?
e.g. A shadow removes the contradiction as to whether the object is in front or elevated.
What is atmospheric perspective?
Due to the light scattering by the atmosphere, objects that are a great distance away appear hazy.
What is linear perspective?
With the depth, the distance between parallel lines becoms smaller in 2D
What is familiar size?
Previous knowledge of the object’s size is used to determine the absolute depth of the object.
What is texture gradient?
Fine detail on the nearby objects can be seen clearly, but these details cannot be seen on far away objects.
What are th monocular movement produced cues?
- Motion parallax
- Kinectic depth effects
What is motion parallax?
Near objects pass the retinal image plane faster than the distant objects.
What is kinetic depth effects?
When a 3D form of an object can be perceived when the object is moving.
How do we perceive depth and size?
Using oculomotor cues.
Monocular = accommodation
Kinesthetic sensations from ciliary muscles sent to the visual cortex where it is used for interpreting distance/depth
Binocular = convergence
Kinesthetic sensations from EOMs helps in distance/depth perception.
i.e. MR used to converge - the more the MR contracts, the closer the object is.
What causes binocular disparity?
Eyes are located at different positions on the head. Therefore BV gives two slightly different images that are projected on the retina.
What causes stereopsis?
When the disparity of the two images are processed by the visual cortex and put together - it gives stereopsis.
What do binocular disparities allow?
They allow 3D viewing.
If you are looking at an object directly, you will not have any 3D as the fovea of the RE corresponds with the fovea of the LE.
To see something in 3D, you have to look at something just in front or just behind which will stimulate almost corresponding retinal points and this disparity will allow you to see in 3D.
What are the advantages of BSV?
- Stereopsis
- Binocular Summation
- Better space perception
- Eye hand co-ordination
- Better reading
- Seeing objects in camouflage
What jobs would be difficult to do without stereopsis?
Pilot
Brain surgeon
Dentist
Sportsmen, etc.
What is stereoacuity?
How good does it get?
It is the smallest depth that we can detect.
Greater the disparity the greater the depth effect.
What is the best observed stereopsis?
Best observers get as good as 2 seconds of arc
Average is 10 seconds of arc
What factors affect the stereo threshold?
- Practice effects - get better as you do it over and over again
- Luminance - difference in stereoacuity if being done in a light room vs dark room
- Exposure duration - when the px is not given enough time to perceive stereoacuity
- Retinal eccentricity
- Crowding effects
- Motions in depth
- Monocular blur - has more of an impact than binocular blur therefore better to correct both eyes as best as possible.
What is the development of stereopsis?
Absent initially.
Develops around 3-4 months old.
Reaches normal levels around 6months old.
Varies with age.
Is highly dependent on the test as it varies with the test.
Children = 40seonds of arc - Titmus > 9 years old
Adults = 60 seconds of arc - TNO < 70 years old
How is stereopsis tested?
Tested clinically.
Qualitative = evidence of stereopsis - but dont know how good.
Quantitative = measurement of stereoacuity with a graded response which is calibrated for a specific viewing distance
What is total dissociation?
How is it measured?
When the eye gets a different image to each eye.
Measured using stereoscopes - not used
Measured with synoptophores
How does a synoptophore work?
It is a qualitative test. A major amblyoscope.
Each eye has a different viewing tbe and two images are presented, one to each eye.
Ask the px if it looks like the arrows are coming towards you or away from you.
If px says one of those = they have stereopsis.
What is partial dissociation?
When both eyes see the same picture but each eye sees different parts of the image.
How is partial dissociation measured?
Polarisation
- Titmus (wirt)
- Randot
- Mallett unit
Red/Green Glasses
- TNO
Optical
- Lang
What tests are based on no dissociation?
Free space tests
- Perception of real depth
- No 2D stimulation
Langs 2 pen
Frisby
