Stereopsis Flashcards
Visual perception of space can be divided into 2 components:
- Bi-dimensional Localisation
The judgement of the direction of an object (i.e. its position relative to the egocentre) and its angular extent. - Tri-dimensional localisation
The judgement of depth and distance and the appreciation of size.
Bi-dimensional localisation
Our ability to accurately locate the direction of an object with respect to the egocentre depends on the combination of two factors:
1) position of the image on the retina
2) position of the eye with respect to the rest of the body
Any image falling on the fovea will be localised as being?
In the centre of our visual perception.
Thus the fovea is said to have principle visual direction or principle local sign
Each point on the retina has a local sign
The further the image falls on the fovea the further away from the centre of our perception
Retinal positions are considered to have a retinomotor value. What does this mean?
Amount of eye movement needed to foveate peripheral objects. (Change from looking centrally to something in the periphery so that becomes on the fovea)
What are some monocular cues for depth perception (can still recognise depth with just one eye)
- Motion parallax
- Relative size of known objects
- Light and shade
- Geometric perspective
- Surface texture
- Aerial perspective
- Overlapping contours
When testing for stereopsis what do we want to eliminate?
Any monocular cues
Is stereopsis more important for near objects or distance?
Stereopsis is important for producing finely tuned depth perception at near distances (particularly within arms length) when other depth cues are absent.
Stereopsis is less important when viewing objects that are at far distances because the threshold for retinal disparity, which is specified as an angle at the eye (unit = min arc), requires such objects to be separated by great distances.
What are some clinical tests of stereoacuity
Titmus fly test
TNO test
Frisby stereo test
Lang stereo test
Stereo acuity on mallet unit
Titmus fly test
Uses Polaroid filters
Shows large disparities - should be seen by most subjects
Useful for young children asked to pick the flys wings
Animal test - performed if growth stereopsis is present. three rows of five animals. PX asked which one animals stand out.
Circle test - nine squares each containing four circles one circle is imaged at random with the threshold ranging from 800 to 40 seconds of arc. If PX passed other two tests, he is asked to push down circle that stands out.
TNO test (random dot stereogram)
- red-green spectacles
- provides retinal disparities from 15 to 480sec of arc
- booklet containing seven plates
- 2 butterflies one can be seen monocularly and 2 can be seen binoculars with glasses on
First four plates - disparity is large and provides qualitative assessment of stereopsis
Plate 1-3 correct but not 5-8 -> gross stereopsis
Plate 4 incorrect —> record which is the suppressing eye
Plates 5-8 correct —> record stereo acuity as the highest level we are both responses were correct
Lang stereo test
- Simplifies stereopsis screening in children
- Based on random dots and cylindrical gratings
- Single card that can be held easily 40 cm away
- Has three levels of stereopsis. All at low-level.
Results:
Positive —> correct localisation and naming of all hidden object
Negative—> no object detected and eye movements do not indicate recognition of 3-D objects- refer to specialist
Doubtful—> only one hidden object localised and named correctly eyes scanning the plate for object- refer to specialist
.
Frisby stereo test
Three plastic cards. Each has 4 squares and in one of them there is a hidden circle which is seen disparity.
Disparity created by displacement of random shapes by thickness of the plate.
Results:
Determine lowest disparity that PX can manage
Can use range of plate thicknesses and hold different viewing distances
Thinner the plate, greater the distance finer the depth discrimination
