Stereoscopic Vision

1. Stereoscopic Vision

2. Humans are extremely good a judging the relative distances of
objects and can very easily judge which objects are closer and
which are further away.
This ability to perceive depth information within a visual scene is
greatly improved by having two eyes that are spatially separated.
However, depth perception is not solely a binocular phenomenon
there are a number of monocular cues that can also give
information about depth:

1.Motion Parallax
2.Relative size of known objects
3.Light and shade
4.Geometric Perspective
5.Surface texture
6.Overlapping contours

3. Perspective
Overlapping Contours

4. Relative size of known objects

5. Texture

6. Binocular Depth Cues & Stereopsis

7. F
F F
Uncrossed retinal disparity
Crossed retinal disparity
Corresponding points
Retinal Disparity

8. In a binocular subject the eyes are separated horizontally and hence
receive slightly different views of objects at different distances.
The disparity information combined with information derived from the
vergence system provides precise quantitative information about
object distance.
The perception of depth that is produced by binocular retinal disparity
is called STEREOPSIS.
Stereopsis is important for producing finely tuned depth perception at
near distances (particularly within arms length) when other depth cues
are absent.
Stereopsis is specified as an angle at the eye (unit = min arc or sec
arc)

9. The Vieth-Müller Circle

10. F
Left Eye Right Eye
f
l f
r
aP
P
p
1
p
r
q
l q
r
C
l
C
r
Vieth-Müller
Circle
aF

11. aP
The point P, unlike point F, is not
fixated but lies on the V-M circle.
Rays from this point strike the
retina at points p
l
& p
r
.
The convergence angle of this
point = aP
since it lies on the circumference
of the V-M circle :
aP=aF,
Since C
l
& C
r
fall on the same
circle the angles q
l
& q
r
are equal.
Therefore the displacements p
l
&
p
r
are equal and are
corresponding points.
F
P
Left Eye Right Eye
f
l
f
r
p
1
p
r
q
l
q
r
C
l
C
r
Vieth-Müller
Circle
aF

12. Panum’s Fusional Areas

13. Binocular disparity produces stereopsis only if the retinal disparity is
not too great.
If retinal disparity does not exceed a certain limit, then retinal images
are fused with the resultant perception of depth – stereopsis.
The area on the retina that corresponds to this area of binocular
fusion is referred to as Panum’s Fusional Area.
If retinal disparity is too great, binocular fusion does not occur. The
images fall on retinal positions that signal very different positions and
results in physiological diplopia.

14. F
F F
Corresponding points
Retinal Disparity

15. F F
Horopter
Panum’s
Fusional
Area
Diplopia
Diplopia
Diplopia

16. The Vieth-Müller Circle describes in very geometrical terms how
stereopsis might come about.
A more physiologically based concept for describing stereopsis is the
HOROPTER.
The horopter can be simply described as the locus of all points in the
binocular field that are seen as single.
As shown in the previous slide diagram below it can be thought of as
a curved line that passes through the fixation point that plots
corresponding points.

17. F F
Horopter
Panum’s
Fusional
Area Diplopia
DiplopiaDiplopia
Objects that fall close to
the horopter are also fused.
For these stimuli the retinal
disparity falls within
Panum’s Fusional Area and
the result is stereopsis.
For those objects located at
greater distances from the
horopter the disparity is too
great for the images to be
fused and the result is
physiological diplopia.

18. The Neurophysiological Basis of
Stereopsis

19. The Physiological Basis of Stereopsis
 
There is evidence that
stereopsis is coded by
neurons in the primary
visual cortex (V1).
Up to this point in the
visual pathway
information from each
eye is largely segregated.

LGN
Layers 2, 3, 5 ipsilateral
Layers 1, 4 6 contralateral
1
2 & 3
4A
4B
4C{
5A
5B
6
a
b
V1
MAGNO
(1 & 2)
1
2
3
4
5
6

20. Certain cells in V1 however receive inputs from two eyes and are
known as binocular neurons.
The pioneering work on the binocularity of cells in the brain was
carried out by Hubel & Wiesel in the 1960s.
They found that approximately 80% of the neurons in the primary
visual cortex of the cat were driven by both eyes.
In the monkey approximately 60% of neurons are binocular.

21. 0
10
20
30
40
50
60
1 2 3 4 5 6 7
Ocular Dominance
No. of Cells
Contralateral Equal Ipsilateral
Ocular dominance distribution of 233 cells
from the striate cortex of the cat. Each cell
is assigned to an ocular dominance group 1-7
according to the relative response weighting
from the two eyes.
(Wiesel & Hubel 1963)

22. Binocular neurons may act as disparity detectors and such cells are
responsive to stimuli at a specific distance – a simple scheme as to how a
binocular neuron might signal disparity is illustrated below: