V666 Binocular Visual Function

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V666 Binocular Visual Function Arthur Bradley,
Ph.D.
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Chapter 1 Binocular visual fields and the
anatomy of binocularity.
Unlike many animals, most of the visual field of
humans is seen by both eyes simultaneously (the
binocular visual field). This course will
summarize the anatomical consequences of having a
binocular visual field, the potential liability
of a binocular visual field, the visual benefits
of having a binocular visual field, and the
susceptibility of the underlying anatomy and
physiology of binocularity to disruption early in
development. In this chapter we examine the
geometry of the binocular visual field and
underlying anatomy that accompanies binocularity.
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Visual Field of Animal with lateral eyes
Monoc Field 180 degrees Total Field 360
degrees
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Visual Field of Human with Frontal Eyes
Monocular Field 145 degrees Total Visual field
180 Binocular Visual Field 130 degrees
Note if the total horizontal visual field is
gt180 degrees, this means that you can see behind
you. How is this possible?
Humans have frontal eyes and are blind to more
than half of the world. This loss must have come
with some gain.
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Monocular Visual Field
Notice that the temporal field is much larger
(generally around 90 degrees) than the nasal
field (55-65 degrees).
Temporal Field
q90
Visual axis
foveola
q55-65
Nasal Field
There are two possible factors that may limit the
extent of the nasal field in object space, the
nose (1), and in image space, the extent of the
temporal retina (2).
Nose
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Experiment to see what limits the nasal visual
field
If we rotate our eye temporally (same as moving
the nose away), this exposes the far temporal
retina to visual stimuli. Does this expand our
nasal field?
?
Eye rotation
We generally find that enabling stimuli to be
imaged on the far temporal retina fails to
significantly expand the nasal field. Conclusion
the far temporal retina, that normally looks
at the nose, is either not there or not
functional.
?
What can we learn from repeating this experiment
with a nasal rotation?
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Monocular Field
Binocular field sum of 2 nasal fields
foveola
Binocular Field
foveola
Thus, if nasal field is limited by nose and
temporal retina, so is the binocular field.
Monocular Field
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Monocular Crescent
30 deg
30 deg

120-130 degrees
Total Field 180 degrees
Binocular Visual Field
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Anatomy of the binocular visual system
Partial Decussation at the chiasm
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Visual Anatomy of Animal with Lateral Eyes
1. Complete Decussation 2. Single images
arriving in each hemisphere. No need to keep two
images in registration, and no possibility of RE
and LE images arriving at different locations in
same hemisphere and thus generating diplopia.
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Retinal Projections temporal retina (red)
projects ipsilaterally and the nasal retina
(blue) contralaterally.
Optic nerve
Optic nerve
Chiasm
Optic tract
Optic tract
LGN
Optic radiations
Optic radiations
Corpus callosum
Primary Visual Cortex (Broadmans area 17, V1)
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LGN laminae
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C
This Nissel stained section shows that the cell
bodies of the LGN neurons are organized into 6
discrete laminae. Layers 1 2 contain
Magnocellular (large) cell bodies, while 3-6
contain smaller parvocellular neurons.
I
4
C
3
I
P
2
I
P
C
P
1
P
M
M
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The previous slide shows that the LGN is a
strictly laminated structure. But that is only
part of the story. The input from the two eyes
is also strictly laminated. This is revealed by
injecting a radioactive amino acid into one eye,
and then examining the pattern of staining in the
R and L LGNs. The stain is carried anterogradely
along the retinal ganglion cell axons.
Right LGN
Left LGN
Notice that the staining pattern is different in
each LGN. Which eye was injected with
radioactive dye?
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Schematic showing how the neural images from the
right eye nasal retina and the left eye temporal
retinas image objects in the right visual field
and project to the same left LGN and are
organized into adjacent layers in the LGN. These
two neural images are very similar since the same
region of the visual field projects to these two
retinal sites.
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Orderly interleaving of Contra- and Ipsi- inputs
from LGN into layer IV in area V1 of cortex
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Simulation of Ocular Dominance Columns
Left
Right
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Simulation of Ocular Dominance Columns
Cortical Binocular Neural Image
Retinal Image

• Neural images from the two eyes are cut into
  slabs and interdigitated in visual cortex area V1.

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Notice that equally wide stained (dark) and
unstained (light) bands can be seen indicating a
precise and orderly interleaving of the right and
left eye signals in V1, and each eye connects to
equal numbers of cortical cells (each band is
about 0.5 mm wide).
Using the same radioactive amino acid labeling
used to view the ocular projections into the LGN,
we can also view the ocular projections in layer
IVc of V1 because the label is able to pass
between the retinal ganglion cell axon terminals
and the dendrites of the LGN neurons
(transneuronal transport). It then travels
anterogradely along these neurons and can then be
seen in the input layer of V1.
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In this example a monkey was trained to fixate
the center of this bull-eye pattern. While
viewing this monocularly, the animal was injected
intravenously with a radioactive glucose which is
absorbed by the metabolically active cells. The
picture below is a flattened section of primary
visual cortex showing the pattern of radioactive
staining. Notice the polar-to-cartesian
transform. Also notice the black/white banding
in the active cortex. This shows the ocular
dominance banding. When the same experiment is
run with binocular viewing, there is no banding.
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Although Hubel and Wiesel described the ocular
dominance columns as 0.5 mm wide, more recent
data (Horton) from monkey shows that they range
from 400 to 700 microns.
Narrow OD columns
Wide OD columns
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Evidence of ocular dominance columns in humans
(Cheng et al, 2001). 1. FMIR imaging maximum
resolution for fMIR is about 0.5 mm, which is
about the size of an ocular dominance column.
The data below show evidence of Ocular Dominance
columns in human primary visual cortex stimulated
with either the right eye (blue) or the left eye
(yellow).
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Achieving binocularity in the LGN the R and L
eye images are carefully aligned but separated
into different layers. In the input layer of V1
(layer IVc) the two neural images are still
monocular, but now precisely interleaved. The
next neuron in the visual pathway (layer IVc
projects to layers 23) receive converging input
from the R and L eyes, and thus these neurons now
respond to both eyes. We classify these neurons
as binocular and at this point there are no
longer two distinct neural images but one single
image. Therefore, it is at this point that
neural fusion of the R and L eye images occurs,
and this fusion of two images into one is the
basis for our single (fused) binocular vision.
Bonocular neurons
Layer 2/3
As we shall see, when this fusion cannot happen,
we can end up seeing double (diplopia).
Monocular neurons
Layer IVc
OD
OS
Optic Radiations LGN axons
Monocular LGN axons
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Much of what we know about the binocular visual
system comes from studies of the physiological
properties of single neurons in the visual cortex.
In this picture (from David Hubels book) we see
an electrode inside of the visual cortex with
several pyramidal cells.
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Ocular Dominance Histograms
Monkey
Cat
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Two examples showing two electrode tracks (red
arrows) through V1.
Track through layers 23
Track through layer 4
Cortical Layers
Anatomy
0.5 mm
Physiology
Ocular Dominance
1mm
Hubel and Wiesel developed a 7 point scale to
describe the relative ability of the
contralateral and ipsilateral eye to activate
individual cells in the brain. A group 1 cell
can only be activated by the contralateral eye,
group 7 only by the ipsilateral eye. The two
groups are described as monocular cells, groups
2-6 are all binocular in that they can be
activated by either eye. Group 4 cells are
equally activated by either eye. The above
diagram shows the ocular dominance category of
each cell encountered on the two electrode
tracks, one through layer IV and one through
layers 2 3. Notice that binocular cells are
not observed in layer IVc, but the rest of the
cortex is dominated by binocular cells.
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X
Visual Axes
Neural basis of diplopia Two neural images
instead of one.
LE Esotrope
When a target is imaged on to different locations
in the two eyes, it projects to different
locations in visual cortex, and the end result is
that both separate neural images exist
simultaneously in the cortex, and therefore we
see the target in two separate directions at once
(diplopia).
X
X
Foveal projection
X
X
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Role of Corpus Callosum A special case of
binocularity
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Binocular Vision mediated by Corpus Callosum
Visual axes
?
Fixation point
Right Field Left Hemisphere
Left Field Right Hemisphere
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How can the neural signal generated by objects
that lie between the visual axes ever converge
onto a single binocular neuron?
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Consequences of abnormal anatomy
Loss of binocular Visual Function as a result of
saggital sections in Corpus Callosum or Chiasm
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Section Chiasm saggitally (rare accident)
Section Callosum (once done to treat severe
epilepsy)
Lose all contralateral connections. Thus lose
all vision in monocular crescents, and small
region between visual axes and farther than
fixation point. Normal binocular field becomes
monocular, but one (small) area survives.
Lose stereopsis (binocularity) in the region of
the visual field between the two visual axes.
Subjects had normal stereopsis to right and left
of fixation, but were stereo-blind directly in
front and behind fixation.