3. OPTIC NERVE
Axons of the retinal
ganglionic cells.
Macular fibres – pass
straight into temporal
part of optic disc.
Temporal fibres – above &
below as arcuate fibres.
Nasal fibres – to nasal half
of disc as superior &
inferior part radiating
fibres.
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4. OPTIC CHIASMA
Flattened structure above
pituitary fossa.
Fibres from nasal half of
retinae decussate &
temporal half remains same.
Visual impulses from
temporal half of visual field
cross while from nasal half
remains same.
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5. OPTIC TRACTS
From Posterolateral
angle of Chiasma to
lateral geniculate
body.
Contains temporal
fibres of same side &
nasal fibres of
opposite side.
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6. LATERAL GENICULATE
BODIES
Retinotopic
projection.
Optic tract fibres project
detailed spatial
representation of retina
on LGB.
Lamellar structure of
LGB.
6 Layers.
1,4,6 receives from
nasal half.
2,3,5 from temporal
half.
In each layer point to
point representation.
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7. LATERAL GENICULATE
BODIES
Magnocellular &
Parvocellular layers.
1& 2 are Magnocellular
receives fibres from M
ganglion cells of retina.
3-6 are Parvocellular
receives from P ganglion
cells of retina
Functions.
Relay station.
Visual perception & to
“Gate” the transmission of
signals. – how much to pass
to cortex.
10-20% from retina.
80-90% from visual cortex
& other brain regions.
Signals from 2 eyes are
kept apart in LGB.
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8. OPTIC RADIATIONS
Axons of LGB relay
cells to visual cortex
on same side.
Maintains Retinotopic
organization.
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9. PROCESSING & ANALYSIS OF VISUAL
IMPULSE IN THE VISUAL CORTEX
Retinotopic organization
Functional anatomy & organization of visual
cortex
Visual areas (Classical nomenclature)
Primary visual cortex
Peristriate cortex
Parastriate cortex
Modified nomenclature of visual areas
Histological layers of primary visual cortex
10. RETINOTOPIC ORGANIZATION
Visual cortex – cortical
retina.
Its only in Visual
Cortex that impulses
from corresponding
points of 2 retina
meet.
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12. MODIFIED NOMENCLATURE
OF VISUAL AREAS
V1 – prim visual
cortex (Brodman 17)
V2 – greater part of
Brodmann 18
V3 – narrow strip of
area 18.
V4 – area 19.
V5 – Middle temporal
area.
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13. HISTOLOGICAL LAYERS OF
PRIMARY VISUAL CORTEX
6 layers.
I-III – thin, contains
pyramidal cells.
IV – thickest, contains
stellate cells.
Subdivided into a ,b , cα
& cβ
V & VI – relatively
thin.
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14. PHYSIOLOGICAL CONSIDERATION OF
VISUAL CORTEX
Concept of receptive field of striate cortex
Columnar organization of the striate cortex
Orientation column
Ocular dominance column
The colour blobs
15. CONCEPT OF RECEPTIVE FIELD
OF STRIATE CORTEX
Simple cells
Complex cells
Hyper complex cells
Retinal ganglion cells &
LGB respond to both
diffuse & spot stimuli but
cortex respond to straight
line, bar or edge in proper
spatial orientation.
So depending on receptive
field peculiarities cell types
are..
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16. SIMPLE CELLS
Mainly in layer IV.
Form the first relay station.
Respond to lines, bars & edges
only in particular orientation.
This orientation most effective
in evoking in response is
“receptive field axis
orientation”
Thus it detects not only lines &
borders but also orientation.
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17. COMPLEX CELLS
Above & below layer IV
in area 17,18,19.
They respond to lateral
movement of linear
stimulus without
change in orientation.
They receive signals
from both the eyes.
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18. Complex cells
Thus play an
important role in
detection of lines,
bars, edges especially
when moving.
So simple & complex
cells together called
“Features Detectors”
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19. Hyper complex cells
In layer II & III of
17,18 & 19.
Require line stimulus
of specific length.
Thus play a role in
detection of specific
length, angle & shapes.
Types (by Hubel &
Wiesel)
2 lower
2 higher.
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20. Columnar organization of the
striate cortex
Visual cortex organized
into vertically oriented
functional modules –
hypercolumns.
They respond to lines of
all orientation from
particular region in space.
Each include 3 types of
columns.
Orientation column
Ocular dominance
column
The colour blobs
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21. Orientation column
Like sensory cortex visual
cortex divided into vertical
columns from pial surface to
white matter .i. e vertical
grouping of cells with
identical orientation
specificity.
As one from one column to
next orientation preference
changes 5-10 degree.
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22. Ocular dominance column
Simple cells in layer IV
receives input from single eye
& complex & hyper complex
cells of layer above & below IV
receives binocular inputs.
So group of binocular complex
& hyper complex cells in layer
II,III,IV & VI & cells in layer IV
that receives inputs from same
eye is called ocular dominance
column.
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23. The colour blobs
Among primary visual
columns are special
column like areas called
colour blobs which
responds specially to
colour signals.
These are primary areas
for deciphering colours.
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24. Concept of serial & parallel
processing of visual information
Parallel processing pathways
Magnocellular pathways
Parvocellular pathways
Serial processing of visual information
Serial processing in the retina
Serial analysis of visual image in the visual cortex
25. Parallel processing pathways
2 types of cells found entire pathway from
retina to cortex.
Large ( Magnocellular)
Small (Parvocellular)
Like 2 separate lanes of road.
So different features of images analysed by
these pathways.
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28. Serial processing of visual
information in cortex.
Sequence from simple to
complex to hyper complex
cells forms a system of
serial analysis with more &
more details being
presented.
As simple cells are
monocular & complex cells
are binocular , complex
cells are at advance stage.
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31. Light sense.
It’s awareness of light.
Minimum brightness
required to evoke
sensation of light.
Human eye can work
normally over wide
range of illumination
by visual adaptation.
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32. Dark adaptation.
Def – ability of eye to
adapt to decreased
illumination.
Time taken to adapt is
dark adaptation time.
Rods used more in
dim light ( scotopic
vision) than cones
( photopic vision)
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33. Dark adaptation curves
Grapg shows visual
threshold falls
progressively in dark
room for ½ hour until
relatively constant
value is reached.
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34. Dark adaptation curves
Graph shows increase in
sensitivity of retina in 2
steps –
First rapid, short & small
due to cone adaptation
Second slow, long & large
due to rods adaptation.
Breakpoint is cone-rod
break or alpha point.
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35. Mechanisms of dark
adaptation.
Visual pigment
mechanism. – reversal of
mechanism of light
adaptation i. e.
regeneration of visual
pigments.
Change in Pupillary size. –
dilation about 30 times
Neural mechanism – visual
chain in retina.
Vit-A deficiency.
Elevates threshold for
dark adaptation due to
depletion of
photosensitive
pigments.
Night blindness –
Nyctalopia.
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36. Light adaptation.
Def – process by
which retina adapt
itself to bright light.
Very quick, over in 5
min.
It’s merely
disappearance of dark
adaptation.
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37. Mechanism.
Neural adjustment –
responsible for transient
effect.
Visual pigment
mechanism – reduction
of rhodopsin & cone
pigment due to bleaching
in light.
Pupillary mechanism –
constriction.
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38. The form sense
Ability to discriminate
between shapes of
objects.
Cones are imp.
Snellen’s chart is
measure of form
sense.
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39. visual acuity
Measurements of
threshold of
discrimination of 2
separate targets is
visual acuity.
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40. Components of visual acuity.
Minimum visible-
visibility or
detection.
The ability to
determine whether
object is present in
empty field
Depend on size, shape
& illumination.
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41. Components of visual acuity.
Resolution –
Discrimination of 2 separate
points.
Minimum separation is
minimum resolvable.
Measurement of threshold of
discrimination is function of
fovea centralis called ordinary
visual acuity.
Distance measured by angle
subtended by 2 points at nodal
point of eye. (MAR)
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42. Components of visual acuity.
Recognition.
Component by which we not
only discriminate spatial
characteristics but also
identifies pattern with
previous experience.
Spatial resolution with
cognitive components.
E. g- identification of faces.
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43. Components of visual acuity.
Minimum
discriminable or
hyperacuity.
Spatial distinction.
Threshold is lower
than ordinary acuity.
Vernier acuity.
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44. Snellen’s test types
Visual acuity is same as
minimum resolvable.
Principle – 2 points seen
separate only if they form
an angle of 1 min at nodal
point.
Each line form angle of
1min
Each letter form an angle
of 5 min.
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45. Critical flicker fusion frequency
When light stimuli given
intermittently produces
flicker sensation.
If frequency of it increased it
fuses to form continuous
stimulation
This frequency is Critical
flicker fusion frequency.
Temporal resolving power
of visual system.
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46. Contrast sensitivity.
Ability to perceive
slight change in
luminance between 2
regions separated by
borders.
Loss of contrast is
more important than
loss of acuity.
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47. Encoding of contrast.
At border between
light & dark.
In lighted portion
center is illuminated &
on cell ganglion cell
activity increased.
In darkened portion
surround is illuminated
& on cell ganglion cell
activity decreased.
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