EYE PHYSIOLOGY

1. VISUAL PATHWAYS VISUAL
CORTEX AND HEMIANOPIAS
BY
Dr.Nusrat Tariq

2. VISUAL PATHWAY

4. Visual Pathways
1.New System-
The optic tracts synapse in the thalamus and then pass to the visual
cortex
2. Old System
The optic tracts synapse in the hypothalamus(suprachiasmatic
nucleus ), then into the midbrain (pretectal nuclei ), superior
colliculus, and then the thalamus (ventral lateral geniculate nucleus).
In humans the new system is responsible for perception of virtually
all aspects of visual form, colors, and other conscious vision.
In many primitive animals, even visual form is detected by the older
system, using the superior colliculus in the same manner that the
visual cortex is used in mammals.

5. VISUAL PAYHWAY
•Vision is generated by photoreceptors in the retina.
•The visual nerve signals leave the retinas through the
optic nerves.
At the optic chiasma, the optic nerve fibers from the
nasal halves of the retinas cross to the opposite sides .
• The fibers from the nasal halves of the retinas join the
fibers from the opposite temporal retinas to form the
optic tracts.
•The fibers of each optic tract then synapse in the dorsal
lateral geniculate nucleus of the thalamus.
• From lateral geniculate nucleus , geniculocalcarine fibers
pass by way of the optic radiation (also called the
geniculocalcarine tract) to the primary visual cortex in the
calcarine fissure area of the medial occipital lobe.

6. Visual fibers also pass to several older areas of the brain
(1)from the optic tracts to the suprachiasmatic nucleus
of the hypothalamus, to control circadian rhythms
that synchronize various physiological changes of the
body with night and day;
(2) into the pretectal nuclei in the midbrain, to elicit
reflex movements of the eyes to focus on objects of
importance and to activate the pupillary light reflex;
(3) into the superior colliculus, to control rapid
directional movements of the two eyes;
(4) into the ventral lateral geniculate nucleus of the
thalamus and surrounding basal regions of the brain, to
help control some of the body’s behavioral functions.

7. OPTIC NERVE:
 Formed by the axons of ganglionic cells.
 leaves the eye through optic disk
 It consists of nerve axons from both the temporal
and the nasal retina
 Runs from the retina to the optic chiasma.
OPTIC CHIASMA:
 The area of crossing of the optic nerve fibers
 Axons from the nasal retina from both eyes cross
to the other side.
 Axons from the temporal retina's do not cross .
.

8. OPTIC TRACT:
• Formed by uncrossed fibers of optic nerve on the
same side and crossed fibers of optic nerve from the
opposite side.
• Due to crossing of medial fibers in optic chiasma:
The left optic tract carries impulses from temporal
part of left retina and nasal part of right retina, i.e.
it is responsible for vision in nasal half of left
visual field and temporal half of right visual field.
The right optic tract contains fibers from nasal half of
left retina and temporal half of right retina. It is
responsible for vision in temporal half of left visual
field and nasal half of right visual field

9. LATERAL GENICULATE BODY
• Majority of the fibers of optic tract terminate
in lateral geniculate body, which forms the
subcortical center for visual sensation.
• From here, the geniculocalcarine tract or
optic radiation arises.
• This tract is the last relay of visual pathway.

10. FUNCTION OF THE DORSAL LATERAL
GENICULATE NUCLEUS OF THE THALAMUS
Two principal functions:
1) It relays visual information from the optic tract to the
visual cortex. This relay function is so accurate that
there is exact point-to-point transmission from the
retina to the visual cortex.
2)The second major function of the dorsal lateral
geniculate nucleus is to “gate” the transmission of
signals to the visual cortex—that is, to control how
much of the signal is allowed to pass to the cortex.
.

11. The nucleus receives gating control signals from
two major sources:
(1)Corticofugal fibers returning in a backward
direction from the primary visual cortex to the
lateral geniculate nucleus.
(2) Reticular areas of the mesencephalon.
Both of these sources are inhibitory and, when
stimulated, can turn off transmission through
selected portions of the dorsal lateral geniculate
nucleus

13. Lateral Geniculate Nucleus
Uncrossed fibers end in layers Layers II, III, and
V (from ventral to dorsal).
 crossed fibers end in layers I, IV, and VI
receive signals from the of the opposite eye.

14. The dorsal lateral geniculate nucleus is divided in to:
1. Layers I and II called magnocellular layers
because they contain large neurons. These neurons
receive their input almost entirely from the large
type M retinal ganglion cells.
This magnocellular system provides a rapidly
conducting pathway to the visual cortex.
This system is color blind, transmitting only black
and white information.
Its point-to-point transmission is poor because
there are not many M ganglion cells, and their
dendrites spread widely in the retina.

15. 2. Layers III through VI called parvocellular
layers contain large numbers of small to
medium-sized neurons.
These neurons receive their input almost
entirely from the type P retinal ganglion cells
that transmit color and convey accurate
point-to-point spatial information.
 This system provides a moderate velocity
of conduction rather a high velocity

16. VISUAL CORTEX :
Divided into:
 primary visual cortex
 secondary visual areas

18. 1- Primary Visual Cortex.
 The primary visual cortex(called visual area I or the
striate cortex) lies in the calcarine fissure area (occipital
pole).
This area is the terminus of direct visual signals from the
eyes.
The peripheral retinal representation occupies the
anterior part of visual cortex.
Macular representation occupies the posterior part of
visual cortex near occipital pole.
The upper portion of the retina is represented
superiorly and the lower portion is represented inferiorly.

19. THE PRIMARY VISUAL CORTEX
the primary visual cortex has six major layers:
I
II
III
IV: a, b, cα, cβ
V
VI

21. Vertical Neuronal Columns in the Visual Cortex. The
visual cortex is organized structurally into several million
vertical columns of neuronal cells, with each column
having a diameter of 30 to 50 micrometers.
After the optic signals terminate in layer IV, they are
further processed as they spread both outward and
inward along each vertical column unit.
“Color Blobs” in the Visual Cortex:
• Special column-like areas Interspersed among the
primary visual columns .
• Receive lateral signals from adjacent visual columns and
are activated specifically by color signals.
•The primary areas for deciphering color.

22. 2-Secondary Visual Cortex.( visual association areas).
lie lateral, anterior, superior, and inferior to the primary
visual cortex.
 Secondary signals are transmitted to these areas for
analysis of visual meanings. concerned with the
interpretation of visual impulses
 Brodmann’s area 18 , which is where all signals from the
primary visual cortex pass next. Therefore, Brodmann’s
area 18 is called visual area II, or simply V-2.
The other, more distant secondary visual areas have
specific designations—V-3, V-4, and so forth—up to more
than a dozen areas. The importance of all these areas is
that various aspects of the visual image are progressively
dissected and analyzed.

27. Binocular Vision

28. Binocular Vision
Interaction of Visual Signals
from the Two Separate Eyes

29.  Visual signals from the two eyes are relayed in the lateral geniculate
nucleus.
 These signals remain separated from each other when they arrive in
layer IV of the primary visual cortex which is interlaced with stripes of
neuronal columns.
 The signals from one eye enter the columns of every other stripe,
alternating with signals from the second eye.
 This cortical area deciphers whether the respective areas of the two
visual images from the two separate eyes are “in register” with each
other—that is, whether corresponding points from the two retinas fit
with each other.
 The deciphered information is used to adjust the directional gaze of
the separate eyes so that they will fuse with each other (i.e., be
brought into “register”).
 The information observed about degree of register of images from
the two eyes also allows a person to distinguish the distance of
objects by the mechanism of stereopsis

30. TWO MAJOR PATHWAYS FOR ANALYSIS OF VISUAL
INFORMATION pathways in the secondary visual
areas :
1. THE FAST“POSITION” AND “MOTION” PATHWAY:
Analysis of Third-Dimensional Position, Gross Form, and Motion
of Objects .
2. THE ACCURATE COLOR PATHWAY:
Analysis of Visual Detail and Color.

32. Functions of visual cortex
Functions of primary visual cortex.(Brodmann’s
area 17).
1-Analysis of visual information by
(1) the fast“position” and “motion” pathway
(2) the accurate color pathway
2-Analysis of Contrasts in the Visual Image
3-Detects Orientation of Lines and Borders
by“Simple” Cells.

33. 4-Detection of Line Orientation When a Line Is Displaced Laterally
or Vertically in the Visual Field by “Complex” Cells.
5-Detection of Lines of Specific Lengths, Angles, or Other Shapes
6-Detection Of Color.
Functions of secondary visual cortex/ visual association
areas/Brodmann’s area 18 and 19/ peristriate ares.
1-various aspects of the visual image are progressively dissected and
analyzed.
2-Relates visual information received by primary visual cortex to past
experience.
3- person recognize and appreciates what he is seeing.

34.  Removal of the primary visual cortex in the human being causes:
• loss of conscious vision—that is, blindness.
• such “blind” people can still, at times, react subconsciously to
changes in light intensity, to movement in the visual scene, or,
rarely, even to some gross patterns of vision. These reactions
include turning the eyes, turning the head, and avoidance.
• This vision is believed to be subserved by neuronal pathways that
pass from the optic tracts mainly into the superior colliculi and
other portions of the older visual system.
 Removal of secondary visual cortex causes:
 DYSLEXIA or word blindness.Person does not understand meaning
of seen words.
 Person is unable to perceive shape size and meaning of objects.