Discuss the visual pathway and how humans see objects

1. VISUAL PATHWAY
SCK

2. Visual pathway consists of
Six components:
1. Optic nerve
2. Optic chiasma
3. Optic tract
4. Lateral geniculate body
5. Optic radiation
6. Visual cortex.

3. OPTIC NERVE
• Optic nerve is formed by the axons of ganglionic
cells.
• Optic nerve leaves the eye through optic disk.
• The fibers from temporal part of retina are in
lateral part of the nerve and carry the impulses
from nasal half of visual field of same eye.
• The fibers from nasal part of retina are in medial
part of the nerve and carry the impulses from
temporal half of visual field of same eye.

4. OPTIC CHIASMA
• Medial fibers of each optic nerve cross the
midline and join the uncrossed lateral fibers of
opposite side, to form the optic tract.
• This area of crossing of the optic nerve fibers
is called optic chiasma.

5. OPTIC TRACT
• Optic tract is formed by uncrossed fibers of optic nerve on the same side and
crossed fibers of optic nerve from the opposite side.
• All the fibers of optic tract run backward, outward and towards the cerebral
peduncle.
• While reaching the peduncle, the fibers pass between tuber cinereum and
anterior perforated substance.
• Then, the fibers turn around the peduncle to reach the lateral geniculate body in
thalamus. Here, many fibers synapse while few fibers just pass through this and
run towards superior colliculus in midbrain.
• Fibers from fovea do not enter superior colliculus. Some fibers from fovea of each
side pass through the optic tract of same side and others through the optic tract of
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.
• 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.

6. 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.
• Some of the fibers from optic tract do not synapse in lateral
geniculate body, but pass through it and terminate in one of
the following centers:
I. Superior colliculus: It is concerned with reflex
movements of eyeballs and head, in response to optic
stimulus
II. Pretectal nucleus: It is concerned with light reflexes
III. Supraoptic nucleus of hypothalamus: It is concerned
with the retinal control of pituitary in animals. But in
human, it does not play any important role.

7. OPTIC RADIATION
• Fibers from lateral geniculate body pass through
internal capsule and form optic radiation.
• The fibers between lateral geniculate body and
visual cortex are also called geniculocalcarine
fibers.
• Optic radiation ends in visual cortex

8. VISUAL CORTEX
• Primary cortical center for vision is called visual
cortex, which is located on the medial surface of
occipital lobe.
• It forms the walls and lips of calcarine fissure in
medial surface of occipital lobe.
• There is a definite localization of retinal
projections upon visual cortex.
• The peripheral retinal representation occupies the
anterior part of visual cortex. Macular
representation occupies the posterior part of
visual cortex near occipital pole.

9. Areas of Visual Cortex and their
Function
• Three areas are present in visual cortex:
1. Primary visual area (area 17), which is
concerned with the perception of visual
impulses.
2. Secondary visual area or visual association
area (area 18), which is concerned with the
interpretation of visual impulses.
3. Occipital eye field (area 19), which is
concerned with the movement of eyes

11. APPLIED PHYSIOLOGY – EFFECTS OF LESION
AT DIFFERENT LEVELS OF VISUAL PATHWAY
• Loss of vision in one visual
field is known as anopia.
• Loss of vision in one half of
visual field is called
hemianopia.

12. Hemianopia is classified into two types:
1. Homonymous hemianopia
2. Heteronymous hemianopia.
Homonymous hemianopia: Homonymous hemianopia means loss of vision in the same halves
of both the visual fields. Loss of vision in right half of visual field of both eyes is known as right
homonymous hemianopia. Similarly, left homonymous hemianopia means loss of vision in left
half of visual field of both eyes.
Heteronymous hemianopia: Heteronymous hemianopia means loss of vision in opposite halves
of visual field. For example, binasal heteronymous hemianopia means loss of vision in right half
of left visual field and left half of right visual field (nasal half of both visual fields). Bitemporal
heteronymous hemianopia is the loss of sight in left side of left visual field and right side of right
visual field (temporal half of both visual fields).

13. Effects of Lesion of Optic Nerve
• Lesion in one optic nerve will cause total
blindness or anopia in the corresponding
visual field. Lesion occurs due to increased
intracranial pressure.

14. Effects of Lesion of Visual Cortex
• Macular sparing In all the conditions mentioned above, total blindness
does not occur because, the macular vision is not lost.
• This phenomenon in which the macular vision is retained (unaffected) in
conditions of hemianopia is called macular sparing.
• Macular sparing occurs because of the following reasons:
1. Fibers from macula project into the visual cortex of both sides.
2. Fibers from macular region are projected into both anterior and posterior
parts of each visual cortex.
Only the bilateral lesion of visual cortex causes total blindness.