The human eye functions similarly to a camera, with light entering through the cornea and lens, which focus an image onto the retina. Photoreceptor cells in the retina, namely rods and cones, detect light and initiate a signal along the optic nerve to the brain for visual processing. Rods function under low light and do not detect color, while cones require higher light levels and detect color. The lens adjusts its curvature through ciliary muscle contraction and relaxation to focus on near and far objects. Information from the retina is transmitted to the visual cortex via the lateral geniculate nucleus for interpretation and perception of vision.

1. Physiology of Vision

2. Eye
• like a camera
• cornea and lens focus an image of distant
objects on retina “film”
• contraction of ciliary muscles changes shape of
lens to bring objects into focus
• adjustment of pupil diameter helps maintain
proper light exposure to retina

3. The Eye is a camera
• The human eye is a camera!
– Iris - colored annulus with radial muscles
– Pupil - the hole (aperture) whose size is controlled by the iris
– What’s the “film”?
– photoreceptor cells (rods and cones) in the retina

4. Image Formation
Digital Camera
The Eye
Film

5. Monocular Visual Field: 160 deg (w) X 135 deg (h)
Binocular Visual Field: 200 deg (w) X 135 deg (h)

6. Processes for Image Formation
• refraction of light rays by cornea and lens
• accommodation of the lens
• constriction of the pupil
accommodation and pupil size are controlled by
smooth muscle fibers of ciliary muscle and iris
(intrinsic eye muscles)

7. Refraction
• the bending of light as it passes at an oblique
angle from one medium (such as air) to
another (such as water)

8. Refraction (cont.)
• anterior and posterior surfaces of cornea refract
light
• both surfaces of lens further refract light into exact
focus on retina
• images are inverted (upside down) and reversed
right to left
brain learns early in life to coordinate visual images
with location of object

9. Refraction (cont.)
• 3/4 of the focusing occurs on the cornea
• lens is responsible for fine-tuning of image
• convex surface of the lens causes light waves to
converge (come to a point)
• concave surface of lens causes light waves to
diverge (fan out)
• normal eye shape causes light waves to be
sharply focused upon retina

10. Light Refraction
• light waves of distant objects travel at almost
parallel angles - focused on retina by cornea
and flatter lens
• light waves of nearer objects reach eye in a
more divergent line - the closer the object, the
more divergent the lines

11. © Stephen E. Palmer, 2002
Visual Cortex
aka:
Primary visual cortex
Striate cortex
Brodman’s area 17
Cortical Area V1
Thalamus
LGN
Striate
cortex
(V1)
Dorsal
Stream
Parietal
visual
cortex
Temporal
visual
cortex
Eye Optic
nerve
Extrastriate
cortex
Ventral
Stream

12. Visual Pathway
• begins in photoreceptors of retina
- stimulated by image focused on retina
• receptor potentials travel via optic nerve to
lateral geniculate nucleus in thalamus then on
to visual cortex on occipital lobe
• processing of visual information occurs along
entire pathway

14. Photoreceptors
• rods - 20 million
- stimulated by low intensity light
• cones - 6 million
- stimulated by high intensity light of color
- three types of cones
- named for different appearance of their outer
segment
- divided into outer and inner segment

15. Photoreceptors
layer of rods
and cones

16. The Retina
Cross-section of eye
Ganglion cell layer
Bipolar cell layer
Receptor layer
Pigmented
epithelium
Ganglion axons
Cross section of retina

17. Retina up-close

18. Rods (cont.)
• predominant type of photoreceptors
• found in all areas of retina except fovea centralis
• extremely sensitive to light
• in dim light rods are the only photoreceptor
stimulated
• do not distinguish color
• all night images are black and white
• image produced is not sharp

19. Cones
tapered or cone-shaped
• Outer segment contains:
- pigment-containing saccules
• Inner segment contains:
- many mitochondria
- cell nucleus
- large synaptic base which most likely
contains neurotransmitter glutamate

20. Cones (cont.)
• fovea centralis contains a high concentration of
cones
• depression on fovea centralis increases
exposure of cones to light waves (sharpest
image)

21. Cones (cont.)
• Photopigments:
- blue-green
- green-sensitive
- red-sensitive

22. © Stephen E. Palmer, 2002
Cones
cone-shaped
operate in high light
color vision
Two types of light-sensitive receptors
Rods
rod-shaped
operate at night
gray-scale vision

23. © Stephen E. Palmer, 2002
Distribution of Rods and Cones
.
0
150,000
100,000
50,000
0
20 40 60 80
20
40
60
80
Visual Angle (degrees from fovea)
Rods
Cones Cones
Rods
Fovea
Blind
Spot
#
Receptors/mm
2
Night Sky: why are there more stars off-center?

24. PHYSIOLOGY OF VISION
• Rays falling on optic disc – no visual sensation
–Blind spot (of Mariotte)
• Light falling on retina causes two essential
reactions
– Photochemical – rods and cones
– Electrical

25. RHODOPSIN (Visual Purple)
• Visual pigment in the outer segment of rods
and cones
• Responsible for scotopic vision
• Chromoprotein

26. RHODOPSIN (Visual Purple)
• Reactive part, chormophore – 11 cis retinal
• Embedded by three chemical bonds inside a single
transmembrane protein – opsin
• Role in transduction cascade – to activate transducin
on absorption of light (Phototransduction)
• Caretonoid (Vit A )

27. RHODOPSIN CYCLE
• Absorption of a photon of light leads to initial
photoreaction
• Conformational changes of protein result in
activation of transducin
• Due to photoreaction rhodopsin loses its color –
bleaching
• Cis-trans isomerization of protein occurs during
bleaching
• Many intermediates formed during the cycle

29. Rhodopsin Cycle

30. VISUAL IMPULSE
• Photoreceptors convert light energy into
electrical signals
• Transmit signals to bipolar cells
• Communicate with Retinal ganglion cells
• Axons of ganglion cells relay in Lateral
geniculate body

32. Parvocellular system
• P cells constitute 90 percent of retinal ganglion cells
• Smaller; thinner axons of small calibre
• Predominant type in macular region
• Color sensitive ; physiological response- color opponent type with
with high spatial frequency resolution
• Respond transiently to constant stimulation with short latencies
• Color selective
• Axons terminate in parvocellular layers of lateral geniculate nucleus
(layers 3-6)

33. Magnocellular system
• Larger cells
• Thicker, larger axons
• Fast conducting
• Transmit high temporal motion related information of low spatial
frequency
• Unrelated to color
• Axons synapse in magnocellular layers of lateral geniculate nucleus
(layers 1 and 2)

34. LATERAL GENICULATE BODY
• Regulates neural information from the retina to and from
the cortex
• Organizes information into different layers based on
– Eye stimulus received from Originating retinal ganglion cells
• Lateral geniculate nucleus – comprised of multiple layers
with each receiving input only from one eye
• Ipsilateral input – layers 2, 3 and 5
• Contralateral input – layers 1, 4 and 6

35. VISUAL CORTEX
• 2 areas:
– Primary visual cortex
• Striate cortex (V1)
• Transforms information received from lateral geniculate
nucleus
• Distributes it to separate domains in secondary visual
cortex
– Secondary visual cortex
• Extrastriate cortex (V2)
• Transmits information to higher visual areas

41. Accommodation
• process by which the curvature or thickness of
the lens is increased for near vision
• divergent waves tend to focus behind the
retina unless accommodation increases
refracting power of the eye

43. Near Point
• minimum distance at which an object can
be brought into clear focus
• 4 inches in young adult
• increasing distance at which an object can
be brought into clear focus is primarily due
to loss of elasticity and hardening of the
lens, therefore its ability to accommodate
- this condition is called presbyopia

44. Near response
Three-part response that consists of :
1 accommodation.
2 convergence of the visual axes.
3 pupillary constriction.

45. Thank you