1. Visual Sensory System

2. Wavelengths of Light Electromagnetic Spectrum Light: Light can be:
2. Stream of photons
Photon: one quantum of energy
Light can be:
1. absorbed
2. diffracted
3. reflected
4. transmitted
5. refracted

3. Light Can Be: 1. Absorbed: 2. Diffracted: 3. Reflected:
light energy is taken up by encountered material
2. Diffracted:
light energy can be bent or scattered
3. Reflected:
light energy can be redirected back to its source
4. Transmitted:
light energy can be transmitted through material
5. Refracted:
light energy can be altered as it passes through material

4. Sensory System: Vision
Cornea
protects eye
refracts light
Iris
colored muscle
regulates pupil size
Pupil
regulates light input
Lens
focuses images on retina
Ciliary Muscles
controls shape of lens
accomodation
Fovea
point of central focus
contains most cones
birds of prey/rodent variation
Retina
contains photoreceptors
The Structure of the Human Eye

5. Accommodation

6. Astigmatism

7. The Retina (make up the optic nerve) Rods Cones 100-120 million
sensitive to dim light
black/white discrimination
large numbers on the periphery
Cones
4-6 million
used for color vision
located near the fovea
red, green, and blue cones

8. Rod/Cone Distribution

9. The Retina Ganglion cells Horizontal cells Bipolar cells
Amacrine cells
Photoreceptors
Rods
Cones

10. Visual Pathway
Light to rods/cones to bipolar cells to ganglion cells to LGN cells to Striate Cortex

11. Photoreceptor Action Bipolar cell Active Not Active In the Dark:
rods are depolarized
rods release glutamate
glutamate is inhibitory
bipolar cells are inhibited
In the Light:
rods are hyperpolarized
no glutamate is released
bipolar cells are not
inhibited (disinhibition)
bipolar cells undergo
spontaneous activity
Glutamate
(-)
Rod
cell
LIGHT
DARK

12. Light Hyperpolarizes the Rods

13. Rhodopsin Photopigment
made up of retinal and opsin
spans the disc membrane
acts as a G-protein

14. Light Transduction LIGHT DARK trans-retinal transformed to cis-retinal
cis-retinal and opsin form rhodopsin
rhodopsin activates guanylate cyclase (GC)
GC increases the synthesis of cGMP
cGMP opens Na+ channels
rod cell depolarizes
increases the release of glutamate
(darkness adjustment–waiting for rhodosin)
cis-retinal transformed to trans-retinal
trans-retinal and opsin dissociate
now active opsin activates transducin
transducin activates PDE
PDE breaks down cGMP to 5’-GMP
5’GMP closes Na+ channels
rod cell hyperpolarizes
reduces the release of glutamate

15. Rhodopsin Cascade LIGHT Rod cell disc Inside Rod cell Outside
Rhodopsin molecule
LIGHT
Rod cell
disc
Inside Rod cell
Outside
1 photon of light can block the entry of 1,000,000 Na+ ions

16. Lateral Inhibition
Brightness Contrast is created in part by the wiring of the visual system.
Horizontal cells run perpendicular to the photoreceptors.
These lateral connections inhibit neighboring cells.
This antagonistic neural interaction between adjacent regions of the retina creates brightness contrast

17. Lateral Discrimination
Result of Lateral Inhibition:
Each strip has a uniform color,
but all look lighter on the left.
Brightness Contrast

18. Receptive Fields Visual Field:
the whole area of the world that you can see at any time
Right/Left Visual Field:
the part of your visual field only to the right or left
Receptive Field:
the part of the visual field that only one neuron responds to

19. Receptive Field of a Photoreceptor

20. Receptive Field of a Ganglion cell

21. Determining Receptive Fields
Specific stimulus
presentation
Specific cell
recording

22. On-Center Off-Surround Receptive Field

23. Off-Center On-Surround Receptive Field

24. Lateral Geniculate Nucleus (LGN)6 5
Parvocellular layers
layers 3-6
smaller cells 4 3 2 1
Magnocellular layers
layers 1-2
larger cells

25. LGN Mapping
Input from the right visual field is mapped on the left LGN.
Input from the left visual field is mapped on the right LGN.
LGN layers 1-6

26. Orientation Sensitive Cells
LGN
Receptive Field
Orientation Sensitive Cells
Motion Sensitive Cells

27. How Receptive Fields Sum to Images
There is convergence
of information as you move
from retina to the visual cortex

28. Striate Cortex

29. Striate Cortex Mapping

30. Orientation and Ocular Dominance
Hubel et al. 1978
Column: a vertical arrangement of neurons
Orientation Columns: systematic, progressive change in preferred orientation
Ocular Dominance Columns: preferential response to one eye stimulation

31. Orientation and Ocular Dominance

32. Color Vision

33. The Trichromatic Theory (Young-Helmholtz)
Can get any color by mixing just three wavelengths
Blue-sensitive cones
Green-sensitive cones
Red-sensitive cones
Each type of cone would have a direct path to the brain
Discriminate among wavelengths by the ratio of activity
across the three different types of cones
To see purple:
60% (of maximum) blue-sensitive cone response
50% red-sensitive cone response
5% green-sensitive cone response
Many areas of the retina lack the diversity to follow this rule

34. Trichromacy

35. Opponent-Process Theory
We perceive color in terms of paired
opposites:
red versus green
blue versus yellow
white versus black
Blue-Yellow Opponent Bipolar cell
Excited by:
short wave or blue light
Inhibited by either:
long wave or red light
medium wave or green light
but strongest by a mix of two-yellow
When Excited – blue perception
When Inhibited – yellow perception

36. Support for the Opponent-Process
Monkey LGN cell 1
Monkey LGN cell 2
inhibited
excited
excited
inhibited

37. Negative Afterimage