Download In the Light

Visual
Sensory
System
Wavelengths of Light
Electromagnetic Spectrum
Light:
1. Wave
2. Stream of photons
Photon: one quantum
of energy
Light can be:
1. absorbed
2. diffracted
3. reflected
4. transmitted
5. refracted
Light Can Be:
1. Absorbed:
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
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
Accommodation
Astigmatism
The Retina
(make up the optic nerve)
Rods
Cones
•100-120 million
•sensitive to dim light
•black/white discrimination
•large numbers on the periphery
•4-6 million
•used for color vision
•located near the fovea
•red, green, and blue cones
Rod/Cone Distribution
The Retina
•Ganglion cells
•Horizontal cells
•Bipolar cells
•Amacrine cells
•Photoreceptors
•Rods
•Cones
Visual Pathway
Light to rods/cones to bipolar cells to ganglion cells to LGN cells to Striate Cortex
Photoreceptor
Action
Not Active
Bipolar
cell
Active
In the Dark:
•rods are depolarized
•rods release glutamate
•glutamate is inhibitory
•bipolar cells are inhibited
Glutamate
(-)
In the Light:
•rods are hyperpolarized
•no glutamate is released
•bipolar cells are not
inhibited (disinhibition)
•bipolar cells undergo
spontaneous activity
DARK
Rod
cell
LIGHT
Light Hyperpolarizes the Rods
Rhodopsin Photopigment
Rhodopsin:
made up of retinal and opsin
spans the disc membrane
acts as a G-protein
Light Transduction
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)
LIGHT
•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
Rhodopsin Cascade
Rhodopsin molecule
LIGHT
Rod cell
disc
Inside Rod cell
1 photon of light can block the entry of 1,000,000 Na+ ions
Outside
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
Lateral Discrimination
Result of Lateral Inhibition:
Each strip has a uniform color,
but all look lighter on the left.
Brightness Contrast
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
Receptive Field of a Photoreceptor
Receptive Field of a Ganglion cell
Determining Receptive Fields
Specific stimulus
presentation
Specific cell
recording
On-Center
Off-Surround
Receptive
Field
Off-Center
On-Surround
Receptive
Field
Lateral Geniculate Nucleus (LGN)
6
5
Parvocellular layers
layers 3-6
smaller cells
4
3
2
Magnocellular layers
layers 1-2
larger cells
1
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
LGN
Receptive Field
Orientation
Sensitive Cells
Motion
Sensitive Cells
How Receptive Fields Sum to Images
There is convergence
of information as you move
from retina to the visual cortex
Striate Cortex
Striate
Cortex
Mapping
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
Orientation and Ocular Dominance
Color Vision
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
Trichromacy
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
Support for the Opponent-Process
Monkey LGN cell 1
inhibited
Monkey LGN cell 2
excited
excited
inhibited
Negative Afterimage