1 1 The Major Senses zThere are 6 major senses yvision yhearing ytouch ytaste ypain ysmell zThe list can be extended with balance, joint senses and others.
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11
The Major Senses
There are 6 major senses vision hearing touch taste pain smell
The list can be extended with balance, joint senses and others
Vision has been studied most extensively
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Vision
Purpose of the visual system transform light energy into an electro-
chemical neural response represent characteristics of objects in
our environment such as size, color, shape, and location
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Light - The Visual Stimulus
Gammarays
X-raysUltra-violetrays
Infraredrays
RadarBroadcastbands
ACcircuits
Visible light
PrismWhitelight
400 500 600 700
10 -5 10 -3 10 -1 10 1 10 3 10 5 10 7 10 9 10 11 10 13 10 15 10 17
Wavelength in nanometers (billionths of a meter)
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Light - The Visual Stimulus
Light can be described as both a particle and a wave
Wavelength of a light is the distance of one complete cycle of the wave
Visible light has wavelengths from about 400nm to 700nm
Wavelength of light is related to its perceived color
55
Structure of the Eye
The eye works like a camera, using a lens to focus light onto a photo-sensitive surface at the back of a sealed structure.
Light rays
Cornea
Pupil
Blind spot
Opticnerve
Retina
Fovea (pointof central focus)
Lens
Iris
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Organization of Retina
5 cell types Photoreceptors
rods and cones Horizontal Cell Bipolar Cell Amacrine Cell Ganglion Cell
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Organization of Retina
Toopticnerve
Ganglioncell
Amacrinecell
Bipolarcell
Horizontalcell
Cone Rod
Light
Light
Cross section of retina shown vastly magnifiedin the diagram to the right
Photochemical is located hereBack of the eye
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Function of Photoreceptors
The photoreceptors transduce the energy in light into a neural response
This occurs when light entering the eye is absorbed by photopigment molecules inside the photoreceptors
When light interacts with the photopigment, it results in the photoreceptor becoming more negatively charged (hyperpolarization)
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Distribution of Rods and ConesCones - concentrated in center of
eye (fovea) approx. 6 million
Rods - concentrated in periphery approx. 120 million
Blind spot - region with no rods or cones
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Distribution of Rods and Cones
Th
ou
san
ds
of
rod
s p
er
squ
are
mil
lim
ete
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Blind spotFovea
60 40 20 0 20 40 60
180
140
100
60
20
0
180
140
100
60
20
060 40 20 0 20 40 60
Distance on retina from fovea (degrees)
FoveaBlind spot
Th
ou
san
ds
of
con
es p
er
squ
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mil
lim
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Distance on retina from fovea (degrees)
Fovea
Blind spot
Rods Cones
1111
Differences Between Rods and ConesCones
allow us to see in bright light allow us to see fine spatial detail allow us to see different colors
Rods allow us to see in dim light can not see fine spatial detail can not see different colors
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Receptive Fields and Rod vs. Cone Visual Acuity
Light
Spots of light Spots of light
Receptivefields
Ganglioncells
Bipolarcells
Photo-receptors(cones)
Photo-receptors(rods)
Pigmentedepithelium
Light
(a) Fovea (b) Periphery of retina
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Receptive Fields and Rod vs. Cone Visual AcuityCones - in the fovea, one cone often
synapse onto only a single ganglion cellRods - the axons of many rods synapse
onto one ganglion cellThis allows rods to be more sensitive in
dim light, but it also reduces visual acuity
1414
Color Vision
Our visual system interprets differences in the wavelength of light as color
Rods are color blind, but with the cones we can see different colors
This difference occurs because we have only one type of rod but three types of cones
1515
Color Mixing
Two basic types of color mixing subtractive color mixture
example: combining different color paints
additive color mixtureexample: combining different color lights
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Additive Color Mixture
By combining lights of different wavelengths we can create the perception of new colors
Examples: red + green = yellow red + blue = purple green + blue = cyan
1717
Trichromatic Theroy of Color VisionResearchers found that by mixing only three
primary lights (usually red, green and blue), they could create the perceptual experience of all possible colors
This lead Young and Helmholtz to propose that we have three different types of photoreceptors, each most sensitive to a different range of wavelengths
1818
Sensitivity Curves for the Three Types of Cones
Physiological studies revealed that Young and Helmholtz were correct
We have three types of cones
Light of different wavelengths will stimulate these cone types by different amounts
“Blue”cones
“Green”cones
“Red”cones
Wavelength in nanometers(billionths of a meter)
Rel
ativ
e re
spo
nsi
ven
ess
of
con
es
1919
Trichromacy and TVAll color televisions are based on the fact
that normal human color vision is trichromatic
Although we perceive the whole range of colors from a TV screen, it only has three colored phosphors (red, green, and blue)
By varying the relative intensity of the three phosphors, we can fool the visual system into thinking it is seeing many different colors
2020
Opponent Process Theory of Color Vision
Some aspects of our color perception are difficult to explain by the trichromatic theory alone
Example: afterimages if we view colored stimuli for an
extended period of time, we will see an afterimage in a complementary color
2222
Opponent-Process Theory
To account for phenomena like complementary afterimages, Herring proposed that we have two types of color opponent cells red-green opponent cells blue-yellow opponent cells
Our current view of color vision is that it is based on both the trichromatic and opponent process theory
2424
Visual Pathway
Axons of the ganglion cells come together to form the optic nerve
Half of optic nerve fibers cross into opposite hemisphere and synapse onto LGN (lateral geniculate nucleus)
LGN neurons synapse onto primary visual cortex
2525
Overview of Visual System
The eye is like a camera, but instead of using film to catch the light we have rods and cones
Cones allow us to see fine spatial detail and color, but can not function well in dim light
Rods enable us to see in dim light, but at the loss of color and fine spatial detail
Our color vision is based on the presence of 3 types of cones, each maximally sensitive to a different range of wavelengths
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