The Major Senses
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1 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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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. Vision. Purpose of the visual system transform light energy into an electro-chemical neural response - PowerPoint PPT Presentation
Transcript of The Major Senses
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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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VisionPurpose 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 Radar
Broadcastbands
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 StimulusLight can be described as both a particle
and a waveWavelength of a light is the distance of
one complete cycle of the waveVisible light has wavelengths from about
400nm to 700nm Wavelength of light is related to its
perceived color
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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
CorneaPupil
Blind spotOpticnerve
RetinaFovea (pointof central focus)
Lens
Iris
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Organization of Retina5 cell types
Photoreceptorsrods 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 PhotoreceptorsThe photoreceptors transduce the energy
in light into a neural responseThis 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
Thou
sand
s of
rods
per
sq
uare
mill
imet
er
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
Thou
sand
s of
con
es p
er
squa
re m
illim
eter
Distance on retina from fovea (degrees)
Fovea
Blind spot
Rods Cones
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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
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Color VisionOur visual system interprets differences in
the wavelength of light as colorRods are color blind, but with the cones
we can see different colorsThis difference occurs because we have
only one type of rod but three types of cones
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Color MixingTwo basic types of color mixing
subtractive color mixtureexample: combining different color paints
additive color mixtureexample: combining different color lights
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Additive Color MixtureBy combining lights of
different wavelengths we can create the perception of new colors
Examples: red + green = yellow red + blue = purple green + blue = cyan
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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
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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
0.00
0.50
1.00
1.50
400 450 500 550 600 650 700
“Blue”cones
“Green”cones
“Red”cones
Wavelength in nanometers(billionths of a meter)
Rel
ativ
e re
spon
sive
ness
of
con
es
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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
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Opponent Process Theory of Color VisionSome 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
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ComplementaryAfterimages
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Opponent-Process TheoryTo 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
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Visual Pathway
RetinaOptictract
Opticchiasm
Opticnerve
Visual areaof the thalamus
Visualcortex
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Visual PathwayAxons of the ganglion cells come
together to form the optic nerveHalf of optic nerve fibers cross into
opposite hemisphere and synapse onto LGN (lateral geniculate nucleus)
LGN neurons synapse onto primary visual cortex
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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
