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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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

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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

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

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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

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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 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

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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

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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

“Blue”cones

“Green”cones

“Red”cones

Wavelength in nanometers(billionths of a meter)

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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 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

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ComplementaryAfterimagesComplementaryAfterimages

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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

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Visual Pathway

RetinaOptictract

Opticchiasm

Opticnerve

Visual areaof the thalamus

Visualcortex

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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

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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