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The Major Senses

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

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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

  2. 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

  3. White light Prism Visible light 400 500 600 700 Ultra- violet rays Broadcast bands AC circuits Gamma rays Infrared rays X-rays Radar 10 10 10 10 10 10 10 10 10 10 10 10 13 15 11 17 9 5 1 3 7 -1 -5 -3 Wavelength in nanometers (billionths of a meter) Light - The Visual Stimulus

  4. 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

  5. Retina Lens Fovea (point of central focus) Iris Cornea Pupil Light rays Optic nerve Blind spot 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.

  6. Organization of Retina • 5 cell types • Photoreceptors • rods and cones • Horizontal Cell • Bipolar Cell • Amacrine Cell • Ganglion Cell

  7. Back of the eye Cross section of retina shown vastly magnified in the diagram to the right Photochemical is located here Light Light To optic nerve Ganglion cell Amacrine cell Bipolar cell Horizontal cell Cone Rod Organization of Retina

  8. 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)

  9. Distribution of Rods and Cones • Cones - concentrated in center of eye (fovea) • approx. 6 million • Rods - concentrated in periphery • approx. 120 million • Blind spot - region with no rods or cones

  10. Rods Cones Blind spot Fovea 180 140 100 60 20 0 180 140 100 60 20 0 Blind spot Fovea Fovea Thousands of cones per square millimeter Thousands of rods per square millimeter Blind spot Distance on retina from fovea (degrees) Distance on retina from fovea (degrees) Distribution of Rods and Cones

  11. Differences Between Rods and Cones • Cones • 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

  12. Light Spots of light Spots of light Receptive fields Light Ganglion cells Bipolar cells (a) Fovea (b) Periphery of retina Photo- receptors (cones) Photo- receptors (rods) Pigmented epithelium Receptive Fields and Rod vs. Cone Visual Acuity

  13. Receptive Fields and Rod vs. Cone Visual Acuity • Cones - in the fovea, one cone often synapse onto only a single ganglion cell • Rods - the axons of many rods synapse onto one ganglion cell • This allows rods to be more sensitive in dim light, but it also reduces visual acuity

  14. 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

  15. Color Mixing • Two basic types of color mixing • subtractive color mixture • example: combining different color paints • additive color mixture • example: combining different color lights

  16. 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

  17. Trichromatic Theroy of Color Vision • Researchers 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

  18. 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 Relative responsiveness of cones Wavelength in nanometers (billionths of a meter)

  19. Trichromacy and TV • All 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

  20. 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

  21. ComplementaryAfterimages

  22. 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

  23. Visual area of the thalamus Optic nerve Optic chiasm Optic tract Retina Visual cortex Visual Pathway

  24. 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

  25. 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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