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 zVision has been studied most extensively

2 2 Vision zPurpose of the visual system ytransform light energy into an electro-chemical neural response yrepresent characteristics of objects in our environment such as size, color, shape, and location

3 3 Light - The Visual Stimulus Gamma rays X-rays Ultra- violet rays Infrared rays Radar Broadcast bands AC circuits Visible light Prism White light Wavelength in nanometers (billionths of a meter)

4 4 Light - The Visual Stimulus zLight can be described as both a particle and a wave zWavelength of a light is the distance of one complete cycle of the wave zVisible light has wavelengths from about 400nm to 700nm zWavelength of light is related to its perceived color

5 5 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 Optic nerve Retina Fovea (point of central focus) Lens Iris

6 6 Organization of Retina z5 cell types yPhotoreceptors xrods and cones yHorizontal Cell yBipolar Cell yAmacrine Cell yGanglion Cell

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

8 8 Function of Photoreceptors zThe photoreceptors transduce the energy in light into a neural response zThis occurs when light entering the eye is absorbed by photopigment molecules inside the photoreceptors zWhen light interacts with the photopigment, it results in the photoreceptor becoming more negatively charged (hyperpolarization)

9 9 Distribution of Rods and Cones zCones - concentrated in center of eye (fovea) yapprox. 6 million zRods - concentrated in periphery yapprox. 120 million zBlind spot - region with no rods or cones

10 Distribution of Rods and Cones Thousands of rods per square millimeter Blind spot Fovea Distance on retina from fovea (degrees) Fovea Blind spot Thousands of cones per square millimeter Distance on retina from fovea (degrees) Fovea Blind spot RodsCones

11 Differences Between Rods and Cones zCones yallow us to see in bright light yallow us to see fine spatial detail yallow us to see different colors zRods yallow us to see in dim light ycan not see fine spatial detail ycan not see different colors

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

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

14 Color Vision zOur visual system interprets differences in the wavelength of light as color zRods are color blind, but with the cones we can see different colors zThis difference occurs because we have only one type of rod but three types of cones

15 Color Mixing zTwo basic types of color mixing ysubtractive color mixture xexample:combining different color paints yadditive color mixture xexample: combining different color lights

16 Additive Color Mixture zBy combining lights of different wavelengths we can create the perception of new colors zExamples: yred + green = yellow yred + blue = purple ygreen + blue = cyan

17 Trichromatic Theroy of Color Vision zResearchers found that by mixing only three primary lights (usually red, green and blue), they could create the perceptual experience of all possible colors zThis 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 zPhysiological studies revealed that Young and Helmholtz were correct zWe have three types of cones zLight of different wavelengths will stimulate these cone types by different amounts “Blue” cones “Green” cones “Red” cones Wavelength in nanometers (billionths of a meter) Relative responsiveness of cones

19 Trichromacy and TV zAll color televisions are based on the fact that normal human color vision is trichromatic zAlthough we perceive the whole range of colors from a TV screen, it only has three colored phosphors (red, green, and blue) zBy 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 zSome aspects of our color perception are difficult to explain by the trichromatic theory alone zExample: afterimages yif we view colored stimuli for an extended period of time, we will see an afterimage in a complementary color

21 Complementary Afterimages

22 Opponent-Process Theory zTo account for phenomena like complementary afterimages, Herring proposed that we have two types of color opponent cells yred-green opponent cells yblue-yellow opponent cells zOur current view of color vision is that it is based on both the trichromatic and opponent process theory

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

24 Visual Pathway zAxons of the ganglion cells come together to form the optic nerve zHalf of optic nerve fibers cross into opposite hemisphere and synapse onto LGN (lateral geniculate nucleus) zLGN neurons synapse onto primary visual cortex

25 Overview of Visual System zThe eye is like a camera, but instead of using film to catch the light we have rods and cones zCones allow us to see fine spatial detail and color, but can not function well in dim light zRods enable us to see in dim light, but at the loss of color and fine spatial detail zOur color vision is based on the presence of 3 types of cones, each maximally sensitive to a different range of wavelengths