Vision Chapter 8 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 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 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 Organization of Retina 5 cell types Photoreceptors rods and cones Horizontal Cell Bipolar Cell Amacrine Cell Ganglion Cell more negatively charged hyperpolarization 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 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 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 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 Color Mixing Two basic types of color mixing subtractive color mixture example combining different color paints additive color mixture example combining different color lights Trichromatic Theory 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 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 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 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 Opponent Process Theory To account for phenomena like complementary afterimages Herring proposed that we Our current view of color vision is that it is based on both the trichromatic and opponent have two types of color opponent cells red green opponent cells blue yellow opponent cells process theory Seeing Forms and Patterns Enhancement of Contours Enhancement of Contrast at contours background Seeing Forms and Patterns Enhancement of Contours Contours are sudden changes in brightness or color that separate objects from the Lateral inhibition is mechanism of the effect Lateral inhibition occurs when a neuron is inhibited by neighboring neurons in proportion to the activity of the neighbors Neuron A is completely surrounded by active neighbors and will be Neuron B is only partially surrounded by active neighbors and will thus highly inhibited be more active than A Neuron C is receives less inhibitory input relative to neuron D and will be more active than D Neuron D is the least active of all Result Exaggeration at the contour border Seeing Forms and Patterns The Detection and Integration of Stimulus Features How does the visual system detect and integrate all of the features of a visual field Hubel Wiesel 1962 Different neurons in the primary visual cortex respond preferentially to different patterns E g edges bars color movement angle Treisman s Two stage theory Feature detection automatic and parallel processing Feature integration requires controlled attention and occurs serially Only one stimulus at a time e g the X can be processed Seeing Forms and Patterns Gestalt Principles of Grouping Law of Pr gnanz Perception of the world is reduced to the simplest possible form Every form is composed of a target figure embedded in a background Targets are perceived based on the distance among the elements Targets are perceived based on how similar the elements are to one We see targets as whole and ignore gaps in their borders Figure ground ground Proximity Similarity another Closure Seeing Forms and Patterns Wholes Influence the Perception of Parts Illusory Contours The simplest interpretation of Figure 1 is that there is a white triangle atop a triangle with a black frame in agreement with the laws of good form and closure Unconscious Inference The use of sensory input to draw conclusions regarding the nature of what is present without awareness Illusory contours are used when they are needed Stimulus b in Figure 2 is more complex than a Thus people see a square in b via the illusory contour but see a square in a based simply on the frame alone How Do We See in 3D Depth perception Binocular vision eye convergence and binocular disparity Monocular vision Motion parallax Pictorial cues Occlusion Relative image size for familiar objects Linear perspective Texture gradient Position relative to horizon Differential lighting of surfaces