The Visual System “Seeing is believing” Good ideas are “bright” This section is an “overview”
Vision- Spectrum of Electromagnetic Energy Insects can see shorter wavelengths Fish and reptiles can see longer wavelengths
Vision Wave amplitude Wave length • Hue • referred to as ‘color’, determined by the wavelength of light reflected from a surface • Saturation • The degree to which light waves producing a color are of the same wavelength; the purity of a color • Brightness • Dependent on the intensity of light reflected from a surface and corresponds to the amplitude of the light wave
The Eye • MAIN PURPOSES • Channel light to the neural tissue that receives it (retina) • House that tissue… • The Cornea – bends light waves so the image can be focused on the retina. • The Pupil – iris opening that changes size depending on the amount of light in the environment • The Iris – Its muscles control the size of the pupil
The Eye The Lens – changes shape to bring objects into focus The Retina- the neural tissue lining the inside back surface of the eye; absorbs light, processes images and sends visual info to brain The Fovea – Central area of retina; greatest density of photoreceptors (rods and cones) The Optic Nerve- Sends visual information to the brain. The Blind Spot (optic disc) – Where the optic nerve leaves the eye. There are no photoreceptor cells here
Receptors only 10% of light makes it! • Rods (elongated – 100-125 million) • Allow humans to see in black, white, and shades of gray in dim light • Mostly in the periphery • Take 20 – 30 minutes to fully adapt to darkness • Cones (stubbier – 5-6.4 million) • Enable humans to see color and fine detail in adequate light, but that do not function in dim light • Mostly in the fovea • Adapt fully to darkness in 2 – 3 minutes
Adaptation • Dark Adaptation – the process in which the eyes become more sensitive to light in low illumination • Light Adaptation – the process in which the eyes become more sensitive to light in high illumination Due to chemical changes in cones and rods
Receptors in the Human Eye Cones Rods Number 6 million 120 million Location in retina Center Periphery Sensitivity in dim light Low High Color sensitive? Yes No Vision- Receptors
Sight What does the retina do? What does the iris do? How do rods differ from cones in function? What is difference threshold?
Pathways from the Eyes to the Visual Cortex 90% of axons from retina synapse here Cells here are highly specialized
Seeing in Color • Longer wavelengths are perceived as RED • Shorter wavelengths are perceived as VIOLET • Humans can discriminate among approx. 1 million colors.
Subtractive Color Mixing – works by removing some wavelengths of light, leaving less light than was originally there. Additive Color Mixing - works by superimposing lights, putting more light in the mixture than exists in any one light by itself.
Visual Information Processing • Trichromatic (three color) Theory • Young and Helmholtz • three different retinal color receptors • red • green • blue
Cones and Color Vision In humans, some cones are sensitive to blue, some to green, and some to red. When more than one kind of cone is stimulated, we perceive other colors. Human color vision differs from that of many animals. Fish have no cones and see no color. Dogs have two types of cones and see fewer colors than humans. Birds have four or more cones and see a wide variety of colors.
Opponent-Process Theory • The theory states that color perception depends on receptors that make antagonistic responses to three pairs of colors • red vs. green • yellow vs. blue • white vs. black
Afterimages • An afterimage is the visual impression that remains after the original image is removed. • The afterimage is the complementary color of the color of the original image • Most afterimages last a few seconds up to a minute. • They occur when the cones of the eye become tired after staring intensely at a single color. • The size of an afterimage depends of the distance from which it is viewed.
Color Blindness • Color blindness: when people do not have normal color vision due to a malfunction in the cones • Partial color blindness is common; total color blindness is rare. • Most people who are color blind have red-green color blindness. • Color blindness is almost always inherited. • Males are more likely to inherit color blindness. • There is no cure for color blindness, but it does not cause any great impairment.
Color-Deficient Vision • People who suffer red-green blindness have trouble perceiving the number within the design • ** Watch Video**
Color Q’s 1. What does the Trichromatic Theory say? 2. What are feature detectors? What might someone have if they can’t detect the shape of a tree? 3. What does the Opponent Process theory say?
Reversible Figure – a drawing that is compatible with two interpretations that can shift back and forth Your experience is SUBJECTIVE • Perpetual Set – a readiness to perceive a stimulus in a certain way • Inattentional blindness – the failure to see fully visible objects
Perceptual Set Kiss the Guy
Feature Analysis • Is the process of detecting specific elements in visual input and assembling them into a more complex form P T I R T Pattern of stimulation in retina T High Level Analyzers Decision Feature Detectors
Cell’s responses Stimulus Visual Information Processing • Feature Detectors • neurons in the visual cortex that respond selectively to specific features • Shape, angle, movement (they’re fussy) Visual Agnosia – inability to recognize objects May be because damage somewhere along the visual pathway
Feature Analysis • The theory ASSUMES Bottom-Up Processing – a progression from individual elements to the whole Others argue for Top-Down Processing – a progression from whole to the element Th mn wnt t the stre ad bght ktchp wth a tn dllr bll.
Subjective Contours • Is the perception of contours where none actually exist
Gestalt Psychology • Gestalt = “Form” or “Shape” “The whole can be greater than the sum of its parts” • Uses mainly Top-Down Processing 6 PRINCIPLES to FOLLOW EXPLAIN and GIVE EX. FOR EACH
Figure and Ground • The Figure is what is being looked at, and the ground is the background against which it stands. • You can’t perceive the drawing both ways at once…
Proximity • What do you see? • Things that are near one another seem to belong together. ************** ************** ************** • You are more likely to see three rows of *’s than 14 vertical columns of *’s
Closure • You may complete figures that have a gap in them
Similarity • People tend to group stimuli that are similar
Simplicity • Viewers tend to organize elements in the simplest way possible • You could view this as a complicated 11 sided shape… or a rectangle overlapping a triangle.
Continuity • Viewers tend to see elements in ways that produce smooth continuation
Appreciating Art and Illusion • Read the article and for EACH time period and/or artist note HOW they have used their knowledge of vision and the brain to create their art.
Visual Cliff Summarize the Visual Cliff Study – pg. 118
Depth Perception • The ability to see objects in three dimensions although the images that strike the retina are two dimensional. • Allows us to judge distance.
How do we transform two-dimensional objects to three-dimensional perception? • Binocular Cues: depth cues that depend on two eyes • Monocular Cues: depth cues that depend on one eye
Binocular Cues • Retinal Disparity: a binocular cue for seeing depth. • The closer an object comes to you the greater the disparity is between the two images. • Convergence • Rotation of eyes to focus on single object Pen together two eyes- try with one