Vision Sensation & Perception Part 3 - Vision Visible light is the form of electromagnetic radiation our eyes are designed to detect. However, this is only a narrow band of the range of energy at different wavelengths, called the electromagnetic spectrum. Visible light is the portion from 350 to 750 nm along that spectrum. Human eyes are able to detect only those wavelengths, but other animals can detect a wider range of wavelengths. Functional Organization of the Eye Light enters the eye through the cornea. The cornea is part of the larger sclera, or the membrane that contains the gelatinous substance of the eye. Both the sclera and the cornea are extremely sensitive to touch. Next, light goes through the pupil, a circular hole formed by the iris. The iris controls the pupil size in reflective response to light intensity. Functional Organization of the Eye The pupil can contract down to as little as 2 mm in diameter in response to bright light, or dilate as much as 8 mm in diameter and in dim light. As the light passes through the lens, it undergoes further refraction or bending. Passing through the curved cornea refracts the light at first; then the lens refraction serves to sharpen the focus. Functional Organization of the Eye The lens can change its curved shape in order to focus on near (more bulge) or distant objects (flatter shape). Those changes of lens shape are called accommodation. Finally, the light is focused on the retina, the layer of cells on the interior surface of the eye. 1
Accomodation The Retina The retina, a network of neurons covering the back surface of the eye, is composed of three layers: The first layer, nearest the lens, consists of the ganglion cells, whose axons compose the optic nerve. The amacrine cells, the horizontal cells, and the bipolar cells compose the second layer. The amacrine and horizontal cells make single lateral connections among the cells of the middle layer of the retina, and the bipolar cells make connections between the layers of cells in the retina. The Retina The third layer is composed of the rods and cones, which are the photoreceptors that actually accomplish transduction, or the conversion of the light energy into neural transmission. Due to this arrangement of cells, the retina is sometimes described as being inverted, in that light must pass through the first two layers before it can be detected by the rods and cones. The Retina There are 120 million of the long and thin rod cells located mostly in the periphery of the retina, and 8 million of the short and thick cone cells which are concentrated in the foveal region of the retina in each eye. The fovea is a small region of the retina, directly in the line of sight. 2
The Retina Both rods and cones contain photopigments, or chemicals that initiate the transduction of light. Rods are sensitive to any wavelength of light. The cones are maximally sensitive to particular wavelengths of colored light. While each column in the fovea is connected to its own ganglion cell, many rods in the periphery share connections to individual ganglion cells. Thus, the peripheral bipolar cells collect information from several rods, while the foveal bipolar cells collect information from only one cone. From the Eye to the Brain After light hits the photopigments in the photoreceptive cells, they convert the visual stimulus into a neural impulse. That impulse travels up through the other two layers of neurons. All of the axons of the ganglion cells of each eye come together to form the optic nerves. From the Eye to the Brain The optic nerves from both eyes meet at the base of the brain to make the optic chiasma. There, the neurons from the nasal side of each eye across over to the opposite hemisphere, while the neurons on the temporal side remain on the side where they originated. From the Eye to the Brain From the optic chiasmus, the ganglion cells go to the thalamus, and from here the signals travel to the primary visual cortex in the occipital lobe. Further processing occurs in the visual cortex as information associated with color, depth, pattern, and form is discerned. 3
How We See: Rods and Cones There are two separate systems for vision. The first uses the rods, which are color insensitive and able to function in dim light. The second uses columns, which require brighter light and are able to detect color. Individuals without rods or without cones are day-blind or night-blind, respectively. Day-blind Day-blind people have poor visual acuity, find the stimulation of bright light painful, and are colorblind, or achromatic, because they are missing their color sensitive cones. Night-blind Night-blind persons have difficulty seeing in low-light conditions because they are lacking the rods, which are normally responsible for vision under such conditions. Normal individuals are also achromatic and dim light when light levels are too weak to activate the color-sensitive cone cells of the retina. Rods and Cones The rods and cones are not evenly distributed on the retina. The cones are most concentrated in the fovea, and the rods are fairly evenly distributed nearly everywhere else. The blind spot is where the neurons exit the eye; no photoreceptors cells are there. Sighting Stars Question: Given the distribution of rods and cones across the surface of the retina, how would one best accomplish the citing a very dim stars in the nighttime sky? 4
Sighting Stars Answer: Astronomers know that the best way to locate dim stars is to use one s peripheral field of vision. Once the star has been located, a telescope can be directed in the direction of the star to amplify the light to a level sufficient enough to allow the cones at the fovea to detect its presence. The Distribution of Rods and Cones on the Retina A demonstration with crayons Seeing by Light and Darkness The physical dimensions of life such as intensity and wavelengths are transferred into psychological dimensions of brightness and color. This occurs via how our bodies and minds react to these physical stimuli. Seeing by Light and Darkness The amount of light that reaches your eyes from an object is called retinal luminance, however this measurement isn t directly transferred into a measurement of brightness. Rather, brightness is the psychological impression of that light intensity, based on the height of the wavelength, or amplitude of the wavelength. Seeing by Light and Darkness Our ability to sense brightness is not only based on these properties of light, but also on the physical properties of the eye, and where the stimulus falls on the retina. When we go from bright sunlight into a darkened room our vision undergoes dark adaptation, a process that takes about 30 minutes, and involves the adaptation of both the cones and rods. Seeing by Light and Darkness During dark adaptation our pupils open by a factor of 16, but our rods and cones increase their sensitivity to light by a factor of as much as 100,000. 5
Hue, Saturation, and Brightness Hue is the psychological property derived from wavelength, and it corresponds to the everyday word color. Normal individuals can see colors ranging from violet (350 nm) to red (700+ nm). Saturation refers to the vividness of a color. Vivid colors are saturated, and pale or washed-out colors are unsaturated. Hue, Saturation, and Brightness Brightness, the psychological dimension related to the amplitude of the light wave and discussed above, completes the collection of color properties Relationships among Colors Humans can perceive about 150 hues. When brightness and saturation are factored in, humans can perceive over 7 million colors of varying hue, brightness, and intensity. Computer screens can display over 16 million colors or more than two times the number humans could possibly perceive. Relationships among Colors Colors can be mixed either additively or subtractively. Additive mixture of colors results when colors, like colored spotlights, combine to produce new colors.in such additive mixtures, the resulting color is the sum of the component light beams wavelengths. If three or more colors are combined, white light results. Relationships among Colors Subtractive mixture is when objects that reflect light are combined, for example, when an artist mixes paint pigments. Theories of Color Vision Trichromatic Theory Opponent-Process Theory 6
Trichromatic Theory How does the eye resolve all of the colors? The Young-Helmholtz, or Trichromatic Theory, first proposed by Thomas Young and revived by Hermann von Helmholtz, relies on primary colors to explain color vision It proposes three separate cone types, each receptive to either red, green, or blue. Trichromatic Theory Some physiological data supports the trichromatic theory, as does evidence of patterns of color blindness. For instance, genes that direct cones to develop either red, green, or blue colorsensitive pigments have been found. However, this theory alone cannot explain all visual processing. Opponent-Process Theory The opponent-process theory, originally proposed by Ewalt Hering, proposes six opposing colors (counting black-andwhite) on three separate cone types whose actions stimulate the sensation of one of the opposing pairs. The three pairs are red-green, blue-yellow, and black-white. Inhibition and afterimage data support this theory Theories of Color Vision It appears that both trichromatic theory and opponent-process theory have merit. Trichromatic theory best describes how color vision occurs at the level of the retina, whereas opponent-process theory describes color vision processes that occur at the level of the ganglion cells and higher in the nervous system. 7
Visual Perception The Ames Room Since much of visual perception is a psychological process, optical illusions can distort the meaning of sensations. Therefore there are several factors that can influence our perceptions of constantly changing sensory information. Visual Perception The Müller- Lyer Illusion Perceptual Constancies Perceptual constancies occur when our brains correct or modify our rapidly changing sensory inputs to give us a more constant perception of the world. Size Constancy Size constancy ensures that as we watch our friend walk off into the distance, although the image of the person that is projected onto our retina is rapidly decreasing in size, we do not perceive that our friend is actually shrinking. 8
Size Constancy The knowledge that as the proximal stimulus (in the internal sensory image) changes, the distal stimulus (the external object being perceived) does not, allows us to correct our sensations and maintain constant perceptions Lightness Constancy Lightness constancy refers to the fact that as the illumination of an object changes, our perception of its brightness and color remain the same. For example, a white sheet of paper perceived in bright sunlight will still appear white and of approximately the same shade when perceived in the shade of a tree. Shape Constancy Shape constancy refers to our tendency to perceive the shape of an object as being constant even though our retinal image of the object is changed. Shape Constancy For example, a whiteboard at the front of the classroom is perceived as being rectangular by a student, regardless of the students position in the classroom. This perceptual constancy occurs despite the fact that as the student changes vantage points, the retinal image of the whiteboard changes shape. Depth Perception Shape Constancy Depth, or distance from surface, judgments are made constantly. Walking and reaching require them, as do driving and playing games. Depth judgments are made with one eye (monocular) or with both eyes (binocular). 9
Monocular depth cues yield the perception of depth in two dimensions because they rely on one eye only. A. Relative size is the perception that distant objects appear smaller than objects that are closer to us. Objects that are closer to us have a larger image on our retinas, while objects at a distance, have a smaller retinal image. B. Texture gradient is a change in both the relative size of objects and the density of the distribution of objects in the picture plane. The closer objects are to us, the larger they appear and the less densely distributed they appear. Conversely, distant objects appear small and densely distributed. For example, a field of sunflowers viewed from a distance may appear to be a homogenous block of yellow; when viewed up-close, the same field would be perceived with much greater textural detail as each individual flower is perceived. C. Interposition: when one object is partially secured from our view by another object, we perceive that the partially obscured object is more distant than the one obscuring it. The Ponzo Illusion D. Linear perspective: depth is perceived from linear perspective when parallel lines appear to converge in the distance at the vanishing point. For example, when one stands on a railroad track and looks off into the distance, the rails appear to come together at the point where the tracks seem to disappear from sight. 10
E. Location in the picture plane: We use location in the picture plane to give us information about death. When objects are below the horizon for higher objects appear to be more disciplined. However, when objects are above the horizon, the higher objects appear to be closer. Vision Depth Perception in a Picture F. Aerial perspective: In aerial perspective, we use atmospheric conditions to give us information about depth. Distant objects should appear less clear to us because our perceptions of them are clouded by atmospheric haze and dust. Conversely, close objects should appear relatively clear because there is little intervening haze or dust to obscure our view of them. G. Motion parallax: Motion parallax refers to the apparent difference in speed and direction of objects that are viewed from a moving vantage point. These differences inform us of the relative depth of moving objects. A good example of motion parallax comes from our perceptions while riding in a car with our eyes fixated on some point in the distance. Objects that are closer than this fixation point will appear to be moving in a direction opposite to the direction we are moving, and objects more distant than the fixation point will appear to be moving in the same direction as we are. Another depth cue comes from the fact that for all objects in our field of vision, the closer they are to us, the more quickly they will appear to be moving. 11
Binocular Depth Cues The binocular depth cues require both eyes and occur because of the slightly different view that each eye has of something due to the separation of the eyes. The technical term for such a vision is stereopsis. Sometimes, individuals may lose the ability to fuse the images from each eye and suffer double vision or diplopia. Binocular Depth Cues A. Binocular Convergence: Because our two eyes are in slightly different places in our skulls, when we turn to look directly at an object, such that its image strikes each fovea, our right and left eyes must rotate slightly inward. The closer an object is to us, the more each I will have to rotate. Because the degree of rotation is tied to the distance and object is from our bodies, our brain uses this information to calculate depth. Binocular Depth Cues B. Binocular Disparity: Because each eye has a slightly different view of the world, because they are set a specific distance from one another, that information can be used to accurately judge depth. Form Perception There are two kinds of theories of perception: some attempt to describe how we recognize forms, whereas others attempt to describe how we recognize patterns of forms. The ability to do both it s very important to the task of making sense of our environment. Form Perception Form perception is the ability to distinguish an object from the background behind, while pattern recognition allows us to distinguish one object from another based on the features of the objects. 12
Gestalt Approach The Gestalt approach is based on the notion that the whole is greater than the sum of its parts. The figure is a percept that stands out from its ground. Figure-ground concept is usually unambiguous. But there are some situations in which the figure and ground are purposely interchangeable. These are called reversible figures. Feature Detection The feature- detector approach to feature perception is a psychophysiological approach based on the work of Nobel laureates Hubel and Wiesel. Hubel and Wiesel used single- cell recording techniques to trace the route that information follows as it travels from specific cells in the retina to cells in the visual cortex. They discovered three kinds of visual cortical neurons: simple, complex, and hypercomplex. Feature Detection The simple cells of the cortex fire in response to features such as lines at specific orientations in the receptive field, specific light-dark boundaries, and lines of specific thickness. The complex cells of the cortex fire in response to stimulation of specific combinations of simple cells. 13
Feature Detection Because the complex cells receive input from several simple cells, they are thought to function as detectors for very complex and specific visual features. For example, complex cells are insensitive to bars of light unless they are oriented in a specific fashion. Feature Detection Hypercomplex cells are sensitive to particular length of stimulus lines. Taken together, these three types of visual cortical neurons (simple, complex, and hypercomplex) may provide the basis for pattern recognition. Pattern Recognition The forms we perceive daily fall into certain patterns although simple patterns may be easily broken down into their component features, complex patterns such as faces are not easily reduced to elementals features. Therefore featuredetection theories alone do not adequately explain the phenomenon of perception. Pattern recognition theories attempt to explain how complex stimuli are perceived Template-Matching Approaches Template-matching theories propose that we store in our minds templates or prototypes for specific patterns. According to template-matching theories, perception of patterns involves comparing the stimuli we perceive to stored templates. When a sufficient match is made between stimulus and the template, recognition of the stimulus occurs. Feature-Matching Approaches Feature-matching theories are analogous to Hubel and Wiesel s approach in that they search for features instead of templates. Problems for Theories of Matching in Pattern Perception Problems remain for both templates and feature matching approaches. Chief among these problems is that they do not take the role of the environment, or context of the stimulus, into account. Such context effects have a long history and include superior recognition of items when they are in the appropriate context. Example: Letters in words (wordsuperiority effect) 14
Vision Form Perception Pattern Recognition Motion Perception Gunner Johansson (1975) performed an experiment in which he attached a small lightbulb to each of the major joints of the person s body had been filmed the person walking. He showed experimental participants only the lights and not the person. When they saw just a single frame, they did not know what they were seeing. Motion Perception But when the participants saw even two frames of film (1/10 of a second) they were able to identify a person walking. A similar scenario in which lights were attached to both men and women allowed most people to judge the sex of the moving figure by the patterns of motion. Deficits in Perception In some individuals, misperceptions can stem from true deficits in perception called agnosias. Agnostics can sense, but are unable to perceive, certain stimuli. A person suffering from a visual-object agnosia may see objects but not be able to identify them. For example, they may see an apple but are unable to recognize it as an apple. Deficits in Perception Cases, such as those of visual agnostics, clearly demonstrate the difference between sensation and perception. Another type of visual agnosia is prosopagnosia, an inability to perceive human faces. People who suffer from prosopagnosia may be able to recognize the faces of animals but not human faces. 15