Understanding Light and Vision

1. :: Slide 1 :: :: Slide 2 :: Sensation is the stimulation of the sense organs. Perception is the selection, organization, and interpretation of sensory input. Light waves vary in amplitude, that is, their height, and in wavelength, which depends on the distance between the peaks. The lights humans normally see are a mixture of different wavelengths. Hence, light can also vary in its purity, which depends on how varied the mixture of wavelengths is. :: Slide 3 :: :: Slide 4 :: Saturation refers to the relative amount of whiteness in a color. As whiteness declines, saturation increases. When light passes through a prism, it is separated into its component wavelengths. What most people call light includes only the visible light spectrum; that is, the wavelengths that humans can see. :: Slide 5 :: :: Slide 6 :: The visible spectrum for humans is only a slim portion of the total range of wavelengths. Other animals have different capabilities. For example, many insects can see shorter wavelengths than humans can see. These wavelengths are in the ultraviolet spectrum. Many fish and reptiles can see longer wavelengths than humans can see. These wavelengths are in the infrared spectrum. The eye has two main purposes, providing a “house” for the neural tissue that receives light, the retina, and channeling light toward the retina. The eye is composed of the cornea, a transparent window where light enters the eye, the lens, which is a crystalline structure that lies right behind the cornea and focuses the light rays on the retina. The iris is the colored ring of muscle around the pupil (the black center of the eye), which constricts or dilates depending on the amount of light present in the environment, and changes the size of the pupil. The size of the pupil regulates the amount of light by constricting to let in less light and vice versa.

2. :: Slide 7 :: :: Slide 8 :: A number of common visual deficiencies are caused by focusing problems or defects in the lens. In nearsightedness, close objects are seen clearly but distant objects appear blurry. The focus of light from distant objects falls a little short of the retina. Nearsightedness occurs when the cornea or lens bends light too much, or when the eyeball is too long. . In farsightedness, distant objects are seen clearly but close objects appear blurry. The focus of light from close objects falls behind the retina. This focusing problem typically occurs when the eyeball is too short. :: Slide 9 :: :: Slide 10 :: The retina is the neural tissue lining the inside back surface of the eye; it absorbs light, processes images, and sends visual information to the brain. Axons from the retina to the brain converge at the optic disk, a hole in the retina where the optic nerve leaves the eye. If an image falls on this hole, it can’t be seen…the blind spot. :: Slide 11 :: :: Slide 12 :: The retina contains two types of receptors, rods and cones. Rods play a key role in night vision because they are more sensitive than cones to dim light. Cones play a key role in daylight vision and color vision. Cones do not respond well to dim light, but in bright light they provide more sharpness and detail than rods. This is a demonstration of what happens to light when it hits the retina.

3. :: Slide 13 :: :: Slide 14 :: Light striking the rods and cones triggers the firing of neural signals that pass into the cells in the retina. Signals move from receptors to bipolar cells to ganglion cells, which in turn send impulses along the optic nerve. These axons carry visual information, and depart the eye through the optic disk. How does visual information get to the brain? Axons leaving the back of each eye form the optic nerves, which project into the brain’s relay center, the thalamus. The optic pathways then travel from the thalamus to the primary visual cortex in the occipital lobe at the back of the brain. :: Slide 15 :: :: Slide 16 :: Here you can see that the optic nerves from the inside half of each eye crisscross at the optic chiasm and then project to the opposite half of the brain. This arrangement ensures that signals from both eyes go to both hemispheres of the brain. All visual input eventually reaches the occipital lobe of the cortex. Researchers have investigated how cortical cells respond to light by placing microelectrodes in the visual cortex of animals, such as cats, to record action potentials from individual cells. They would flash spots of light in the retinal receptive fields that the cells were thought to monitor, but the cells rarely responded. David Hubel and Torsten Wiesel discovered the reason for this lack of response by accident – the cells in the visual cortex responded to lines, edges, and more complicated stimuli – rather than small spots of light. :: Slide 17 :: :: Slide 18 :: A vertical line elicits rapid firing in the cell, as shown in the microelectrode recording. [Click to continue] A horizontal line elicits no response; the cell fires at its baseline rate. [Click to continue] A diagonal line elicits moderate firing in the cell. The key point is that cells in the visual cortex seem to be highly specialized. They have been characterized as feature detectors, neurons that respond selectively to very specific features of more complex stimuli. This video shows Hubel and Wiesel’s work on the visual cortex in cats.

4. :: Slide 19 :: :: Slide 20 :: This color solid shows how color varies along three perceptual dimensions. Brightness increases from the bottom to the top of the solid, hue changes around the solid’s perimeter, and saturation increases toward the periphery of the solid. Color is largely a function of wavelength. Lights with the longest wavelengths appear red, while those with the shortest appear violet. As the color solid demonstrates, people can perceive many different colors. Most of these variations are the result of mixing a few basic colors. There are two kinds of color mixing: subtractive and additive. Additive color mixing works by superimposing lights, putting more light in the mixture than exists in any one light by itself. To see an example, click each projector to turn it on. :: Slide 21 :: :: Slide 22 :: Subtractive color mixing works by removing some wavelengths of light, leaving less light than was originally present. For example, you can mix yellow and blue paints to make green. Paints yield subtractive mixing because pigments absorb most wavelengths, selectively reflecting specific wavelengths that give rise to particular colors. In the middle of the 19th century, the parallels between additive color mixing and human color perception inspired the trichromatic theory of color vision. This theory holds that the human eye has three types of receptors sensitive to the specific wavelengths associated with red, green, and blue. The impetus for the trichromatic theory was the demonstration that a light of any color can be matched by the additive mixture of three primary colors. :: Slide 23 :: :: Slide 24 :: But what about yellow? Is it just reddish-green? Ewald Hering, proposed opponent process theory, which holds that color perception depends on receptors that make antagonistic responses to three pairs of colors: red vs. green, yellow vs. blue, and black vs. white. While researchers argued about which was right for almost a century, most psychologists now agree that it takes both theories to explain color vision. Do you see a seal balancing a ball in front of a trainer? ...or two ballroom dancers? A reversible figure is a drawing that is compatible with two different interpretations that can shift back and forth. This shifting is caused by information given to you about the drawing, called a perceptual set. A perceptual set creates the shift in how you interpret sensory input.

5. :: Slide 25 :: :: Slide 26 :: The Gestalt principle maintains that the whole can be greater than the sum of its parts. One example of gestalt is the phi phenomenon – the illusion of movement created by presenting visual stimuli in rapid succession. Another example is the principle of figure and ground, shown in this image. Whether you see two faces or a vase depends on which part of the drawing you see as figure, and which part you see as ground. The Gestalt principles of form perception include proximity, closure, similarity, simplicity, and continuity. :: Slide 27 :: :: Slide 28 :: Depth perception involves interpretation of visual cues that indicate how near or far away something is. Two types of clues are used to make judgments of distance: monocular cues (clues from a single eye) and binocular cues (clues from both eyes together). Binocular cues include retinal disparity (objects within 25 feet project images to slightly different locations on the left and right retinas; thus each eye sees a slightly different view of the object) and convergence, feeling the eyes converge toward each other as they focus on a target. Monocular cues are clues about distance based on the image in either eye alone. Pictorial depth cues are cues about distance that can be given in a flat picture. Optical Illusions involve an apparently inexplicable discrepancy between the appearance of a visual stimulus and its physical reality. The Muller-Lyer illusion is one famous visual illusion, shown here. Of the two vertical lines shown here, which one looks longer? The lines are actually the same size – because the line on the left appears to be the outside of a building, it is perceived to be closer, and therefore shorter. :: Slide 29 :: :: Slide 30 :: This animation illustrates the illusions of Victor Vasarely, who challenged viewers to think about the process of perception. The stimulus for the auditory system is sound waves, which are actually vibrations of molecules. Sound waves must travel throughout some physical medium, such as air. Like light waves, sound waves are characterized by their amplitude (loudness), wavelength (pitch), and purity (timbre). Also as with light, characteristics of sound interact in sound perception.

6. :: Slide 31 :: :: Slide 32 :: Like your eyes, your ears channel energy to the neural tissue that receives it. The human ear can be divided into three sections: the external ear, the middle ear, and the inner ear. Sound is conducted differently in each section. The external ear consists of the pinna, which collects sound. The middle ear consists of a mechanical chain made up of three tiny bones in the ear, the hammer, anvil, and stirrup, known collectively as the ossicles. The inner ear consists of the cochlea, a fluid-filled, coiled tunnel that contains the hair cells, the auditory receptors. The hair cells are lined up on the basilar membrane. Hermann von Helmholtz (1863) proposed that perception of pitch corresponds to the vibration of different portions, or places, along the basilar membrane. Thus, different places have different pitches, like keys on a piano. Other researchers (Rutherford, 1886) proposed an alternate model called frequency theory, which holds that perception of pitch corresponds to the rate, or frequency, at which the entire basilar membrane vibrates, causing the auditory nerve to fire at different rates for different frequencies. Thus, according to this theory, the brain detects the frequency of a tone by the rate at which the auditory nerve fires. :: Slide 33 :: :: Slide 34 :: Like with research in theories of color vision, researchers argued about these two competing theories for almost a century. It turns out that both are valid – in part. The two were reconciled by Georg von Bekesy, 1947, with his traveling wave theory. Basically, von Bekesy said that the whole basilar membrane does move, but the waves peak at particular places, depending on frequency. This animation shows how the basilar membrane vibrates in response to sound frequencies. Taste (gustation) has as its physical stimulus chemical substances that are dissolvable in water. Receptors for taste are clusters of cells found in the taste buds, which line the trenches around tiny bumps on the tongue. These cells absorb chemicals, trigger neural impulses, and send the information throughout the thalamus and on to the cortex. The four primary tastes are sweet, sour, bitter, and salty, with uneven distribution on the tongue. Clearly, taste results from a complex blend of these 4, as well as learned and social processes. :: Slide 35 :: :: Slide 36 :: This is an ABC video that describes a type of gum that eliminates the perception of sweetness in an attempt to decrease the reward, or positive response, associated with eating sweet foods. Smell (Olfaction) operates much like the sense of taste. The physical stimuli are chemical substances carried in the air that are dissolved in fluid, the mucus in the nose. Olfactory receptors are called olfactory cilia and are located in the upper portion of the nasal passages. Humans can distinguish among about 10,000 odors, but for some reason have a hard time attaching names to odors quite frequently.

7. :: Slide 37 :: :: Slide 38 :: This is an ABC news feature that discusses how smell influences behavior and possible links between the ability to smell and disease. The physical stimuli for touch are mechanical, thermal, and chemical energy that impinges on the skin. The skin has at least 6 types of sensory receptors, which are routed throughout the spinal column to the brainstem. Temperature is registered by free nerve endings in the skin that are specific for cold and warmth. Pain receptors are also mostly free nerve endings which transmit information to the brain via two types of pathways...the fast pathway that registers localized pain and relays it to the brain in a fraction of a second, and the slow pathway that lags a second or two behind and carries less localized, longer-lasting aching or burning pain. :: Slide XX :: :: Slide XX :: Left blank Left blank :: Slide XX :: :: Slide XX :: Left blank Left blank