CHAPTER 5 PSYCH 100 SENSATION & PERCEPTION Sensing Our World: Basics of Sensation • Sensation is the process by which physical stimuli is sensed by our sensory organs and are converted into neural impulses sent to the brain. • The brain uses this information sensed to create our experiences of vision, touch, hearing, taste, smell, and so on. This is our perception! • Perception is the process the brain uses to integrate, organize, and interpret sensory information to create representations of the world. Can reflect reality, or not… • Sensory receptors in our sensory organs convert sources of sensory stimuli, such as light and sound, into neural impulses the brain can use to create sensations. • Psychophysics is the study of relationships between the features of physical stimuli, such as the intensity of lights and sounds, and the sensations we experience in response to these stimuli. • Absolute threshold is the smallest amount of stimulus a person can reliably detect. • Difference threshold (or just noticeable difference) is the minimum difference or change in a stimulus that can be detected. • Weber’s Law states that the amount of change in a stimulus needed to occur for that difference to be detected is given by a constant ratio of the original stimulus. Absolute Thresholds Sense Stimulus Receptors Threshold Vision Light energy Rods and cones in The flame from a single the eyes candle flickering about 30 miles away on dark night Hearing Sound waves Hair cells in the inner ear The ticking of a watch placed about 20 feet away from a listener in a quiet room Taste Chemicals on tongue Taste buds on the tongue 1 teaspoon sugar dissolved in 2 gallons water Smell Chemical substances that enter the nose Receptor cells in About one drop of perfume the upper nostrils dispersed in a small house Touch Movement or pressure on skin Nerve endings in the skin The wing of a bee falling on the cheek from about 1 centimeter away • Signal-detection theory states that the threshold for detecting a given event depends not only on the features of that event but on other factors as well, including background stimulation (noise) and on the biological and psychological characteristics of the receiver. • Sensory adaptation occurs when sensory systems become less sensitive to constant or unchanging stimuli. Vision: Seeing the Light The visible spectrum for humans is only a small portion of the total range of wavelengths. 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 color spectrum is often remembered with the name Roy G. Biv, which stands for red, orange, yellow, green, blue, indigo, and violet. Red has the longest wavelength (about 700 billionths of a meter, or nanometers), while Violet has the shortest, at about 300 nanometers. The Eye • The eye has two purposes, providing a “housing” for neural tissue that receives light, the retina, and channeling light to the sensory receptors (rods and cones) in 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. Lens Retina Iris Fovea (point of central focus) Path of light Pupil Cornea Blind spot Optic nerve (to visual cortex of brain) Visual cortex 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. Light Blind Spot • The retina has two types of receptors, rods and cones. • Rods play a key role in night vision because they are more sensitive 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. Rod Light Cone Bipolar Cell Optic Nerve Fibers Receptor Cells (rods and cones) Rod Light Cone Optic Disk (and blind spot) Ganglion Cell Light striking the rods and cones triggers firing of neural signals that pass into the cells in the retina. Signals move from receptors to bipolar cells to ganglion cells, which send impulses along the optic nerve. These axons carry visual information, and depart the eye through the blind spot. Visual Pathways 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. The Visual Cortex • 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 to record action potentials from individual cells. • The cells in the visual cortex respond to lines, edges, and more complicated stimuli, rather than to small spots of light. • Neurons that respond to specific features of a visual stimulus are called feature detectors. • David Hubel and Torsten Wiesel discovered this by accident. The Visual Cortex A vertical line elicits rapid firing in the cell, while a horizontal line elicits no response, the cell fires at its baseline rate. A diagonal line elicits moderate firing in the cell. 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. Yet the brain is able to piece together these particular visual features to form impressions of whole objects & patterns. Color Vision • Trichromatic theory of color vision believes the human eye has three types of receptors sensitive to the specific wavelengths associated with red, green, and blue-violet. • When the three primary colors of light – red, green, and blue-violet – are combined, they form white. • A light of any color can be matched by mixing of the three primary colors. Color Vision • But what about yellow? Is it just reddishgreen? Ewald Hering, proposed opponentprocess 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. Color Blindness Research finds that more males than females are affected by color-vision limitations. This may be because there is a sex-linked genetic defect on the x-chromosome, of which males only have one. Hearing: The Music of Sound Hearing The stimulus for the auditory system is sound waves, which are vibrations of molecules. Sound waves must travel throughout some physical medium, such as air. Sound waves are characterized by amplitude (loudness) and wavelength (pitch). Hearing The vertical size of the wave (the amplitude) is related to the volume of the auditory stimulus, while the wavelength (indicated in the frequency) is related to the pitch of the auditory stimulus. The Ear • The 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 outer ear funnels sound to the eardrum, which is a tight membrane (similar to a snare drum) that vibrates when struck by sound waves. • 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. Outer Ear Malleus (hammer) Incus (anvil) Middle Ear Inner Ear Auditory cortex Cochlea (partially unfolded) Semicircular canals Stapes (stirrup) Cochlea Hair cells Auditory nerve (to auditory cortex of brain) Sound waves Organ of Corti Eardrum Ossicles Oval window Basilar membrane Eardrum Hearing • Place Theory – the perception of pitch corresponds to the vibration of different portions, or places, along the basilar membrane. Different places have different pitches, like keys on a piano. This helps account for higher-pitched sounds that can be coded by the brain for location. • Frequency Theory – the 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. Then the brain detects the frequency of a tone by the rate at which the auditory nerve fires. • Volley principle - the firing in volleys of groups of neurons along the basilar membrane. The brain interprets the firing of these neurons to interpret sounds between 1,000 and 4,000 cycles per second. Our Other Senses: Chemical, Skin, and Body Senses Smell • The physical stimuli giving rise to odors are chemical substances carried in the air that are dissolved in fluid- the mucus in the nose. • Olfactory receptors are located in the upper portion of the nasal passages. Chemical information is sent via the olfactory nerve directly to the brain via the olfactory bulb. Olfaction (smell) is the only one of the five sensory systems that does not go through the thalamus. • Humans can distinguish among some 10,000 odors, and have over 5 million different odor receptors. Olfactory bulb Olfactory nerve Nasal passage Receptor cells in olfactory membrane Taste • Taste operates much like the sense of smell. Taste 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 to the thalamus and on to the cortex. • The five primary tastes are sweet, sour, bitter, salty, and umami, with uneven distribution on the tongue. • Taste results from a complex blend of these five tastes, as well as learning and social processes. Tongue Taste Buds Projects on tongue that contain taste buds Touch • Your skin is your largest sensory organ, and it contains sensory receptors for touch, pressure, warmth, cold, and pain. • Information is transmitted from the receptors in the skin to the somatosensory cortex, which is located in the parietal lobe in the brain. • Pain receptors transmit information to the brain via two types of pathways: a fast pathway registers localized pain and relays pain signals to the brain in a fraction of a second. A slower, thinner pathway lags a second or two behind and carries less localized, longer-lasting aching or burning pain. Heat Cold Light touch Nerve Pain Touch (Hair) Light touch Strong pressure Pain • The gate-control theory of pain believes that a neural “gate” in the spinal cord opens to allow pain messages to travel to the brain or closes to shut them out. • Bottleneck at the “gate” may block pain. • Endorphins also help close the “pain gate.” Kinesthesia • Body sense that provides information about • Movement of body parts • Relative position of body parts to each other • Receptors located in joints, ligaments, tendons, skin, and muscles Vestibular Sense Semicircular canal Nerve leading to brain Hair cells Fluid in semicircular canals Semicircular Canals Vestibular Sacs Membrane in vestibular sac Hair cells Nerve leading to brain Allow us to maintain our balance and equilibrium, involves messages received by hair-cell receptors in the inner ear in response to forces of gravity when we tilt out head or move our head through space. The semicircular canals are of particular importance to this system. Motion sickness occurs when the information from your visual system and vestibular sense are in conflict with each other. Perceiving Our World: Principles of Perception A perceptual set refers to the tendency for one’s perceptions to be influenced by expectations or preconceptions. The experience that you have had can influence how you perceive the world around you. Duck or Rabbit? Perception • The way we perceive visual information can occur in one of two ways – top-down or bottom-up processing. • In bottom-up processing, the brain recognizes meaningful patterns by piecing together bits and pieces of sensory information. • In top-down processing, the brain identifies patterns as meaningful wholes rather than as piecemeal constructions. Gestalt psychologists maintain that the whole can be greater than the sum of its parts. The principle of figure and ground is shown in this image. What you see depends on which part of the drawing you see as figure and which part you see as background. Reversible Figure Ambiguous Figure By shifting figure and ground, your perception can shift from perceiving the ambiguous figure as a young woman or as an old woman. A reversible figure is a drawing that is compatible with two different interpretations that can shift back and forth. Young Woman Old Woman Gestalt Principles Proximity Continuity Closure Shape Constancy • Depth perception is the interpretation of visual cues that indicate how near or far away something is. • Two types of clues are used to make judgments of distance: • Binocular cues (both eyes) include retinal disparity (objects within 25 feet project images to slightly different locations on the left and right retinas; each eye sees a slightly different view of the object) and convergence- the eyes converge toward each other as they focus on a target. • Monocular cues (one eye) are clues about distance based on the image in either eye alone. Motion Perception • Our perception of movement depends on perceptual cues: • The path of the image as it crosses the retina • The changing size of an object in relation to the observer • We perceive a car moving faster than our own vehicle when we perceive it becoming smaller as it moves further ahead of us on the road • Optical Illusions involve an apparently inexplicable discrepancy between the appearance of a visual stimulus and its physical reality. • The Müller-Lyer illusion is one famous visual illusion, shown here. Of the two vertical lines shown here, which one looks longer? Ponzo Illusion Impossible Figure Stroop Effect Thank you! Any Questions? Start reading your next chapter!