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Sensation and Perception

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1 Sensation and Perception
Chapter 4 Sensation and Perception

2 Sensation and Perception: The Distinction
Sensation : stimulation of sense organs Perception: selection, organization, and interpretation of sensory input Psychophysics = the study of how physical stimuli are translated into psychological experience Sensation and perception are hard to separate, because people automatically start organizing incoming sensory stimulation the moment it arrives. For theory and research, however, the distinction between the two is useful. Sensation is the stimulation of sense organs…for example absorbing energy from light by the eyes. Perception is the selection, organization, and interpretation of sensory input…translating the sensory input into something meaningful. Look at a photo of a rose in your text, your eyes sense the light reflecting from the page; what you perceive, however, is the picture of the rose. Psychophysics is the study of how physical stimuli are translated into psychological experience, thus psychologists in this area are interested mainly in sensation and perception.


4 Psychophysics: Basic Concepts
Sensation begins with a detectable stimulus Transduction – the conversion of external physical stimuli to neural impulses the brain can understand. Fechner: the concept of the threshold Absolute threshold: detected 50% of the time. Just noticeable difference (JND): smallest difference detectable Weber’s law: size of JND proportional to size of initial stimulus Sensation begins with a detectable stimulus…but what counts as detectable depends on whom or what is doing the detecting. Gustav Fechner (1860) questioned, for any given sense, what is the weakest detectable stimulus? The concept of the threshold is implicit in Fechner’s question. A threshold is defined in the text as a dividing point between energy levels that do and do not have a detectable effect…example, automatic lights turn on when a threshold is reached. An absolute threshold is the minimal amount of stimulation that an organism can detect…depends on the boundaries of an organism’s sensory capabilities…researchers discovered, however, that there is no single stimulus intensity that results in a jump from no detection to 100% detection every time…thus researchers use the concept of the absolute threshold…the stimulus intensity that can be detected 50% of the time. Fechner was also interested in the smallest difference in the amount of stimulation that a specific sense can detect…the just noticeable difference (JND): smallest difference detectable. Research on the JND by Ernst Weber illustrated that the size of a JND is a constant proportion of the size of the initial stimulus…in general, as stimuli increase in magnitude, the JND becomes larger.


6 Psychophysics: Concepts and Issues
Psychophysical Scaling: Fechner’s Law - larger and larger increases in stimulus intensity are required to produce JNDs in the magnitude of sensation. Signal-Detection Theory: Sensory processes + decision processes Subliminal Perception: Existence vs. practical effects Sensory Adaptation: Decline in sensitivity Being asked whether one light that has twice the energy of another is twice as bright is an example of scaling. Fechner’s Law asserts that larger and larger increases in stimulus intensity are required to produce JNDs in the magnitude of sensation. Signal detection theory holds that the detection of sensory information is influenced by two things…1) noise in the system (irrelevant stimuli in the environment that elicit neural activity), and 2) decision making processes. Signal detection theory was important in that it emphasized factors other than stimulus intensity influencing detectability (in contrast to Fechner’s ideas). Many researchers, using very different methods, have demonstrated that perception can occur without awareness. Many people believe that advertisers attempt to place subliminal messages in ads, while others say that people are just reading things into ads, like seeing familiar shapes in the clouds. Regardless, research shows that the effects of subliminal perception are relatively weak and of little practical impact. Prolonged stimulation may lead to sensory adaptation, or a decline in sensitivity to the stimulus…you don’t smell the skunk that sprayed you yesterday, but everyone else does…the pool is only cold at first…etc.

7 Vision: The Stimulus Light = electromagnetic radiation
Amplitude: perception of brightness Wavelength: perception of color purity: mix of wavelengths perception of saturation, or richness of colors. Light is electromagnetic radiation that travels as a wave…the wave travels quickly…the speed of light. Light waves vary in height or amplitude (which affects brightness) and in wavelength (which affects color) or distance between peaks. Light can also vary in its purity, which has to do with how many different wavelengths are mixed together. Purity influences the perception of saturation, or richness of colors.



10 The Eye: Converting Light into Neural Impulses
The eye: housing and channeling Components: Cornea: where light enters the eye Lens: focuses the light rays on the retina Iris: colored ring of muscle, constricts or dilates via amount of light Pupil: regulates amount of light 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.

11 Figure 4.7 The human eye

12 The Retina: An Extension of the CNS
Retina: absorbs light, processes images, and sends information to the brain Optic disk: where the optic nerve leaves the eye/ blind spot Receptor cells: Rods: black and white/ low light vision Cones: color and daylight vision Adaptation: becoming more or less sensitive to light as needed Information processing: Receptive fields -the collection of rod and cone receptors that funnel signals to a particular visual cell in the retina. Lateral antagonism (contrast) - occurs when neural activity in a cell opposes activity in surrounding cells The retina is a piece of neural tissue that lines the back of the eye…it absorbs light, processes images, and sends 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. The visual receptor cells in the axon are the rods (for black and white and low light vision) and the cones (for color and daylight vision). Adaptation, or becoming more or less sensitive to light as needed, occurs in part due to chemical changes in the rods and cones. Receptive fields are the collection of rod and cone receptors that funnel signals to a particular visual cell in the retina. Lateral antagonism, or lateral inhibition, occurs when neural activity in a cell opposes activity in surrounding cells.



15 The Retina and the Brain: Visual Information Processing
Light -> rods and cones -> neural signals -> bipolar cells -> ganglion cells -> optic nerve -> optic chiasm -> opposite half brain -> Main pathway: lateral geniculate nucleus (thalamus) -> primary visual cortex (occipital lobe) magnocellular: where is it (ex. brightness) parvocellular: what is it (ex.color) Second pathway: superior colliculus ->thalamus -> primary visual cortex (motion) The Retina and the Brain: Visual Information Processing Light striking the rods and cones triggers neural signals to move to bipolar cells then to ganglion cells, then along the optic nerve to the optic chiasm, where the optic nerves from the inside half of each eye cross over and project to the opposite half brain. This crossing ensures that signals from both eyes go to both hemispheres of the brain. After the crossing, 2 visual pathways exist. The main pathway goes through the lateral geniculate nucleus in the thalamus and on to the primary visual cortex in the occipital lobe. The other goes through the superior colliculus to the thalamus and on to the primary visual cortex. The main visual pathway is subdivided into two subspecialty pathways, the magnocellular channel and the parvocellular channel. These channels engage in parallel processing, which involves simultaneously extracting different kinds of information from the same input, with the parvocellular channel handling perception of color, for example, and the magnocellular channel handling brightness.



18 Hubel and Wiesel: Feature Detectors and the Nobel Prize
Early 1960’s: Hubel and Wiesel Microelectrode recording of axons in primary visual cortex of animals Discovered feature detectors: neurons that respond selectively to lines, edges, etc. Groundbreaking research: Nobel Prize in 1981 Later research: cells specific to faces in the temporal lobes of monkeys and humans Hubel and Wiesel: Feature Detectors and the Nobel Prize In the early 1960’s H and W started research using microelectrode recording of axons in the primary visual cortex of animals…initially, they had little success getting neurons to fire by having the cats look at flashing spots of light. Accidentally, they introduced a straight line light…rapid firing occurred in the visual cortex. They went on to discover that the visual cortex has feature detectors in it, neurons that respond selectively to very specific features of complex stimuli…lines, edges, etc. This was groundbreaking research, which won them the Nobel Prize in 1981. Later research has demonstrated that there are cells in the temporal lobes of monkeys and humans (along the visual pathway) that specifically respond to pictures of faces…grandmother cells.



21 Visual Information Processing
Trichromatic (three color) Theory Young and Helmholtz three different retinal color receptors red green blue


23 Theories of Color Vision
Trichromatic theory - Young and Helmholtz Receptors for red, green, blue – color mixing – F 4.21 Opponent Process theory – Hering 3 pairs of antagonistic colors – negative afterimages red/green, blue/yellow, black/white Current perspective: both theories necessary Color vision defects Color vision defects: simulations Young and Helmholtz, in the mid 1800’s, came up with the first theory of color vision…trichromatic theory. This theory holds that the human eye has three types of receptors with differing sensitivities to different light wavelengths…one for red, one for green, and one for blue. All colors can be seen, according to this theory, because of color mixing. But what about yellow? Is it just reddish-green? Edward Hering, in 1878, proposed opponent process theory…which holds that color perception depends on receptors that make antagonistic responses to three pairs of colors…red on,green off; yellow on,blue off; black on,white off. This not only takes care of yellow, but also explains the phenomenon of complimentary afterimages. While researchers argued about which was right for almost a century, most psychologists now agree that it takes both theories to explain color vision.



26 Perception: Perceiving Forms, Patterns, and Objects
Reversible figures – F 4.23, F 4.31 Perceptual sets – readiness to perceive a stimulus in a particular way – ambiguous stimuli – F 4.24 – effects of motivational factors Inattentional blindness/change blindness – Feature detection theory - bottom-up processing. Form perception - top-down processing Subjective contours Gestalt psychologists: the whole is more than the sum of its parts Reversible figures and perceptual sets demonstrate that the same visual stimulus can result in very different perceptions A reversible figure is a drawing that is compatible with two interpretations that can shift back and forth (see following slides for depiction). A perceptual set is a readiness to perceive a stimulus in a particular way. Inattentional blindness involves the failure to see fully visible objects or events in a visual display. According to feature detection theory, people detect specific elements in stimuli and build them up into recognizable forms…bottom-up processing. Subjective contours is a phenomenon whereby contours are perceived where none actually exist, attributed to top-down processing. Form perception involves top-down processing…clearly emphasized by the Gestalt psychologists, who demonstrated that the whole is more than the sum of its parts.



29 Principles of Perception
Gestalt principles of form perception: figure-ground, proximity, similarity, continuity, closure, and simplicity Recent research: Distal (stimuli outside the body) vs. proximal (stimulus energies impinging on sensory receptors) stimuli. Perceptual hypotheses Context The Gestalt principles of form perception include figure-ground, proximity, similarity, continuity, closure, and simplicity. The Gestalt emphasis is still felt in the study of perception, as they had useful insights that have stood the test of time, raised important issues, etc. More recently, researchers have focused on the way we distinguish between distal (stimuli outside the body) and proximal (stimulus energies impinging on sensory receptors) stimuli. People may develop perceptual hypotheses about the distal stimulus that may be responsible for the proximal stimulus…the effects of context, etc.







36 Depth and Distance Perception
Binocular cues – clues from both eyes together retinal disparity convergence Monocular cues – clues from a single eye motion parallax accommodation pictorial depth cues 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 may involve motion parallax (having images of objects at different distances moving across the retina at different rates), as well as feeling the accommodation or change in the shape of the lens as the eye focuses. Other monocular cues are pictorial depth cues…cues about distance that can be given in a flat picture, visually depicted on the next slide.

37 Stability in the Perceptual World: Perceptual Constancies
Perceptual constancies – stable perceptions amid changing stimuli Size Shape Brightness Hue Location in space Perceptual constancies are tendencies to experience a stable perception in the face of continually changing stimuli. For example, when a person walks toward you, they get larger perceptually…do you think they are growing? No. Constancies for size, shape, brightness, hue, and location in space have been shown.




41 Random Dot stereograms

42 Autostereogram – a wave

43 Autostereogram – What is it?

44 Emphasis on linear perspective during the Western Renaissance

45 Optical Illusions: The Power of Misleading Cues
Optical Illusions - discrepancy between visual appearance and physical reality Famous optical illusions: Muller-Lyer Illusion, Ponzo Illusion, Poggendorf Illusion, Upside-Down T Illusion, Zollner Illusion, the Ames Room, and Impossible Figures Cultural differences: Perceptual hypotheses at work - website with visual illusions and other visual effects Art and Illusion – pages Optical Illusions involve an apparently inexplicable discrepancy between the appearance of a visual stimulus and its physical reality. Famous optical illusions include those listed. Cultural differences in susceptibility to illusions such as Muller-Lyer and Poggendorf demonstrate the importance of perceptual hypotheses.



48 Hearing: The Auditory System
Stimulus = sound waves (vibrations of molecules traveling in air) Amplitude (loudness) Wavelength (pitch) Purity (timbre) Wavelength described in terms of frequency: measured in cycles per second (Hz) Frequency increase = pitch increase 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. Wavelength is described in terms of frequency and is measured in cycles per second, or hertz (Hz). In general, the higher the frequency (more cycles per second), the higher the pitch. Amplitude is a description of sound pressure and is measured in decibels (db). Perceived loudness is higher with increasing decibel level.


50 The Ear: Three Divisions
External ear (pinna): collects sound. Middle ear: the ossicles (hammer, anvil, stirrup) Inner ear: the cochlea a fluid-filled, coiled tunnel contains the hair cells, the auditory receptors lined up on the basilar membrane The Ear: Three Divisions 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.

51 Figure The human ear

52 Figure 4.50 The basilar membrane

53 The Auditory Pathway Sound waves vibrate bones of the middle ear
Stirrup hits against the oval window of cochlea Sets the fluid inside in motion Hair cells are stimulated with the movement of the basilar membrane Physical stimulation converted into neural impulses Sent through the thalamus to the auditory cortex (temporal lobes) The hair cells are lined up on a membrane that runs the length of the cochlea called the basilar membrane. Sound waves cause the bones of the middle ear to hit against the oval window, a covered opening of the cochlea, which sets the fluid inside in motion. The hair cells are stimulated with the movement of the basilar membrane and convert this physical stimulation into neural impulses that are then sent throughout the thalamus to the auditory cortex, located mostly in the temporal lobes.

54 Theories of Hearing: Place or Frequency?
Hermann von Helmholtz (1863) Place theory Other researchers (Rutherford, 1886) Frequency theory Georg von Bekesy (1947) Traveling wave theory - the whole basilar membrane does move, but the waves peak at particular places, depending on frequency. 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. 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.

55 Auditory Localization: Where Did that Sound Come From?
Two cues critical: Intensity (loudness) Timing of sounds arriving at each ear Head as “shadow” or partial sound barrier Timing differences as small as 1/100,000 of a second Auditory localization involves locating the source of a sound in space. Two cues appear critical, intensity (loudness) and timing of sounds arriving at each ear. A sound in your left ear produces a greater intensity in your left ear, as opposed to your right ear. Also, your head produces a “shadow” or partial sound barrier. People are evidently pretty good at using these cues, as humans have been shown to detect timing differences as small as 1/100,000 of a second.


57 The Chemical Senses: Taste
Taste (gustation) Physical stimulus: soluble chemical substances Receptor cells found in taste buds Pathway: taste buds -> neural impulse -> thalamus -> cortex Four primary tastes: sweet, sour, bitter, and salty Taste: learned and social processes 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.


59 The Chemical Senses: Smell
Smell (Olfaction) Physical stimuli: substances carried in the air dissolved in fluid, the mucus in the nose Olfactory receptors = olfactory cilia Pathway: Odor molecule, Nasal Passage, Nasal Cavity, Olfactory cilia (transduction)-> neural impulse ->olfactory nerve (axons pass through ethmoid bone)-> olfactory bulb (brain)->olfactory tract (inside bulb)->olfactory cortex in temporal lobe->limbic system Does not go through thalamus Retro-nasal Olfaction -> food in your mouth goes through the back of your mouth to your nasal cavity where you smell it. The Chemical Senses: Smell 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. The olfactory receptors synapse directly with cells in the olfactory bulb at the base of the brain. Olfaction is the only sense, therefore, that is not routed through the thalamus. Odors are not easily classified, and primary odors have not really been delineated. Humans can distinguish among about 10,000 odors, but for some reason have a hard time attaching names to odors quite frequently.


61 Skin Senses: Touch Physical stimuli = mechanical, thermal, and chemical energy impinging on the skin. Pathway: Sensory receptors -> the spinal column -> brainstem -> cross to opposite side of brain -> thalamus -> somatosensory (parietal lobe) Temperature: free nerve endings in the skin Pain receptors: also free nerve endings Two pain pathways: fast vs. slow 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. There, they cross over mostly to the opposite side of the brain, project through the thalamus and onto the somatosensory cortex in the parietal lobe. 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.


63 Other Senses: Kinesthetic and Vestibular
Kinesthesis - knowing the position of the various parts of the body Receptors in joints/muscles Vestibular - equilibrium/balance Semicircular canals Synesthesia – “The man who tasted shapes” MIT Synesthesia Project Kinesthesis involves knowing the position of the various parts of the body. Kinesthetic receptors lie in the joints, indicating how much they are bending, or in the muscles, registering tautness or extension. The Vestibular system responds to gravity and keeps you informed of your body’s location in space. It provides your sense of balance or equilibrium. The semicircular canals make up the largest part of the vestibular system; these are fluid filled canals that contain hair cells similar to those in the basilar membrane. When your head moves, the fluid moves, moving the hair cells, and initiating neural signals that travel to the brain.

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