Sensation and Perception

Sensation and Perception Chapter Four

Sensation vs. Perception • Sensation: the process of detecting a stimulus such as • light waves (vision) • sound waves (hearing) • chemical molecules (smell and taste) • heat or pressure (touch) • orientation or balance (kinesthetic senses) • Perception: the process of integrating, organizing and interpreting sensations

From a sensory point of view – mass of red, white and blue colors and horizontal and vertical lines • Perception allows you to interpret the splotches of color and array of lines as the American flag

Bottom-up processing – sensory analysis that starts at entry levels • Used when we have no prior knowledge • Top-down processing – construction of perception based on experiences and expectations • Used when we do have prior knowledge

Transduction – process by which sensory receptors convert the incoming physical energy of stimuli (like light waves) into neural impulses the brain can understand • Sensations such as “red” and “cold” occur only when the neural impulse reaches the brain

Selective attention – focusing conscious awareness on a particular stimulus • Attention to one thing causes inattention to something else • Cocktail party effect – ability to attend to only one voice among many • Inattentional blindness – failing to see visible objects when our attention is directed elsewhere • Change blindness – failing to notice changes in the environment

Thresholds • Absolute threshold – minimum amount of a stimulus that an observer can reliably detect at least 50% of the time

Thresholds -cont- • Difference threshold – minimal difference needed to notice a stimulus change; called “just noticeable difference” • Noticing when the TV volume is turned down just one notch • Weber’s law – just noticeable difference will vary depending on its relation to the original stimulus • Size of JND is proportional to the strength of the original stimulus • Noticing the addition of a 5-pound weight when bench pressing 50 pounds but not when bench pressing 500 pounds

Thresholds -cont- • Signal detection theory – tries to explain and predict different perceptual mistakes we make • False positive – we think we perceive a stimulus that isn’t there • Mistaking a stranger for someone you know on a crowded street • False negative – not perceiving a stimulus that is present • Not noticing directions on a test that tell you to write in complete sentences

Sensory adaptation – when a constant stimulus is presented for a length of time, receptors fire less frequently and the sensation often fades or disappears • Getting used to new running shoes or the temperature of the water • Does not affect vision

Vision

Vision: From the cornea to the retina • Cornea – light waves first enter the eye here • Clear membrane covering the visible part of the eye • Protects the eye and helps gather and direct incoming light waves • Pupil –opening in the middle of the iris • Changes size to let in different amounts of light • Iris – colored part of the eye • Ring of muscle tissue that contracts or expands to control the size of the pupil • Muscles respond to light and inner emotions – constrict in parasympathetic calm and dilate in sympathetic arousal

Vision: From the cornea to the retina • Lens – transparent structure located behind the pupil that actually focuses and bends light as it enters the eye • Accommodation – change in the curvature of the lens that enables the eye to focus on objects at various distances • Nearsightedness – results when the cornea and lens focus on an image in front of the retina, making distant objects appear blurry • Farsightedness – results when the cornea and lens focus on an image behind the retina, making objects near the eye appear blurry

Vision: The retina • Retina – light-sensitive membrane at the back of the eye where the transduction of light waves into neural messages occurs • Contains millions of sensory receptors for vision • Rods – allow you to see in poorly light environments • Located primarily in the retina’s periphery • Cones- sensitive to colors and bright light • Concentrated in the fovea – a small region in the center of the retina

Vision: The retina • Bipolar cells – specialized neurons that connect the rods and cones with ganglion cells • Ganglion cells – specialized neurons that connect to the bipolar cells • Bundled axons of ganglion cells form the optic nerve • Blind spot – point where the optic nerve leaves the eye and where there are no rods or cones, creating a blind spot in our vision

Feature detectors – nerve cells in the brain that respond to specific features of the stimulus, such as shape, angle or movement

Theories of Color Vision • Trichromatic or three-color theory – theorizes that the retina has three different color receptors – cones that detect the different colors red (long wavelengths), blue (short wavelengths) or green (medium wavelengths) – which when stimulated in combination can produce the perception of any color • Does not explain afterimages and color blindness

Theories of Color Vision • Opponent-process theory – theorizes that ganglion cells process color in opposing pairs of red or green, black or white, and blue or yellow colors • The visual cortex also encodes color in terms of these three opponent pairs • Explains afterimages – visual experiences that occurs after the original source of stimulation is no longer present • When you look at the color red for a long time, you fatigue the sensors for red; when you switch your gaze and look at a blank page, the opponent part of the pair for red will fire and you will see a green afterimage

Theories of Color Vision • Color blindness • Typically caused by deficiency in cones • Most common is related to deficiencies in red-green system

Hearing

Sound Waves • Ear transforms vibrating air into nerve impulses, which our brain decodes as sounds • The amplitude (strength) of sound waves determines their loudness • Waves vary in frequency – number of complete wavelengths that pass a point in a given time • Frequency determines the pitch (highness or lowness) that we experience • Long waves have low frequency, short waves have high frequency

The Outer Ear • Collects sound waves • The pinna • Flap of skin and cartilage attached to each side of our head • Catches sound waves and channels them into the auditory canal • The auditory canal • Sound waves travel down the auditory canal and bounce into the ear drum • The eardrum or tympanic membrane • Tightly stretched membrane located at the end of the auditory canal • Eardrum vibrates when hit by sound waves; vibrations match the intensity

The Middle Ear • Amplifies sound waves • Hammer, anvil and stirrup (collectively called ossicles) • Three tiny bones in the middle ear • Joint action doubles the amplification of sound • Oval window • Small membrane separating the middle ear from the inner ear • Stirrup transmits amplified vibrations to the oval window and oval window relays vibrations to the cochlea

The Inner Ear • Transduces sound waves into neural messages • Cochlea • Spiral-shaped, fluid-filled structure that contains the basilar membrane and hair cells • Basilar membrane • Runs the length of the cochlea • Holds the hair receptors for hearing • Hair cells • Sensory receptors embedded in the basilar membrane • Hair cells transduce the physical vibration of the sound waves into neural impulses

Distinguishing Pitch • Pitch – relative highness or lowness of a sound • Frequency theory – theory differences in pitch are due to the rate of neural impulses traveling up the auditory nerve • We sense pitch because the hair cells fire at different rates (frequencies) in the cochlea • Explains how low-frequency tones are transmitted to the brain • Place theory – theory that differences in pitch result from stimulation of different areas of the basilar membrane • Higher-frequency sounds cause maximum vibrations near the stirrup end of the basilar membrane; lower frequency sounds cause maximum vibrations at the opposite end • Explains how high-frequency tones are transmitted to the brain

Loss of Hearing • Conduction deafness • Caused when the bones in the middle ear are damaged and can’t transmit sound waves to the inner ear • Causes can include tumors, objects in ear canal, infections or otosclerosis (genetic degeneration of the middle ear bones) • Nerve deafness • Caused by damage to the cochlea, hair cells or auditory nerve • Treated with hearing aids or cochlear implants • Causes include infections, genetic defects, exposure to loud noises, trauma, high blood pressure, diabetes and MS

Touch and the Sensory Cortex

Touch • Touch receptors aren’t evenly distributed among the different areas of our bodies • More densely concentrated in the face, hands and lips than on the legs or back • Gate-control theory of pain – the brain regulates pain by sending signals down the spinal cord that either open or close sensory pathways or “gates” • Brain signals gates to open = pain is experienced or intensified • Brain signals gates to close = pain is reduced

Vestibular sense – provides a sense of balance and equilibrium • Inner ear contains receptors that are especially important for maintaining balance • Semicircular canals are filled with fluid and lined with hair like receptor cells that shift in response to motion, providing the brain with important information about the body’s posture and head position

Kinesthetic sense – gives us feedback about the position and orientation of specific body parts

Chemical Senses • Taste (or gustation) • Chemicals from food are absorbed by taste buds on our tongue • Taste buds are located on papillae – bumps you can see on your tongue • Are located all over the tongue and some parts of the inside of the cheeks and roof of the mouth • More densely packed taste buds = more intense taste • Humans sense five different types of taste: sweet, salty, sour, bitter and umami (savory or meaty taste)

Chemical Senses • Smell (or olfaction) • Mucous membrane at the top of each nostril contains receptor cells that absorb airborne chemical molecules • Receptor cells communicate neural messages to the olfactory bulb • Impulses from the olfactory bulb don’t go to the thalamus • Nerve fibers connect to the brain at the amygdala and then to the hippocampus

Perceptual Organization

Gestalt Principles of Organization • Founded by Max Wertheimer in early 1900s • Maintains that we actively process our sensations according to consistent perceptual rules • Rules create whole perceptions (gestalts) that are meaningful, symmetrical and as simple as conditions allow

Figure-ground relationship – organization of the visual field into objects (figures) that stand out from their surroundings (ground) • Your brain organizes black markings in a book as letters and groups them into words and sentences • Letters = figure • White page = ground

Perceptual Grouping • Law of similarity – tendency to perceive objects of a similar size, shape or color as a unit or figure • Organizing a crowd at a football game into home fans, visiting fans, band members and cheerleaders • Law of proximity – tendency to perceive objects that are physically close to one another as a single unit • Grouping visiting fans into a single, homogenous group • Law of closure – tendency to fill in the gaps in an incomplete image • Scoreboard reads “HO E and VISI ORS” and your brain fills in missing M and T to complete the words

Constancy • Size constancy – objects closer to our eyes will produce bigger images on our retinas, but we take distance into account in our estimations of size • Knowing an object doesn’t grow or shrink in size as it moves closer or farther away • Shape constancy – objects viewed from different angles will produce different shapes on our retinas, but we know the shape of an object remains constant • Brightness constancy – we perceive objects as being a constant color even as the light reflecting off the object changes

Shape constancy

Perceived Motion • Stroboscopic effect – images in a series of still pictures presented at a certain speed will appear to be moving • Phi phenomenon – series of light bulbs turned on and off at a particular rate will appear to be one moving light • Autokinetic effect – if people are asked to stare at a spot of light projected steadily onto the same place on a wall of an otherwise dark room, they will report seeing it move

Depth perception • Ability to perceive three-dimensional space and to accurately judge distance • Visual cliff experiment • Supports conclusion that perception in humans is innate and emerges during infancy

Monocular depth cues • Require the use of only one eye to process distance or depth cues

Monocular depth cues • Linear perspective - parallel lines appear to converge toward a vanishing point as they recede into the distance • Carlo Crivelli’s The Annunciation

Monocular depth cues • Aerial perspective – distant objects often appear hazy and blurred compared to close objects • Pieter Bruegel the Elder’s The Harvester

Monocular depth cues • Relative size - if two or more objects are assumed to be similar in size, the object that appears larger is perceived as being closer • George Seurat’s Sunday Afternoon on the Island of La Crande Jatte

Monocular depth cues • Motion parallax - as you move, you use the speed of passing objects to estimate the distance of the objects • On the interstate, nearby telephone poles, fences and roadside signs seem to zip by faster than distant hills

Binocular depth cues • Require the use of both eyes to process distance or depth cues • Convergence – binocular depth cue in which the closer the object, the more the eyes converge, or turn inward • Retinal disparity – binocular depth cue in which separation of the eyes causes different images to fall on each retina • when two retinal images are very different, we interpret the object as being close by; when they are more nearly identical, the object is perceived as being farther away

Sensation and Perception