Sensation & Perception Psychology 2012 – Spring 2004

Sensation & Perception Sensation – detection of basic sensory experiences Sounds, touch, smells Perception – interpretation of sensations in meaningful ways Interpreting sounds as music, touch as the cat rubbing against you, smells as dinner cooking

Sensation & Perception Sensory receptors – specialized cells unique to each sense organ They respond to a particular form of sensory stimulation Transduction – the process of converting different forms of energy into electrical impulses that are transmitted via neurons to the brain Done by the sensory receptors

Sensory Thresholds Absolute threshold – the smallest stimulus that can be detected half the time Difference threshold – the smallest difference between 2 stimuli that can be detected half the time AKA the just-noticeable difference (jnd) Weber’s law – the ability to detect a difference between 2 stimuli depends on the strength of the original stimulus Sensory adaptation – sensory receptor cells become less responsive to a constant stimulus

Vision Wavelength – the distance from one wave peak to another

Parts of the Eye Cornea – clear membrane covering the visible part of the eye; helps gather and direct incoming light Pupil – opening in the middle of the iris that changes size to let in different amounts of light Iris – colored part of the eye; controls the size of the pupil Dim light = iris widens; more light is let in Bright light = iris narrows; less light let in

Parts of the Eye Lens – structure located behind the pupil that focuses light entering the eye Accommodation – process by which the lens changes shape to focus incoming light on the retina Problems with the shape of the eyeball = either nearsightedness (long, thin eyeball) or farsightedness (short, fat eyeball) Retina – membrane located in the back of the eye containing the sensory receptors for vision Rods – receptors of the eye that are highly sensitive to light, but not color; responsible for peripheral & night vision Cones – pointed sensory receptors of the eye that detect color vision and fine details Most cones are concentrated on the fovea (in the center of the retina)

Visual Processing in the Retina Bipolar cells – specialized neurons in the retina that connect with the rods & cones and the ganglion cells Ganglion cells – specialized neurons in the retina that connect to the bipolar cells; axons of these cells make up the optic nerve Each ganglion cell combines, analyzes, and encodes information from the rods & cones before sending info to the brain

From the Eye to the Brain Optic nerve – exits from the back of the eye and carries information to the visual cortex of the brain Made up of a million axons of ganglion cells Optic chiasm – where the optic nerve fibers from each eye meet and partly cross over to the opposite side of the brain Thalamus – where most of the optic nerve axons go; processes information about form, color, brightness, and depth Visual cortex – information from the cortex goes here, where it is decoded and interpreted

Color The perceptual experience of different wavelengths of light Hue – color; different wavelengths We see colors from 400 nanometers (violet) to 700 nanometers (red) White object – reflecting all colors; black object – absorbs all colors; red object – reflects red light Brightness – intensity of color Amplitude (height) of the light wave Saturation – the purity of color (light wave) Red = more saturation than pink

Theories of Color Perception Trichromatic Theory – the sensation of color is a result of the cones being especially sensitive to either red (long wavelengths), green (medium), or blue (short) light Color blindness – inability to sense a certain color of light due to a deficiency in cone sensitivity Opponent-Process Theory – the sensation of color is due to opposing pairs of color receptors (red/green, blue/yellow, and black/white); when one color in the pair is stimulated, the other is inhibited Afterimages – seeing the other color (e.g., green) in a color pair once the original color (e.g., red) is no longer present Integrated Explanation of Color Vision – cones of the retina adhere to the trichromatic theory, while the ganglion cells adhere to the opponent process theory

Hearing/Audition Pitch – highness or lowness of sound (frequency – wavelength) Hertz – units used to measure frequency Loudness – determined by intensity (amplitude - height) of sound wave Decibels – units used to measure loudness Timbre – the complexity of sound (combination of frequencies)

Hearing Outer Ear – part of the ear collecting sound waves Pinna, ear canal, & eardrum Eardrum – membrane at the end of the outer ear that vibrates when hit by sound waves Middle Ear – amplifies sound waves via the hammer, the anvil, and the stirrup Inner ear – part of the ear that changes sound into neural impulses Cochlea – structure that contains sensory receptors for sound Basilar membrane – membrane in the cochlea containing hair cells (receptors for sound)

Distinguishing Pitch Frequency theory – the view that the basilar membrane vibrates at the same frequency as the sound wave involved Works for low frequency sounds Place theory – the view that different frequencies cause larger vibrations at different locations along the basilar membrane Works for higher frequency sounds

Smell Olfactory receptors – where odor molecules are sensed Bundles of axons make up olfactory nerves Olfactory bulb – where sensation of smell is registered; in the olfactory cortex at the front of the brain

Smell Sensitivity Sense of smell varies among animals Dogs have 200 million olfactory rods, spread out in a much bigger nose Humans differ greatly in ability to detect smells The most sensitive people are 20 times more sensitive than the least

Taste/Gustation Taste buds – sensory receptors for taste Located on the tongue and inside the mouth and throat Sweet, salty, sour, & bitter

The Skin & Body Senses Skin senses provide information about our physical interaction with objects in the environment The body senses keep us informed about our position & orientation in space Skin = largest and heaviest sense organ Pacinian corpuscle – receptor beneath the skin involved in sense of touch Touch & temperature – more sensory receptors in certain areas of the skin (hands, faces, lips) than others (back, arms, & legs)

The Skin & Body Senses Pain – the sensation of discomfort or suffering that occurs in varying degrees of intensity Free nerve endings in the skin, muscles or internal organs carry messages of pain to the spinal cord, releasing “substance P” Substance P – neurotransmitter that is involved in transmitting pain messages to the brain Depending on how the brain interprets pain, it can be intensified or reduced Gate-control theory – sensation of pain is controlled by a series of “gates” in spinal cord. Depending on how the brain interprets pain, these gates are opened (more pain sensation) or closed (less pain sensation) Psychological factors increasing pain: anxiety, fear, hopelessness Psychological factors decreasing pain: positive emotions, distraction, feelings of control

The Skin & Body Senses Psychological factors also influence the release of endorphins, which can: Inhibit pain messages in the brain, and Inhibit the release of substance P in the spinal cord Psychological factors can affect muscle tension, blood flow, arousal, & heart rate All of these can affect the experience of pain

The Skin & Body Senses Movement, Position, & Balance Kinesthetic sense – sensation of body parts in relation to one another Proprioceptors – sensory receptors in the joints, the inner ear and the muscles Constantly communicate information to the brain about changes in body position and muscle tension Vestibular sense – sense of balance through responding to changes in gravity, motion, and body position Semicircular canals and vestibular sacs located in the ear Filled with fluid and hairlike receptor cells that sense changes in body position or gravity Vision also affects this sense When vision is at odds with the semicircular canals and vestibular sacs, the result can be dissiness, disorientation, & nausea

Perception The organization and interpretation of raw sensory data Bottom-up processing – analysis that moves from recognizing the parts of a stimulus to the whole stimulus Data-driven processing Used most during ambiguous situations Top-down processing – analysis that moves from the whole stimulus to the parts Conceptually driven processing Drawing on our experiences to arrive at meaningful perceptions

The Perception of Shape Figure-ground relationship – a principle of perceptual organization that states that we automatically separate a stimulus into: 1. The feature that stands out (the figure) from: Figure – the main element of a scene Usually stands out clearly 2. Its less distinct background (the ground) Ground – the background Usually is more fuzzy and less clear than the figure

The Perception of Shape – What is it? Perceptual grouping – when perceiving a form, we tend to group different elements of it together We actively organize elements to try to produce well-defined, whole objects

The Perception of Shape Law of Pragnanz (simplicity) – when several perceptual organizations are possible, we interpret objects in the simplest ways Example – three squares

Depth Perception – How far away is it? The ability to perceive distance of an object and its three dimensional qualities Monocular Cues – only require one eye to notice Relative size – if two or more objects are assumed to be similar in size, the object that appears larger is perceived as being closer Overlap – when one object blocks another object, the partially blocked object is viewed as being farther away Aerial perspective – hazy or slightly blurred objects appear to be farther away

Depth Perception Monocular Cues Texture gradient – crisp and distinct objects appear closer than fuzzy and blended objects Linear perspective – parallel lines seem to meet in the distance; the closer together the lines appear to be, the greater the perception of distance Motion parallax – when moving, nearby objects seem to move by faster than more distant objects

Depth Perception Binocular cues – require both eyes to notice Convergence – the degree to which eye muscles rotate to focus on an object The more the eyes rotate inward, the closer the object is perceived to be Binocular disparity – when retinal image is very different between the two eyes, the object is interpreted as being very close When retinal image is very similar, object is interpreted as being further away

Convergence Repeat

The Perception of Motion – Where is it going? Neural pathways doing a combination of the following 1. Sensing eye muscle activity 2. Sensing the changing retinal image 3. Contrasting the moving object with its stationary background

The Perception of Motion Illusions of motion 1. Induced motion – our tendency to perceive an object moving and a background staying still Even when a background is actually moving Karl Duncker – the dot and the rectangular frame 2. Stroboscopic motion – the illusion that two or more flashing lights are actually one moving light If the flashing of the second light is within 1/10 of a second The brain’s visual system perceives motion, even though there is no movement across the retina

Perceptual Constancies Tendency to perceive familiar objects as unchanging despite actual changes in sensory input Size constancy – the perception that an object remains the same size despite a changed retinal image Shape constancy – the tendency to perceive objects as having a similar shape despite the images cast on the retina Brightness constancy – the tendency to perceive the brightness of an object as the same despite changes in lighting conditions

Perceptual Illusions Stimuli that tend to make us misperceive the true characteristics of objects or images The Muller-Lyer illusion – the misperception of identical length of two lines due to arrows pointing in different directions The moon illusion – the misperception that the moon is larger when it is closer to the horizon

Illusions Repeat

Experience & Perception Perceptual set – the influence of prior assumptions and expectations on perceptual interpretations Observers often interpret ambiguous stimuli in terms of what their expectations are Circular clouds = UFOs, rocks in a river = the Lock Ness Monster

Experience & Perception Our perceptions are a combination of the following: Cues that indicate distance, form, movement, etc. Our expectations, learning experiences, and cultural experiences