Sensation & Perception
Psychology 2012 – Spring 2004
Sensation & Perception
Sensation – detection of basic sensory experiencesSounds, 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 SensitivitySense 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 tasteLocated 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 environmentThe 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 togetherWe 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 waysExample – 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 motion1. 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 interpretationsObservers 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