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