sensation & perception chapter 4. transduction transduction: transformation of one form of...

Sensation & Perception

Chapter 4

Transduction

• Transduction: Transformation of one form of energy into another – especially the transformation of stimulus information into nerve impulses

• Receptors: Specialized neurons that are activated by stimulation and transduce (convert) it into a nerve impulse

• Sensory pathway: Bundles of neurons that carry information from the sense organs to the brain

Sensation and Perception

• Sensation: Neurons in a receptor create an internal pattern of nerve impulses that represent the conditions that stimulated it

• Perception: A process that makes sensory patterns meaningful and more elaborate

• Stimulation Transduction Sensation Perception

Sensory Adaptation

• Sensory adaptation: Loss of responsiveness in receptor cells after stimulation has remained unchanged for a while

• Examples???

Thresholds

• Absolute threshold: Amount of stimulation necessary for a stimulus to be detected (50% of time)

• Difference threshold: Smallest amount by which a stimulus can be changed and the difference be detected (also called just noticeable difference – JND)

Looking at the JND• Weber’s law: The JND increases with the magni-

tude of the stimulus. – The JND is large when the stimulus intensity is high, and

small when the stimulus intensity is low• TV (volume high – turn down a lot, volume low – turn down

little)• Groceries (50 pound bag – need to add more than you would to

a 25 pound bag)

• Steven’s power law: – Used for wider array of stimuli (shock, temperature)– Fills in gaps left by Weber and Fechner

Signal Detection Theory• Signal detection theory: Perceptual judgment as

combination of sensation and decision-making processes – Based on each individual’s sensitivity and response criterion

– Example: holiday weekend on the interstate• False Alarm (brakes and no cop)

• Hit (brakes and cop)

• Miss (no brakes and cop)

• Correct Rejection (no brakes and no cop)

– Lowers response criterion and raises hit rates• Flawed merchandise off the assembly line

• TSA putting weapons in bags

Subliminal Persuasion

• Studies have found that stimuli flashed subliminally on a screen can “prime” a person’s later responses.

• No controlled research has ever shown that subliminal messages delivered to a mass audience influences buying habits.– Subtle, fleeting effect on thinking– No powerful, enduring effect on behavior

The Anatomy of Visual Sensation

Fovea: Area of sharpest vision in the retina

Retina: Light-sensitive layer at the back of the eyeball – transduction occurs here!

• Photoreceptors – Light-sensitive cells in the retina that convert light energy to neural impulses

• Rods – Sensitive to dimlight but not colors

• Cones – Sensitive tocolors but not dim light

• http://www.youtube.com/watch?v=x6Ua5d3wlA0 (6:32)

The Anatomy of Visual Sensation• Optic nerve: Bundle of neurons that carries visual

information from the retina to the brain• Rods/conesbipolar cellsganglion cells

Blind spot: Point where the optic nerve exits the eye and where there are no photoreceptors (money in the bank demo)

Color Blindness & Afterimages• Color blindness: Vision

disorder that prevents an individual from discriminating certain colors– Red-green is most common!

• Afterimages: Sensations that linger after the stimulus is removed

• Fix your eyes on the center for a negative afterimage…

How the Visual System Creates Color

• Visual cortex: Part of the brain – the occipital cortex – where visual sensations are processed

• Brightness: Sensation caused by the intensity of light waves

• Color: Psychological sensation derived from the wavelength of visible light – color, itself, is not a property of the external world

• Electromagnetic spectrum: Entire range of electromagnetic energy, including radio waves, X-rays, microwaves, and visible light

• Visible spectrum: Tiny part of the electromagnetic spectrum to which our eyes are sensitive

How the Visual System Creates Brightness

WavelengthWavelength

ColorColor

Intensity Intensity (amplitude)(amplitude)

BrightnessBrightness

Sensing Colors

• Trichromatic Theory: Idea that colors are sensed by three different types of cones sensitive to light in the red, blue and green wavelengths.– Explains the earliest stage of color sensation.

• Opponent-process Theory: Idea that cells in the visual system process colors in complimentary pairs, such as red or green or as yellow blue.– Explains color sensation from the bipolar cells onward in

the visual system.

Hearing: How Sound Waves Become Auditory Sensations

Tympanic membrane:The eardrum

Pinna Ear Canal Tympanic Membrane Middle Ear Oval Window Cochlea (Basilar Membrane) Auditory Nerve


Cochlea: Where sound waves are transduced


Basilar membrane:Thin strip of tissue with hairs sensitive to vibrations

Cochlea


Auditory nerve: Neural pathway connecting the ear and the brain

Deafness

• Conduction deafness: Results from damage to structures of the middle or inner ear

• Nerve deafness: Linked to a deficit in the body’s ability to transmit impulses from the cochlea to the brain– Usually involves the auditory nerve or higher

auditory processing centers

Hearing: The Physics of Sound

• Loudness: produced by the amplitude of a sound wave

High Amplitude Low Amplitude

- Amplitude: Physical strength of a wave

Hearing: The Physics of Sound

• Pitch: produced by the frequency of a sound wave– Frequency: Number of cycles completed by a wave in a

given amount of time

Low Frequency High Frequency

How Sound Waves Become Auditory Sensations

• Pitch:– Place Theory: different places on the basilar membrane

send neural codes for different pitches

– Frequency Theory: neurons have different firing rates for different sound wave frequencies

• Timbre: Quality of a sound wave that derives from the wave’s complexity

• Auditory cortex: Portion of the temporal lobe that processes sounds

Position and Movement

• Vestibular sense: Sense of body orientation with respect to gravity


• Kinesthetic sense: Sense of body position and movement of body parts relative to each other

Smell

• Olfaction: Sense of smell

• Olfactory bulbs: Brain sites of olfactory processing

• Pheromones: Chemical signals released by organisms to communicate with other members of the species

Taste buds: Receptors for taste (primarily on the upper side of the tongue)

Taste

• Gustation: The sense of taste

The Skin Senses and Pain

• Gate-control Theory: An explanation for pain control that proposes we have a neural “gate” that can, under some circumstances, block incoming pain signals.

• Placebos: Substances that appear to be drugs but are not

• Placebo effect: A response to a placebo caused by subjects’ belief that they are taking real drugs

The Machinery of Perceptual Processing

• Percept: Meaningful product of a perception

• Feature detectors: Cells in the cortex that specialize in extracting certain features of a stimulus

• Binding problem: A major unsolved mystery in cognitive psychology, concerning the physical processes used by the brain to combine many aspects of sensation to a single percept

Bottom-Up and Top-Down Processing

• Bottom-up processing: Analysis that begins with the sense receptors and works up to the brain’s integration of sensory information

• http://www.youtube.com/watch?v=x6Ua5d3wlA0 (50)• Top-down processing: Analysis guided by higher-level

mental processes - emphasizes perceiver's expectations, memories, and other cognitive factors


Perceptual Constancies

• Perceptual constancy: Ability to recognize the same object under different conditions, such as changes in illumination, distance, or location

• Shape, color, size

Perceptual Illusions

Do you see or ?

Perceptual Ambiguity and Distortion• Illusions: Distortion of a stimulus pattern,

shared by others in the same perceptual environment– More likely when:

• stimulus is unclear• info is missing• elements combined in unusual ways• familiar patterns aren’t apparent

• Ambiguous figures: Images that are capable of more than one interpretation

The Gestalt Approach• Gestalt psychology: View that much of perception

is shaped by innate factors built into the brain (nature)– The whole pattern is greater than the sum of its parts.

• Figure: Part of a pattern that commands attention

• Ground: Part of a pattern that does not command attention; the background

The Gestalt Approach

• Subjective contours: Boundaries that are perceived but do not appear in the stimulus pattern

• Closure: Tendency to fill in gaps in figures and see incomplete figures as complete

The Gestalt Laws of Perceptual Grouping

SimilaritySimilarity

ProximityProximity

ClosureClosure

ContinuityContinuity

PrägnanzPrägnanz

Common FateCommon Fate

Binocular Cues – two eye depth cues

• Binocular Convergence: lines of vision from each eye converge at different angles on objects at different distances– Can feel eye muscles change as you focus at different

distances

• Retinal Disparity: difference in perspectives of the 2 eyes (greater disparity for nearby objects – provides us with depth information)

Monocular Cues – one eye depth cues

• Monocular Cues:– Linear Perspective: parallel lines appear to meet in the

distance


• Monocular Cues:– Relative Motion: objects closer to you move faster than

those further away from you


• Monocular Cues:– Relative Size: 2 objects the

same size; the one that appears larger = closer to us


• Monocular Cues:– Interposition: hidden objects are more distant than those

objects that hide them


• Monocular Cues:– Texture Gradient: as object gets further away from us,

the texture gets smoother

Theoretical Explanations for Perception• Learning-based inference: View that perception is

primarily shaped by learning, rather than innate factors (nurture); opposite of Gestalt

• What determines how successful we will be in forming an accurate percept?– Context, expectation, perceptual set – each influenced by culture

– Perceptual set: Readiness to detect a particular stimulus in a given context

Cultural Influences on Perception

B

A

Which box is bigger, A or B?

Muller-Lyer Illusion

• When 2 objects make the same size image on the retina, and we judge one to be farther away than the other, we assume that the more distant one is larger.

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