4. sensation & perception

SENSATION & PERCEPTION

Sensation & Perception Defined

• Sensation – stimulus detection process by which our sense organs respond to and translate environmental stimuli into nerve impulses that are sent to the brain

• Perception – the active process of organizing this stimulus input and giving it meaning

Stages of Sensory Processing

1. Sensation is received by sensory receptors.2. Receptors translate stimulus properties into nerve

impulses (transduction).3. Feature detectors analyse stimulus features.4. Stimulus features are reconstructed into neural

representation.5. Neural representation is compared with previously

stored information in brain.6. Matching process results in recognition and

interpretation of stimulus.

SENSORY PROCESSES

Psychophysics – scientific study of physical characteristics of stimuli and sensory capabilities

Stimulus Detection: The Absolute Threshold

• The lowest intensity at which a stimulus can be detected 50% of the time

• < Absolute threshold > sensitivity

Approximate Absolute Threshold for YOU

Sensory Modality Absolute Threshold

Vision Candle flame seen at 30 mi (48.2 km) on a clear, dark night

Hearing Tick of a watch under quiet conditions at 20 ft (6 m)

Taste 1 teaspoon of sugar in 2 gal (3.7 L) of water

Smell 1 drop of perfume diffused into the entire volume of a large apartment

Touch Wing of a fly or bee falling on a person’s cheek from a distance of 1 cm

Signal Detection Theory

Signal Detection Theory

• Before: “each person has a more or less fixed level of sensitivity for each sense”

• Now: fixed AT is inaccurate• Why?• Range of uncertainty (decision criterion) – a

standard of how certain people must be that a stimulus is present before they will say they detect it

Signal Detection Matrix

Conclusions in SDT

1. At low stimulus intensities, both the P’s and situation’s characteristics influence the decision criteria

2. Bold Ps (people who frequently say “yes”): hits = false alarms than conservative Ps

3. Rs can influence Ps to become bolder or more conservative by manipulating the rewards & costs for giving correct or incorrect responses

4. Perception is, in part, a decision.

Subliminal Stimuli: Can They Affect Behavior?

• Subliminal stimulus – stimulus that is so weak or brief that although it is received by the senses, it cannot be perceived consciously

• YES, to a limited extent.

The Coca-Cola-popcorn-subliminal-message controversy

• James Vicary (late 50s) – arranged to have subliminal messages flashed on a theater screen during a movie

• Popcorn sales – 50%

• Coke sales – 18%

FALSE

• Influence to consumer behavior: persuasive stimuli the perceptual threshold > subliminal stimuli sneaking into the subconscious mind

BUT(T)!

• They affect more subtle phenomena (i.e., judgment, attitudes & behaviors)

The Difference Threshold

• The smallest difference between two stimuli that people can perceive 50% of the time (aka just noticeable difference or jnd)

• Weber’s law – the difference threshold/jnd is directly proportional to the magnitude of stimulus with which the comparison is being made

Weber Fractions for Various Sensory Modalities

Sensory Modality Weber Fraction

Audition (tonal pitch) 1/333

Vision (brightness, white light) 1/60

Kinesthesis (lifted weights) 1/50

Pain (heat produced) 1/30

Audition (loudness) 1/20

Touch (pressure applied to skin) 1/7

Smell (India rubber) ¼

Taste (salt concentration) 1/3

Sensory Adaptation

• The diminishing sensitivity to an unchanging stimulus (aka habituation)

• Reduces overall sensitivity

• Frees our senses from the constant and mundane (important to our well-being & survival)

THE SENSORY SYSTEMS

Transduction

• The process by whereby the characteristics of a stimulus are converted into nerve impulses (electrical by nature)

• Electromagnetic energy/light waves – normal stimulus for vision

Vision

The Human Eye

Photoreceptors: The Rods & Cones

Facts about Photoreceptors

• Rods & cones bipolar cells 1 million ganglion cells (axons form the optic nerve)

• Light-sensitive ends point away from the direction of the entering light

• The connection of rods & cones to bipolar cells account for the importance of rods in dim light & visual acuity caused by cones

Facts about Photoreceptors

• Many rods & cones single/same bipolar cells: additive effect of many signals

• Cones in the fovea have “private lines” to single/same bipolar cell: visual acuity – ability to see fine details

• Blind spot

Visual Transduction: From Light Waves to Nerve Impulses

• Photopigment – protein molecules in the rods & cones that help them translate light waves into nerve impulse

• Absorption of light by the photopigment chemical reaction Δ in the rate of neurotransmitter release signal passed on to the bipolar cells ganglion cells optic nerve (bundle of ganglion cells’ axons) visual relay station (thalamus) visual cortex (cerebrum)

Brightness & Dark Adaptation

• Rods & cones sensitivity in low illumination– Rods: sensitive throughout the spectrum except

for red– Cones: sensitive in the greenish-yellow range– Change of fire engines from red to greenish-yellow– Rods are not always ready to fulfill its function

Brightness & Dark Adaptation

• Dark adaptation – the progressive improvement in brightness sensitivity that occurs over time under conditions of low illumination– Photopigments of rods vs photopigments of cones– Used for enhancing the night vision of pilot during

the WW II

Color Vision

• Young-Helmholtz trichromatic theory – assumes that there are three types of color receptors in the retina– Cones sensitive to blue, green and/or red (if all are

activated white light)– Criticisms: perception of yellow & color afterimage

Retinal receptor

Brain

Kinds of color mixture

Color Vision

• Hering’s opponent-process theory – proposes that each of the three cone types respond to two different wavelengths– Blue-yellow cones, red-green cones & black-white

cones

Opponent Process: Afterimage

Color Vision

• Dual-process theory – combines trichromatic & opponent-process theories to account for the color transduction process– Trichromatic theory: cones contain one of the 3

different photopigments sensitive to blue, green & red

– Ganglion cells, neurons in visual relay station & the visual cortex are responsible for opponent-process, not the cones

Opponent-process Mechanisms

Color-Deficient Vision

• Trichromats• Dichromats – color

blind in only one of the systems (blue-yellow or red-green)

• Monochromats – sensitive only to the black-white color system (total color-blindness)

Audition

• Sound wave – a form of mechanical energy & the stimulus for the sense of hearing

• Sound – pressure waves in air, water or some other conducting medium vibration successive waves of compression & expansion among air molecules

Characteristics of Sound Waves

• Frequency – the number of sound waves, or cycles, per second; hertz (Hz) – the technical measure of cycle per second; related to perceived pitch

• Amplitude – the vertical size of the sound waves, amount of compression & expansion of the molecules in the conducting medium; decibels (dB) – a measure of the physical pressures that occur at the eardrum; related to perceived loudness

Auditory Transduction: From Pressure Waves to Nerve Impulse

The Cochlea

Coding of Pitch & Loudness

• Loudness is coded in terms of both the rate of firing in the axon (auditory nerve) & the specific hair cells that are sending messages

Coding of Pitch & Loudness

• Coding of wave frequency (pitch)– Frequency theory of pitch perception – assumes

that nerve impulses sent to the brain match the frequency of the sound wave

– Place theory of pitch perception – suggests that the specific point in the cochlea where the fluid wave peaks & most strongly bends the hair cells serves as a frequency coding cue

Sound Localization

• Humans’ binaural (two-eared) ability to localize sounds

Hearing Loss

• 2 major types of hearing loss (U.S.)– Conduction deafness – involves problems w/ the

mechanical system that transmits sound waves to the cochlea

– Nerve deafness – caused by damage receptors within the inner ear or damage to the auditory nerve itself

Taste & Smell: The Chemical Senses

• Gustation – the sense of taste• Olfaction – the sense of smell

– Their receptors are sensitive to chemical molecules rather than to some form of energy

Gustation: The Sense of Taste

Taste buds – chemical receptors concentrated along the tip, edges & back surface of the tongue

Olfaction: The Sense of Smell

Pheromones – chemical signals found in natural body scents

Menstrual synchrony – the tendency of women who live together or are close friends to become more similar in their menstrual cycle

The Skin & Body Senses

The Tactile Senses

• Four tactile sensations: pressure (touch), pain, warmth & cold

• Mechanoreceptors– Merkel receptors – sensing fine details– Meissner corpuscle – controlling handgrip– Ruffini cylinder – perceiving stretching of the skin– Pacinian corpuscle – sensing rapid vibration & fine

texture

The Body Senses

• Kinesthesis – provides us with feedback about our muscles’ and joints’ positions & movements– Receptors: nerve endings in the muscles, tendons

& joints

• Vestibular sense – sense of body orientation & equilibrium