Sensation and Perception

True or False? • Advertisers are able to shape our buying habits through subliminal

messages.• If we stare at a green square for a while and then look at a white sheet

of paper, we see red. • Touching adjacent cold and pressure spots triggers a sense of wetness.• People who are born without the ability to feel pain may die by early

adulthood. • Without their smells, a cold cup of coffee may be hard to distinguish

from a glass of red wine. • Infants just learning to crawl do not perceive depth • Persons who have sight in only one eye are totally unable to gauge

distances. • A person who is born blind but gains sight as an adult cannot recognize

common shapes and forms. • If required to look through a pair of glasses that turns the world upside

down, we soon adapt and coordinate our movements without difficulty.• Laboratory evidence indicates that some people do have ESP.

Sensation and Perception

• Transformation of stimulus energy into a meaningful understanding– Each sense converts energy into

awareness

Sensation

• Detection and encoding of physical stimuli into neural signals– Occurs at sensory receptors– External stimuli correspond to sensory

modalities– Light waves: ___________– Sound waves : ___________– Pressure, warmth, cold, pain : ___________– Chemical messengers : _________&__________– Body position and movement : _________&_________

Perception

• Organization of sensory information into cognitive awareness environmental stimuli– Occurs in the cerebral cortex

Processing stimuli

• Bottom-up processing– Scientific explanation– Begins with receptors and works up to

integration

• Top-down processing– Understanding stimuli based on prior

experience and expectations– The brain will rapidly interpret stimuli based on

their “most likely” explanation

Thresholds of sensation

• Absolute threshold– Minimum stimulation

needed to detect a

stimulus 50% of the time• Ex. Whisper

• Difference threshold– Minimum difference between two stimuli that

can be detected 50% of the time• Ex. Change in room temperature

Subliminal sensations

• Sensations not strong enough to be perceived– May be processed sometimes– May result in understanding without conscious

awareness

• Useful persuasion method?

Sensory adaptation

• Diminished sensitivity to a continuous stimulus– Allows for focus on relevant stimuli

Vision

• Stimulus: visible light– Wavelength– Intensity

Vision

• Receptive organ: the eye

Photoreceptors

• Rods

• Cones

Visual pathway

• Optic nerve– Exits retina

• Optic chiasm

• Thalamus

• Primary visual cortex

Processing visual stimuli

• Feature detectors in the primary visual cortex respond to specific features in parallel– Form– Movement– Depth– Color

Perception of visual stimuli

• Perception occurs in visual association areas in the occipital, parietal and temporal lobes

Color vision

• Young-Helmholtz trichromatic theory– Red, green, blue color receptors in the retina– Stimulation of one or many results in color sensation

• Opponent-process theory– Colors are analyzed in terms of opponent colors

• Red vs. Green• Yellow vs. Blue• Black vs. White

– One color turns some cells “on” and other cells “off”

Psychology of visual perception

• Top-down processing means that what we see is influenced by biological, psychological and socio-cultural factors

Perceptual organization

• How do we form meaningful perceptions from sensory information?

Gestalt psychology

• Branch of cognitive psychology

• Organization of many sensations into perceptions of wholes

• Based on experience and expectations

• Perceived whole is not always the same as its parts!

Form perception

• Simplification into easily interpretable wholes

• Figure-ground

Form perception

• Grouping principles– Proximity– Similarity– Continuity– Connectedness– Closure

Depth perception

• Distance is perceived with vision and hearing

• Visual depth perception– Binocular cues– Monocular cues

Binocular depth cues

• Retinal disparity– Strongest visual depth cue

Monocular depth cues

• Light and shadow• Relative size and position• Relative height/vertical position• Linear perspective

Auditory location cues

• Intensity and pitch

• Arrival times at each ear

• Clarity

Perceptual constancy

• Cognitive functions that maintain the features of an object, despite changing illumination, color, size, or shape– Based on comparisons between the figure

and ground

Color and lightness constancy

• Consistent color and light intensity, despite changes in illumination

Shape and size constancy

• Familiar objects are perceived as unchanging despite changes in retinal images.

Perceptual interpretation

• Making sense of the perceptions produced by the cortex– Genetics– Experience

• Critical periods• Plasticity and adaptation

Perceptual set

• Psychological predisposition to perceive stimuli in a particular way– Shaped by learned assumptions and beliefs– Affects how we interpret sensory stimuli

• Examples

Other sensory modalities

Hearing

• Stimulus - sound waves– Frequency– Amplitude

The ear

Auditory stimuli

• Bending of hair cells in the cochlea transduces vibrations into neural signals

• Auditory nerve

• Primary auditory cortex

• Auditory association cortex

Touch

• Stimulus - pressure, pain, warmth, cold– Receptors– Other sensations

• Stimuli organized in primary somatosensory cortex

• Perceptions created in somatosensory association cortex

Pain

• Critical alert system

• Subjective– Physiology– Prior experiences– Attention– Context– Culture

Pain

• Gate-control theory

• Pain control/management

Taste

• Stimulus - chemical molecules that impart the sensations of sweet, sour, salty, bitter and umami

• Tastebuds contain taste and touch receptors

Taste perception

• Flavor – Based on taste, olfactory, and touch stimuli

• Begins in brainstem

• Completed in the limbic system

Taste preferences

• Genetic predisposition

• Biological predisposition

• Learned responses

Smell

• Stimulus - chemical molecules

• Receptors in olfactory epithelium– Axons project directly to the olfactory bulb of

the brain– Perception begins in the olfactory bulb,

completed in the limbic system

Kinesthesis & vestibular sense

• Kinesthesis - sense of body position and movement

• Vestibular sense - sense of head postion and movement

• Stimulus - gravity and movement

• Receptors found in muscles (body) and inner ear (head)

Kinesthesis & vestibular sense

• Sensory signals about position and movement are organized in the medulla and cerebellum

• Perception occurs throughout the brain– Brain stem– Temporal cortex