Chapter 3
Sensation and Perception


Sensation
– process of detecting external stimuli and changing those stimuli into nervous system activity

Perception
– cognitive process that involves the selection, organization, and interpretation of stimuli


Sensory threshold
– minimum intensity of a stimulus that will cause the sense organs to operate

Psychophysics
– study of relationships between the physical attributes of stimuli and psychological experiences they produce

Figure 3.1: Examples of absolute threshold values for the five senses (i.e., these stimuli will be detected 50 percent of the time).


Absolute threshold
– physical intensity of a stimulus that a person reports detecting
50% of the time
 Used to see whether a person’s senses are operating properly

•
Signal detection theory
– states that stimulus detection is a decision-making process of determining whether a signal exists against a background of noise


Difference threshold
– smallest difference between stimulus attributes that can be detected

Just noticeable difference (jnd)
– amount of change in a stimulus that makes it just noticeably different from what it was


Occurs when our sensory experience decreases with continued exposure to a stimulus

Dark adaptation
– process in which the visual receptors become more sensitive to light as we spend time in the dark

Light adaptation
– process by which our eyes become more sensitive to dark when we spend time in the light

Figure 3.8: The dark adaptation curve.


Light
– wave of electromagnetic energy

Wave amplitude
– intensity or brightness of light

Wavelength
– distance between any point in a wave and the corresponding point on next cycle (e.g., peak to peak), measured in nanometers (nm)

Determines the color or hue we perceive

Wave purity – refers to characteristic of saturation

Figure 3.2: Representations of light waves differing in wavelength and wave amplitude.

Figure 3.3: The visible spectrum, in which wavelengths of approximately 380-760 nanometers are visible to the human eye and are perceived as various hues.

Figure 3.4: Relationships between physical characteristics of light and our psychological experience of that light.

Figure 3.5: The major structures of the human eye.


Cornea
– outer shell of eye

Protects structures at front of eye

First point where light rays are bent

Pupil
– opening through which light enters eye

Iris
– colored part of the eye that expands or contracts, depending on light intensity

Ciliary muscles
– expand or contract to change shape of the lens to bring image into focus ( accommodation )

Figure 3.6: The major features of the human retina.


The eye is filled with two fluids:
1. Aqueous humor
– provides nourishment to the cornea and other structures at the front of the eye
2. Vitreous humor – fills the interior of the eye, behind the lens, where it functions to keep the eyeball spherical


Begins to take place at the retina, where light energy is transduced to neural energy
Rods
Photosensitive cells that are most active in low levels of illumination and do not respond differently to different wavelengths of light
Cones
Photosensitive cells that operate best at high levels of illumination and are responsible for color vision


Optic Nerve
– formed of fibers from ganglion cells; leaves the eye and starts back toward other parts of the brain

Fovea
– small area of retina with the best visual acuity. It is packed with cones cells
(no rods!).

Blind spot
– where nerve impulses from rods and cones leave the eye

Figure 3.7: This figure provides two ways to locate your blind spot.


Left visual field
– everything off to your left ends up in right occipital lobe

Right visual field
– everything off to your right ends up in left occipital lobe

Optic chiasma
– sorting of which fibers of the optic nerve get directed where largely occurs here

Figure 3.9: Cross Laterality.


Trichromatic theory
– First proposed by
Thomas Young and revised by Herman von
Helmholtz

The eye contains 3 distinct receptors for color

Each responds best to one of 3 primary colors of light: red, blue, and green

By the careful combination of all 3, all other colors can be produced

Figure 3.10: The relative sensitivities of three types of cones to lights of differing wavelengths.


Opponent-process theory
– Ewald Hering proposed this theory in 1870

Three pairs of visual mechanisms that respond to different wavelengths of light
 Blue-yellow processor

Red-green processor

Black-white difference/brightness processor

Each is capable of responding to either of the two hues that give it its name, but not both


In dichromatism, there is a lack of one type of cone (supporting Young-Helmholtz’s theory)

However, color vision defects higher in the visual pathway support the opponentprocess theory

Both theories are probably correct, each in its own way


Reliable, stable differences in color preferences:
1. Women prefer “cool colors,” while men prefer bright, strong colors
2. Women are more likely to have a favorite color
3. Women can name more colors
4. Color matters more to women.


Sound
– series of pressures of air (or some other medium) beating against the ear

Amplitude
– intensity that determines the psychological experience we call loudness
 Zero point on decibel scale (perceived loudness) is lowest intensity of sound that can be detected – absolute threshold

Figure 3.11: Sound waves are manifested as changes in air pressure are produced as the tines of the tuning fork vibrate back and forth.


Frequency
– number of waves exerted for every second of Unit of sound is called hertz (Hz) {20-20,000 Hz}

Pitch
– how high or low a tone is
(determined by wavelength)

Purity
– timbre is character of sound that reflects degree of purity

White noise is a random mixture of sound frequencies

Figure 3.12: Loudness values in decibel units for various sounds.

Figure 3.13: A summary of the ways in which the physical characteristics of light and sound waves affect our psychological experiences of vision and hearing.


Cochlea
– major structure of inner ear

Receptor cells (transducers for hearing) are here

When fluid inside cochlea moves, basiliar membrane is bent up & down, which stimulates receptors ( hair cells )

Neural impulses travel on auditory nerve toward temporal lobe

Figure 3.14: The major structures of the human ear.


Taste = gustation

Four psychological qualities: sweet, salty, sour, and bitter

Taste buds
– receptor cells for taste on tongue

We have about 10,000 taste buds

Figure 3.15: Enlarged view of a taste bud, the receptor for gustation.


Smell = Olfaction

Pheromones
– chemicals that many animals emit that produce distinctive odors that are used as a method of communication between organisms

VNO (vomeronasal organ)
– primary organ used in detection of pheromones. Involved in mating, territoriality, and aggressiveness in animals.

Figure 3.16: The olfactory system, showing its proximity to the brain and transducers for smell — the hair cells.


A square inch of skin contains nearly 20 million cells

Some skin receptor cells have free nerve endings, while others have encapsulated nerve endings

Our ability to discriminate among types of cutaneous sensation is due to a unique combination of responses the receptor cells have to various types of stimulation

Figure 3.17: A patch of hairy skin, showing the layers of skin and several nerve cells.

Figure 3.18: A demonstration that our sense of what is hot can be constructed from sensations of what is warm and cold.


Vestibular Sense
 Tells us about balance, where we are in relation to gravity and about acceleration or deceleration

Receptors are located on either side of the head, near the inner ear (5 chambers)

Over-stimulation may result in motion sickness
 Kinesthetic sense
 Tells us about the position of various parts of our bodies and what our muscles and joints are doing

Receptors are located primarily in our joints, but some information comes from muscles and tendons
 Information from these receptors travels via the spinal cord

They provide examples of reflex reactions

Theories of pain:
1.
Gate control mechanism (high in spinal cord) that opens to let pain messages race to brain or closes to block messages
2.
Cognitive behavioral – pain is influenced by attitudes, expectations and behaviors


Drug therapy

Hypnosis & cognitive self-control

Acupuncture

Placebo
– a substance a person thinks will be helpful in treatment

Counterirritation
– stimulating an area of the body near the location of the pain


Salient detail
– one that captures our attention

Remembered better than peripheral details
(which are part of the perceptual background)

Stimulus factors make some details more compelling than others

Personal factors
– characteristics of a perceiver that influence which stimuli get attended to


Contrast
– extent to which a stimulus is physically different from the other stimuli around it

Most important factor in perceptual selectivity

The more intense a stimulus is, the more likely we are to attend to it

Motion is another dimension for which contrast is important

Repetition can also influence attention


Bottom-Up
Processing

Attend to a stimulus, organize and identify it, and then store it in memory

Top-Down
Processing

Motivation, mental set, and past experience influence perceptual sensitivity

Figure 3.19: How we perceive the world is determined at least in part by our mental set, or our expectations about the world.


A gestalt forms when one sees the overall scheme of things: the whole, totality or configuration.

Gestalt Psychology
– basic principle is figure-ground relationship

Of all the stimuli in your environment, those you attend to and group together are “figures”
 All other stimuli become “ground”

Figure 3.20: (A) A classic reversible figure-ground pattern.

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Proximity

Similarity

Continuity

Common fate

Closure

Figure 3.21: Four Gestalt psychology examples of grouping.

Figure 3.22: An example of subjective contour.


Perceiving stimuli because we want to, expect to, or have experienced them together in the past

How we ultimately organize our experiences depends on both types of processing

Figure 3.23: An example of top-down processing.


Ocular cues are built into our visual system and tell us about depth and distance

Retinal disparity
– each eye gets a somewhat different view of a 3-dimensional object

Convergence
– eyes turning in, toward each other, when something is viewed up close

Figure 3.24: When looking at a three-dimensional object, such as a pen, the right eye sees a slightly different image than does the left eye — a phenomenon called retinal disparity.


Physical cues to depth and distance are those we get from the structure of our environment

Linear Perspective

Interposition

Relative Size

Texture gradient

Patterns of Shading

Motion Parallax

Figure 3.25: At the level of the retina, we experience different images; yet we know we are looking at the same door because of shape constancy.


Perceptual constancies help us organize and interpret the stimulus input we get from our senses. They allow us to see stimuli as constant, regardless of changing conditions.

Size constancy

Shape constancy

Brightness constancy

Color constancy


Illusions
– experiences in which our perceptions are at odds with what we know as physical reality

Illusions remind us that perception is a higher level process than sensation !

Figure 3.26: A few classic geometrical illusions.

Figure 3.27: Impossible figures — examples of conflicting visual information.

Figure 3.29:
Müller-Lyer illusion.


Yes! There is a role of culture in the development of depth perception.

However, with training, most cultural differences in the perception of depth disappear.

Figure 3.28: Which animal — the antelope or the elephant — is the hunter about to spear?