Sensation and Perception

Psychology: A Concise Introduction

2nd Edition

Richard Griggs

Chapter 3

Prepared byJ. W. Taylor V

Windows on the World

We understand the world through our senses, our “windows” on the world

Our reality, in fact, is dependent upon two basic processes: Sensation: Gathering information Perception: Interpreting information

An Example of Misperception

The Journey…

How the Physical World Relates to the Psychological World

How We See and How We Hear

How We Make Sense of What We See

How the Physical World Relates to the Psychological World

How We See and How We Hear

How We Make Sense of What We See

How the Physical World Relates to the

Psychological World

The Detection Question

The Difference Question

The Scaling Question

The Questions

1. The detection question is concerned with the limits on our ability to detect very faint signals How intense does a light have to be

for us to see it?

How intense does a sound have to be for us to hear it?

The Questions

2. The difference question is concerned with limits on our detection abilities, but in this case with our ability to detect very small differences between stimuli What is the smallest difference in

brightness between two lights that we can see?

What is the smallest difference in loudness between two sounds that we can hear?

The Questions

3. The scaling question is concerned with how we perceive the magnitudes (intensities) of clearly detectable stimuli What is the relationship between the actual

physical intensities of stimuli and our psychological perceptions of these intensities?

The Detection Question

Absolute threshold is the minimum amount of energy in a sensory stimulus that is detected 50% of the time

Subliminal stimulus is one that is detected only up to 49% of the time Any effects of subliminal persuasion are short-

lived with no long-term consequences on our behavior

Theoretical and Observed Absolute Thresholds

Four Possible Outcomes in a Signal Detection Study

Present Absent

“Yes” HitFalse Alarm

“No” MissCorrect

RejectionOb

serv

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s

Resp

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Signal

The Difference Question

A difference threshold (also called a just noticeable difference, or jnd) is the minimum difference between two stimuli that is detected 50% of the time

Weber’s Law says that for each type of sensory judgment, the difference threshold is a constant fraction of the standard stimulus value used to measure it

The Scaling Question

Steven’s Power Law states that the perceived magnitude of a stimulus is equal to its actual physical intensity raised to a constant power for each type of judgment For instance, to perceive a light as twice as bright,

its actual intensity has to be increased between and 8 and 9 times

Likewise, if an electric shock is doubled in intensity, we perceive it as being about 10 times more intense

The Scaling Question

Sensory adaptation is the disappearance to repetitive or unchanging stimuli This sensory adaptation has survival value, as

it is more important to detect new stimuli (which may signal danger) than constant stimuli

How We See and How We Hear

How the Eye Works

How We See Color

How the Ear Works

How We Distinguish Pitch

Physical Characteristics of Light and Sound Waves

Wavelength refers to the distance in one cycle of a wave, from one crest to the next With respect to vision, human can

see wavelengths of about 400 to 700 nanometers

Amplitude is the amount of energy in a wave, its intensity, which is the height of the wave at its crest For light waves, amplitude

determines its brightness

A Typical Waveform and its Characteristics

Physical Characteristics of Light and Sound Waves

With respect to auditory stimuli, frequency is the number of times a sound wave cycles in one second, with shorter wavelengths having higher frequencies The frequency determines

the pitch of a sound; that is how high or low the sounds is perceived to be

To understand these physical characteristics, receptor cells must transduce them into neural signals that the brain can use

How the Eye Works

The cornea covers the eye and is the clear covering through which light rays pass

The light rays are further filtered by the pupil through the lens before being passed to the retina at the back of the eye

The lens accommodates the light waves from objects of different distances directly on the retina For nearsighted people, light rays from distant objects are

focused in front of the retina, whereas for farsighted people, light rays from close objects are focused behind the retina

How the Eye Works

The retina is the light-sensitive layer of the eye and has three layers of cells: The ganglion cells are the first layer through which

light rays pass After which light rays pass through the bipolar cells And are finally processed in the receptor cells,

which contain the visual receptor cells rods and cones The approximately 120 million rods are responsible for

seeing in dim light and for peripheral vision The approximately 5 million cones, located in the

center of the retina, called the fovea, are responsible for seeing in bright light and in color

How the Eye Works After being processed in the retina, patterns of

neural impulses describing the visual image are carried through the bipolar cells to the ganglion cells, which bundle together to form the optic nerve Where the optic nerve leaves the eye, there are no

receptor cells, and thus we have a blind spot The optic nerve runs through the thalamus, which acts as

a “relay station” to transmit sensory information to the correct part of the cerebral cortex

Visual information is directed to the occipital lobe, where it is processed

Feature detector cells recognize basic features of the stimulus, which are then coordinated to give it meaning (i.e., to perceive it)

How the Eye Works

How We See Color

The Trichromatic theory contends that there are three types of cones, each activated by a certain wavelength, which corresponds approximately to blue, green, and red

The Opponent-Process theory assumes that there are three types of cell systems that help us see color, and these systems are located at the post-receptor level of processing

The three types of cell systems are red-green and blue-yellow, as well as black-white (to detect brightness)

If one color in a pair is stimulated, the other is inhibited

Subtractive and Additive Mixtures

Demonstration of Complementary Afterimage

Demonstration of Complementary Afterimage

How We See Color

Both theories have validity, each at different levels of visual information processing The Trichromatic theory is

correct in its account of how color information is processed by the cones

The Opponent-Process theory is correct in its account of how color information is processed after it leaves the retina (and is processed by the bipolar, ganglion, and thalamic cells)

How the Ear Works

The outer ear The pinna, which is the external part of the ear,

collects sounds and funnels them through the auditory canal to the tympanic membrane (the eardrum), which marks the boundary between the outer ear and the middle ear

How the Ear Works

The middle ear The malleus, incus, and stapes (also called the

hammer, anvil and stirrup) vibrate in reaction to sound waves from the auditory canal

The stapes’ movement creates vibrations on the oval window, which covers the inner ear

How the Ear Works

The inner ear The cochlea contains in the basilar membrane

about 16,000 hair cells that are the receptor cells for hearing

Fluid in the cochlea is displaced, causing the hair cells to move, in turn causing the sensation of hearing

When these hair cells or auditory nerve fibers are damaged, a person suffers nerve deafness

Conduction deafness is hearing loss due to damage to the mechanical system carrying sound waves to the cochlea

How the Ear Works

How We Distinguish Pitch

Pitch is the quality of a sound perceived as high or low and is determined by the frequency of the sound wave Humans can perceive sound wave frequencies

from about 20 to 20,000 Hertz Place theory contends that there is a

specific place along the basilar membrane in the inner ear that will correspond to a particular frequency.

Frequency theory contends that the frequency of a sound wave is mimicked by the firing rate of the hair cells across the entire basilar membrane

How We Distinguish Pitch

Both theories have validity According to the volley principle, Frequency theory

explains our perception of sound up to about 5000 Hz. Because 5000 times per second is the upper limit for the

firing rate using the volley principle, Frequency theory would not be able to explain how we perceive higher frequencies

Hence, Frequency theory explains the perceptions of lower pitched sounds (< 500 Hz) and Place theory explains how we perceive higher frequencies (> 5000 Hz)

For frequencies between 500 and 5000 Hz, both theories are correct, and hence we have better pitch perception in this range

How We Make Sense of What We See

Bottom-up Processing and Top-down Processing

Perceptual Organization and Perceptual Constancy

Depth Perception

Bottom-up Processing and Top-down Processing

Bottom-up processing is the processing of sensory information as it enters the sensory structures and travels to the brain

Top-down processing is the brain’s use of existing knowledge, beliefs, and expectations to interpret the sensory stimulation Perception is subjective because of top-down processing Perceptual set occurs when we interpret an ambiguous

stimulus in accordance with our past experiences A contextual effect occurs when we use the present

context of sensory input to determine its meaning

Perceptual Organization and Top-down Processing

A Context Effect on Perception

Perceptual Organization

Gestalt means “organized whole” Gestalt psychologists believe that the organized

whole is greater than the sum of its individual pieces of sensory information

The figure-and-ground principle states that the brain organizes sensory input into a figure (the center of attention) and a ground (the background)

Closure is the tendency to complete (i.e., close) incomplete figures to form meaningful objects

Subjective contours are lines or shapes that are perceived to be present but do not really exist

An Example of Figure-Ground Ambiguity

An Example of an Organizational Perceptual Ambiguity

An Example of a Subjective Contour

Perceptual Constancy

Refers to the perceptual stability of Size Shape Brightness Color

For familiar objects seen at Varying distances Different angles Different lighting conditions

Depth Perception

Involves judging the distance of objects from us

Binocular depth cues require the use of both eyes Retinal disparity refers to the fact that as the disparity

between the two retinal images decreases, the distance from us increases (and vice versa)

Monocular depth cues require only one eye Linear perspective refers to the fact that as parallel lines

recede away from us, they appear to converge Interposition refers to the fact that if one object blocks our

view of another, we perceive the blocking object as closer

Visual Illusions

In the Ponzo illusion, two horizontal lines are equal in length, but one appears longer than the other

The convergence of the two lines (i.e., linear perspective) outside the horizontal lines normally indicates increasing distance

Visual Illusions

In the Müller-Lyer illusion, two vertical line segments are equal in length, but the one with arrow feather endings appears to be longer

The line with arrow feather endings has the appearance of a corner that is receding away from you (the corners where two walls meet in a room), while the line with arrowhead endings has the appearance of a corner that is jutting out toward you (the corners where two sides of a building meet)

Thus, it is our past experience with corners that leads the brain to believe that the line with arrow feather endings is farther away