Unit 6Module 12

Sensation & Perception

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Sensation & Perception

How do we construct our representations of the external world?

To represent the world, we must detect physical energy (a stimulus) from the

environment and convert it into neural signals. This is a process called sensation.

When we select, organize, and interpret our sensations, the process is called perception.

Sensation and Perception: The Distinction

Sensation: stimulation of sense organs…for example absorbing energy from light by the eyes.

• Each sense organ contains specialized cells called receptors which detect and then convert light waves, sound waves, chemical molecules, and pressure into neural impulses that are transmitted to the brain.

Perception: selection, organization, and interpretation of sensory input…translating the sensory input into something meaningful.

• For example, from a sensory point of view, the American flag is a mass of red, white, and blue colors and horizontal and vertical lines. Perception is the process by which you interpret these splotches of color and array of lines as the American flag.

Psychophysics = the study of how physical stimuli are translated into psychological experience, , thus psychologists in this area are interested mainly in sensation and perception.

PsychophysicsA study of the relationship between

physical characteristics of stimuli and our psychological experience with them.

Physical WorldPsychological

World

Light Brightness

Sound Volume

Pressure Weight

Sugar Sweet

Figure 4.1 The distinction between sensation and perception

Bottom-up Processing

Analysis of the stimulus begins with the sense receptors and works up to the level

of the brain and mind.

Letter “A” is really a black blotch broken down into features by the brain that we perceive as an

“A.”

Top-Down Processing

Information processing guided by higher-level mental processes as we

construct perceptions, drawing on our experience and expectations.

THE CHT

Figure 4.23 Bottom-up versus top-down processing

Our sensory and perceptual processes work together to help us sort out

complex images.

Making Sense of Complexity

“The Forest Has Eyes,” Bev Doolittle

Transduction The process by which sensory receptors convert

the incoming physical energy of stimuli such as light waves into neural impulses that the brain can understand.

As noted by psychologist Philip Zimbardo, the transduction “process seems so immediate and direct that it fools us into assuming that the sensation of redness is characteristic of a tomato or the sensation of cold is characteristic of ice cream.” In reality, sensations such as “red” and “cold” occur only when the neural impulses reach the brain.

Basic Principles of Sensation

Sensing the World

Senses are nature’s gift that suit an organism’s needs.

A frog feeds on flying insects; a male silkworm moth is sensitive to female sex-attractant odor; and we as human beings are sensitive to sound frequencies that

represent the range of human voice.

Exploring the Senses

What stimuli cross our threshold for conscious awareness?

ThresholdsAbsolute Threshold: Minimum stimulation

needed to detect a particular stimulus 50% of the time.

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0 5 10 15 20 25 Stimulus Intensity (lumens)

Sense Absolute Threshold

Vision Candle flame thirty miles away on a clear, dark night

Hearing The tick of a watch at twenty feet

Smell One drop of perfume throughout a three-room apartment

Taste One teaspoon of sugar in two gallons of water

Touch A bee’s wing falling on your cheek from a height of about half an inch

ThresholdsSignal Detection

Signal-detection theory•Ratio of “hits” to “false alarms”

Signal detection theory

= a theory predicting how and when we detect the presence of a faint stimulus (signal) amid background stimulation (noise). Assumes there is no absolute threshold and that detection depends partly on a person’s experience, expectations, motivation, and altertness.

Subliminal ThresholdSubliminal

Threshold: When stimuli are below one’s absolute threshold for conscious awareness.

Kurt Scholz/ Superstock

Just noticeable difference (JND): smallest difference detectable

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Weber’s LawTwo stimuli must differ by a

constant minimum percentage (rather than a constant amount), to

be perceived as different.

Weber fraction: k = dI/I.

Stimulus Constant (k)

Light 8%

Weight 2%

Tone 3%

The principle that, to be perceived as different, two stimuli must differ by a constant percentage (rather than a constant amount).Example: A weight lifter who is bench pressing 50 pounds would notice the addition of a 5-pound weight. However, the same weight lifter would not notice the extra 5 pounds if he were bench pressing 500 pounds.

Weber’s Law

Sensory adaptation occurs when a constant stimulus is presented for a length of time. When this happens receptors fire less frequently and the sensation often fades or disappears.• Examples:When a jogger first puts on a new pair of running shoes, he/she immediately notices that the new shoes have a different feel from the old shoes. After going on a jog, he/she no longer notices the new shoes.

When a swimmer first dives into a pool, he/she immediately notices the water is chilly. After swimming a few laps, he/she no longer notices the water temperature.

Sensory Adaptation

Interesting Note:

• Sensory adaptation does not affect our vision. The reason is because our eyes constantly shift from one location to another. This ensures that receptor cells in the eyes always receive continuously changing stimuli.

Sensory Adaptation

The focusing of conscious awareness on a particular stimulus.

•Cocktail Party Effect: Your ability to attend to only one voice among many.

Selective Attention

Selective Attention

Selective AttentionIn one driving-simulation experiment, students whose attention was diverted by cell-phone conversation (rather than merely listening to a radio) missed twice as many traffic signals as did those not talking on the phone.

Inattentional Blindness

Video Clip

Selective AttentionSelective Inattention

Change blindness•Change deafness•Choice blindness•Pop-out