Module 18 notes (chapter 6)

Module 18: Basic Concepts of Sensation and Perception — In-Depth Notes

Big Picture

Sensation and perception explain how the brain takes physical information from the outside world and turns it into meaningful conscious experiences.

The brain itself does not directly see light, hear sounds, smell odors, or feel objects. Instead, sensory receptors collect information from the environment and convert it into neural signals that the brain can understand.

A useful way to remember the overall process is:

Environmental stimulus → sensory receptors → transduction → neural signals → brain → perception

LOQ 18-1: What are sensation and perception? What are bottom-up and top-down processing?

Sensation

Sensation is the process by which our sensory receptors and nervous system receive and represent stimulus energies from the environment.

Examples:

• Eyes detect light.

• Ears detect sound waves.

• Skin detects pressure, temperature, and pain.

• Nose detects chemicals associated with smell.

• Tongue detects chemicals associated with taste.

Think of sensation as detecting or receiving information.

Perception

Perception is the process of organizing and interpreting sensory information, allowing us to recognize meaningful objects and events.

For example, your eyes may detect:

• colors,

• lines,

• shapes,

• movement.

Your brain organizes this information and allows you to perceive, “That is a person walking toward me.”

Sensation vs. Perception

Face Blindness Example

Indiana Adams has normal vision but experiences face blindness.

Her sensation works normally because her eyes receive visual information and transmit it to her brain.

Her perception is impaired specifically for faces because her brain has difficulty recognizing who a person is from their face.

She may compensate by recognizing:

• someone’s hair,

• voice,

• body shape,

• walking style or gait.

This demonstrates that seeing something and recognizing what you see are not exactly the same process.

Bottom-Up Processing

Bottom-up processing begins with the sensory receptors and works upward toward the brain’s integration of sensory information.

The brain starts with the individual pieces of sensory information and builds them into a complete perception.

For example, when looking at a dog:

Lines + colors + shapes + movement → organized by brain → DOG

Bottom-up processing is especially important when encountering something new or unfamiliar.

Memory Trick

Bottom-up = Building up

You start with basic sensory information and build toward meaning.

Top-Down Processing

Top-down processing is information processing guided by our higher-level mental processes.

Our brain uses things such as:

• previous experiences,

• knowledge,

• expectations,

• context,

• assumptions,

• emotions

to interpret sensory information.

For example, if handwriting is difficult to read, the words around it may help you figure out what the unclear word says.

Memory Trick

Top-down = Thinking down

Your brain’s existing knowledge influences how you interpret incoming information.

Important Point

Normally, bottom-up and top-down processing work together.

Our perceived world is essentially the brain’s interpretation of incoming sensory information.

LOQ 18-2: What three steps are basic to all sensory systems?

Every sensory system performs three basic steps:

1. Receive

Sensory receptors receive sensory stimulation from the environment.

Examples:

• eyes → light

• ears → sound waves

• nose → chemicals

• skin → pressure/temperature

2. Transform

The sensory system transforms the stimulation into neural impulses.

This process is called transduction.

3. Deliver

The sensory system delivers the neural information to the brain, where it can be processed and interpreted.

Easy Formula

Receive → Transform → Deliver

Transduction

Transduction is the process of converting one form of energy into another form that the brain can use.

You can think of transduction as translation.

The physical world has to be translated into the brain’s electrochemical language.

For example:

Light energy → sensory receptors → neural impulses → brain

Without transduction, the brain would not be able to process environmental stimulation.

Psychophysics

Psychophysics studies the relationship between:

physical characteristics of stimuli

and

our psychological experiences of those stimuli.

Researchers might study questions such as:

• How quiet can a sound be before we cannot hear it?

• How dim can a light become before we cannot see it?

• How much heavier must an object become before we notice?

• How strong must an odor become before we detect it?

LOQ 18-3: Absolute Thresholds vs. Difference Thresholds

Humans can detect only a small portion of the physical energy surrounding us.

For example, we cannot normally detect:

• X-rays,

• radio waves,

• ultraviolet light,

• infrared light,

• extremely high-frequency sounds,

• extremely low-frequency sounds.

Other animals can detect stimuli humans cannot.

Examples:

• Migrating birds can use Earth’s magnetic field.

• Bats and dolphins use sonar.

• Bees can detect forms of light that humans cannot see.

This demonstrates that our sensory experience is not a complete representation of everything that exists around us.

Absolute Threshold

An absolute threshold is the minimum stimulation needed to detect a particular stimulus 50% of the time.

The important number is:

50%

For example, during a hearing test, increasingly quiet tones may be played.

Your absolute threshold is approximately the intensity at which:

You hear it 50% of the time and don’t hear it 50% of the time.

German scientist and philosopher Gustav Fechner studied these thresholds.

Humans can be remarkably sensitive. Under ideal circumstances, people may detect extremely faint:

• lights,

• sounds,

• touches,

• smells.

Signal Detection Theory

Our ability to detect a stimulus does not depend only on how strong the stimulus is.

Signal detection theory predicts how and when we detect the presence of a faint stimulus, or signal, among other background stimulation, or noise.

Detection can be influenced by:

• experience,

• expectations,

• motivation,

• alertness,

• psychological state.

Example: Mammograms

A health professional examining a mammogram is trying to detect a faint signal of cancer among other visual information.

There are two important possibilities:

Hit: correctly identifying the signal.

False alarm: believing a signal is present when it actually isn’t.

Signal detection theory helps explain why different people may respond differently to the exact same stimulus.

It also explains why the same person might respond differently depending on the situation.

Subliminal Stimulation

Subliminal stimuli are stimuli that fall below our absolute threshold for conscious awareness.

In other words, you may not consciously realize that you saw or heard something.

However, your brain may still process some information from it.

Important Distinction

Subliminal ≠ completely ignored by the brain.

The brain may react to information even when the conscious mind does not recognize it.

LOQ 18-4: How are we affected by subliminal stimulation?

Research shows that stimuli presented too quickly or weakly to be consciously perceived can sometimes influence our brain activity, attention, or later responses.

For example, researchers can present an image so briefly that participants cannot consciously identify it, while measurements still show that their brains responded to it.

This demonstrates an important idea:

The brain can process some information without conscious awareness.

However, this does not mean subliminal messages have unlimited power to control people’s behavior.

Their effects tend to be much more limited than popular claims about “subliminal mind control” suggest.

Priming

Priming refers to the activation, often unconsciously, of certain associations that can influence later perception, memory, or behavior.

For example, being exposed to one concept may make related concepts easier to recognize afterward.

Think:

Earlier stimulus → activates association → influences later response

Difference Threshold

The difference threshold, also called the just noticeable difference (JND), is the minimum difference between two stimuli that a person can detect 50% of the time.

Again, remember:

50%

Examples:

• noticing that music became louder,

• detecting that one backpack is heavier,

• recognizing that one light is brighter,

• hearing a slight difference between two musical tones.

Absolute Threshold vs. Difference Threshold

Easy Memory Trick

Absolute = Is it there?

Difference = Did it change?

Weber’s Law

German physician Ernst Weber discovered that our ability to notice differences depends on the percentage of change, rather than simply the amount of change.

Weber’s law: For an average person to perceive a difference, two stimuli must differ by a constant minimum percentage, not a constant amount.

Examples from the module:

• Light intensity → approximately 8% difference

• Weight → approximately 2% difference

• Tone frequency → approximately 0.3% difference

Example

Adding 5 pounds to a 10-pound object would be extremely noticeable.

Adding 5 pounds to a 500-pound object might not be.

The numerical difference is still 5 pounds, but the percentage difference is dramatically different.

Key Idea

JND depends on proportion, not simply amount.

LOQ 18-5: What is the function of sensory adaptation?

Sensory adaptation is our reduced sensitivity to constant, unchanging stimulation.

When stimulation remains unchanged, sensory nerve cells may fire less frequently, causing us to notice the stimulus less.

Everyday Examples

You may initially notice:

• perfume,

• the feeling of clothing,

• a watch on your wrist,

• a fan running,

• the temperature of a swimming pool,

• background noise.

After continued exposure, you become less aware of them.

Rubber Band Example

If you place a rubber band around your wrist, you immediately feel it.

After some time, you barely notice it.

The rubber band did not disappear.

Your sensitivity to the constant stimulation decreased.

Why Sensory Adaptation Is Helpful

Sensory adaptation allows us to focus on important changes in the environment instead of constantly paying attention to unchanging information.

Imagine constantly being consciously aware of:

• your shoes touching your feet,

• your clothing touching your skin,

• the temperature of the room,

• every background noise,

• every smell.

Your brain would become overwhelmed.

Sensory adaptation helps your brain decide:

“Nothing is changing here, so I can focus somewhere else.”

Why Doesn’t Our Vision Completely Adapt?

If sensory adaptation occurs, why don’t objects disappear when we stare at them?

Because our eyes are constantly making tiny movements.

These movements continuously change which receptors receive stimulation.

When researchers artificially stabilize an image so it remains in exactly the same location on the retina, parts of the image may begin to fade from awareness.

Main Purpose

Our sensory systems are particularly sensitive to change and novelty.

This is also why changing stimuli such as phone notifications easily capture our attention.

Major Takeaway

Sensory adaptation frees our attention to focus on informative changes in our environment.

LOQ 18-6: How do expectations, context, motivation, and emotions influence perception?

Our perceptions are not determined entirely by sensory information.

They are also influenced by what is happening inside our minds.

Four especially important influences are:

Expectations + Context + Motivation + Emotion

Perceptual Set

A perceptual set is a mental predisposition to perceive one thing and not another.

Our experiences create expectations, and those expectations influence what we perceive.

This is an example of top-down processing.

Ambiguous Image Example

An ambiguous image could potentially look like either:

• a young woman,

• an old woman.

What you see first may depend on what image you saw immediately beforehand.

Your previous experience creates an expectation that influences your interpretation.

Schemas

Schemas are concepts or mental frameworks that organize and interpret unfamiliar information.

We develop schemas through experience.

For example, you have schemas for:

• restaurants,

• classrooms,

• families,

• animals,

• parties,

• friendships,

• relationships.

When you encounter new information, your brain uses existing schemas to interpret it.

Loch Ness Example

People expecting to see the Loch Ness monster may interpret an unclear object as a monster.

Someone approaching the same image skeptically might interpret it as a tree limb or log.

The sensory information may be identical.

The perceptual interpretation is different.

Stereotypes and Perception

Stereotypes can also influence perception.

Expectations concerning:

• culture,

• ethnicity,

• gender,

• sexual orientation,

• income,

• age,

• ability

can influence how people interpret others.

This illustrates that perception is not always a completely objective reflection of reality.

Context and Perception

Context strongly influences how we interpret sensory information.

The same stimulus may be interpreted differently depending on what surrounds it.

“Eel” Example

If you hear:

”_eel is on the wagon”

you will probably perceive:

“Wheel is on the wagon.”

If you hear:

”_eel is on the orange”

you will probably perceive:

“Peel is on the orange.”

The sensory information is ambiguous, so your brain uses context and top-down processing to fill in the missing information.

Cultural Context

Culture can influence how people interpret visual information.

People from different environments develop different expectations about:

• buildings,

• objects,

• social interactions,

• distances,

• environments,

• everyday activities.

Therefore, people from different cultures can sometimes look at the same visual information and interpret it differently.

Important Point

Perception is influenced partly by experience.

Expectations and Taste

Expectations can even change how food and drinks seem to taste.

McDonald’s Example

Preschool children were more likely to report that french fries tasted better when they believed the fries came from a McDonald’s bag compared with a plain bag.

Their expectations influenced their taste perception.

MIT Beer Experiment

Participants sometimes preferred beer containing a small amount of vinegar when they did not know beforehand that vinegar had been added.

When they were told about the vinegar first, they expected the drink to taste worse—and often perceived it that way.

This demonstrates:

Expectation → influences perception → influences experience

Motivation and Perception

Our goals and desires can influence what we perceive.

Examples

A thirsty person may perceive a water bottle as closer.

A hill may appear steeper when someone is carrying a heavy backpack.

A destination may appear farther away when someone is tired.

Athletes who are performing well may perceive a ball as larger.

Main Idea

We don’t always perceive the physical world completely objectively.

What we want and what we are trying to accomplish can influence how the world appears to us.

Emotion and Perception

Our emotional state can also change how we interpret sensory information.

Sadness

Sad music can make people more likely to interpret ambiguous words negatively.

For example:

mourning instead of morning

die instead of dye

pain instead of pane

Anger

When people are angry, they may become more likely to interpret ambiguous or neutral objects as threatening, such as perceiving an object as a gun.

Anxiety

Someone worried about having a panic attack may interpret:

• a pounding heart,

• shortness of breath,

• other normal physical sensations

as evidence that a panic attack is occurring.

Subliminal Emotion

Even brief exposure to an angry or scowling face may influence how someone evaluates a neutral face afterward.

Main Point

Our emotions can bias how we interpret ambiguous information.

The Major Idea of Module 18

The world we consciously experience is not simply an exact recording of the physical environment.

Our brains actively construct our perceptions.

What we perceive comes from a combination of:

Sensory information coming IN

• ●

Knowledge, expectations, context, motivation, and emotion already in our minds

This is why two people can experience the same physical event but perceive it differently.

Key Terms to Know

Sensation

The process by which sensory receptors and the nervous system receive and represent environmental stimulation.

Sensory Receptors

Specialized cells that detect environmental stimulation.

Perception

The process of organizing and interpreting sensory information.

Bottom-Up Processing

Processing that begins with sensory receptors and builds toward the brain’s interpretation.

Top-Down Processing

Using experience, knowledge, expectations, and other higher-level mental processes to interpret sensory information.

Transduction

Converting physical energy into neural signals the brain can process.

Psychophysics

The study of relationships between physical stimuli and psychological experiences.

Absolute Threshold

The minimum stimulation needed to detect a stimulus 50% of the time.

Signal Detection Theory

A theory explaining how psychological factors influence whether we detect weak signals among background stimulation.

Subliminal Stimulation

Stimulation that occurs below the absolute threshold for conscious awareness.

Priming

The often unconscious activation of associations that can influence later perception, memory, or behavior.

Difference Threshold / JND

The minimum difference between two stimuli that can be detected 50% of the time.

Weber’s Law

The principle that noticeable differences depend on a constant percentage of change, rather than a constant amount.

Sensory Adaptation

Reduced sensitivity to constant, unchanging stimulation.

Perceptual Set

A mental predisposition or expectation that influences what we perceive.

High-Priority Concepts for the Test

The most important distinctions to memorize are:

Sensation = detecting information
Perception = interpreting information

Bottom-up = sensory information → brain
Top-down = brain’s expectations/knowledge → interpretation

Transduction = physical energy → neural signals

Absolute threshold = smallest stimulus detected 50% of the time

Difference threshold/JND = smallest change detected 50% of the time

Weber’s law = noticeable change depends on percentage

Sensory adaptation = decreased awareness of constant stimulation

Perceptual set = expectations influence perception

And for LOQ 18-6, remember:

Expectations + Context + Motivation + Emotion → influence perception

One-Sentence Module Summary

Module 18 explains how our sensory systems detect and convert environmental stimulation into neural information and how our brains actively interpret that information using both incoming sensory data and our experiences, expectations, context, motivations, and emotions.