Week 4 · Chapter

Chapter 4 — Sensation & Perception

Introduction to Psychology Free sample edition

The module's primary reading · ~35–45 minutes · Objective 4 (explain how humans sense and perceive the world and how sleep, dreaming, and psychoactive drugs shape states of consciousness), Objective 3 (describe the biological bases of behavior: neurons and neural transmission, the major neurotransmitters, and the structures and functions of the nervous system and brain) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the Analysis Workshop all build on it.

In plain language

Everything you know about the world arrives through your senses, but your senses do not hand your brain a finished picture. Sensation is the first step, when your eyes, ears, skin, nose, and tongue pick up energy from the world and turn it into signals your nerves can carry. Perception is the second step, when your brain sorts those signals and decides what they mean, like hearing a jumble of sounds as your own name. This week you will learn how psychologists measure the faintest thing you can sense and why a steady smell or sound fades from notice. You will also follow light and sound into the eye and ear, and see why what you see is built by your brain rather than copied from the world.

Why you already care about this

Picture a dark road far from any town. You spot one faint light on a distant hill. The friend beside you squints and sees nothing. Is the light there or not? And if it is, why does one of you see it and the other does not?

Psychologists have measured questions like this for a long time, and the answers are surprising. On a clear, dark night, a person can detect a candle flame about 30 miles away. In a quiet room, a person can detect a clock ticking about 20 feet away. Those two illustrations come from Galanter (1962), and both rest on one careful rule: a stimulus counts as detected when a person reports it 50 percent of the time. Your senses are far more sensitive than most people guess, and the edge of what you can sense is something a scientist can measure.

But detecting is only half the story. You read a messy note from a friend without trouble. You see a door as a rectangle even when you look at it from an angle. You hear one voice in a noisy room. None of that is in the light or the sound. Your brain adds it.

By the end of this chapter you will be able to:

tell sensation apart from perception, and bottom-up processing from top-down processing, in an everyday example;
explain what the absolute threshold and the difference threshold each measure, and use Weber's law to predict when a change will be noticed;
recognize sensory adaptation and say why it is not the same as a threshold;
trace light through the eye to the rods and cones of the retina, and sound through the ear to the hair cells;
name the Gestalt principles and depth cues at work in a scene, and use constancies and illusions to show that perception is constructed.

Carry this sentence with you: your senses detect; your brain decides.

Where we are

Already covered: Week 1 showed that psychology measures mental processes, starting with reaction time; Week 2 taught operational definitions, reliability, and validity; Week 3 followed a signal through neurons and named the occipital lobe as the brain's vision region.
This week: how energy from the world becomes a neural signal (sensation), how the brain turns those signals into an experience (perception), and how psychologists measure both.
How they connect: a sensory receptor is where Week 3's neural message begins, and every threshold this week is an operational definition in Week 2's sense.
Where we're going next: Week 5, Consciousness, asks what it means to be aware of all this sensing and perceiving, and what changes when you sleep.

Each earlier week hands you a tool for this one. In Week 1 you met Wundt's laboratory, where instruments timed how fast a person responded to a signal; that was already a study of sensing. In Week 2 you learned that a fuzzy idea is not measurable until you give it an operational definition, the exact, observable procedure used to measure it. The absolute threshold is a perfect example, because "the faintest thing you can sense" becomes "the weakest stimulus detected on 50 percent of the tries." In Week 3 you watched a neuron fire once its input crossed its threshold. This week begins one step earlier, at the receptor cells that start that message.

1. Separate sensation from perception: what gets in versus what you make of it

Think about a phone camera and a person standing side by side at a concert. Both take in the same light from the stage. The camera stores a grid of colored dots. The person sees a singer, recognizes the song, and notices a friend in the crowd. The camera senses. Only the person perceives.

Psychologists split this into two steps.

Sensation is the process by which sensory receptors detect physical energy, such as light, sound waves, pressure, or chemicals, and turn it into neural signals.
Transduction is the conversion of one form of physical energy, such as light or sound waves, into the neural signals that the nervous system can carry; it happens in the sensory receptors.
Perception is the process by which the brain organizes and interprets the neural signals from the senses into meaningful experience, such as recognizing a face or hearing a word.

Transduction is the bridge back to Week 3. A receptor cell in your eye or ear is where a neural message starts. Light is not a neural signal, and neither is a sound wave. The receptor translates.

Perception can run in two directions. Sometimes your brain builds a whole from the parts. Sometimes what you already know reaches down and shapes what you see.

Bottom-up processing is perception that builds a whole from the features of the stimulus itself, starting with the raw sensory details and working up to recognition.
Top-down processing is perception that is shaped by what you already know, expect, or see around the stimulus, such as reading a smudged word correctly because the rest of the sentence makes it clear.

Worked example — the rain-smudged flyer. A flyer on a campus bulletin board got wet. One line reads "Free tutoring in the l_brary every evening," with one letter washed out.

Bottom-up first. Your eyes take in the shapes that are there: the letters l, then a gap, then b, r, a, r, y. On their own, those features do not settle the missing letter.
Top-down fills the gap. You know English words, you know tutoring often happens in libraries, and the sentence is about a campus. That knowledge settles the word as "library" before you even notice the gap.
Name both. The letter shapes are bottom-up input. Your knowledge of words and campuses is top-down influence. Both worked at once.

In words: you sensed the same smudge anyone would, and you perceived a word because your brain brought what it knows.

Misconception check. Students often use "sensation" and "perception" as if they mean the same thing. It is tempting because the two happen so fast that they feel like one event. The cure is to ask where each step happens. Sensation happens at the receptor and stops at the neural signal. Perception happens in the brain and ends in meaning. If the example involves recognizing, interpreting, or deciding what something is, it is perception.

2. Measure the edge of sensing: absolute and difference thresholds

Some sounds are too faint to hear and some lights too dim to see. Somewhere between "nothing" and "impossible to miss" is an edge. Psychophysics is the study of how the physical properties of a stimulus, such as its intensity, relate to what a person actually senses and reports. Psychophysics measures that edge, and the measuring has rules.

The first rule is that the edge is fuzzy. Play the same faint tone many times and you will catch it on some tries and miss it on others. So psychologists define the edge with a percentage.

The absolute threshold is the weakest amount of a stimulus that a person can detect 50 percent of the time; it marks the boundary between sensing nothing and sensing something.

Notice what that definition does. It turns "the faintest thing you can sense" into a procedure: present a stimulus many times at different strengths, and find the strength you report on half of the tries. That is an operational definition, exactly as Week 2 described.

The second kind of threshold is about change, not about the first flicker.

The difference threshold is the smallest change in a stimulus that a person can detect 50 percent of the time; it is also called the just noticeable difference (JND).

How big must a change be before you notice it? The answer, credited to Weber, is a surprising rule. The change you need is not a fixed amount. It grows with the size of what you started with.

Weber's law is the principle that the difference threshold is a constant proportion of the original stimulus rather than a fixed amount, so a stronger stimulus needs a larger change before the change is noticed.

Worked example — Galanter's two illustrations, classified. Use the two absolute-threshold illustrations from Galanter (1962) and the two threshold definitions.

Start from the definition. An absolute threshold is the weakest stimulus a person detects 50 percent of the time, so it is a measurement taken over many tries, not a single yes or no.
Vision. On a clear, dark night, a person can detect a candle flame about 30 miles away. The dim light reaches the retina, where the rods do most of the work.
Hearing. In a quiet room, a person can detect the tick of a clock about 20 feet away. The sound waves reach the hair cells of the inner ear.
Read the conditions. "Clear, dark night" and "quiet room" are part of the measurement. They remove the background light and sound that would hide a faint stimulus. The same flame would be harder to detect against a lit sky, and the same tick harder to hear in a noisy room.
Absolute or difference? The candle and the clock each go from nothing to something, so they illustrate the absolute threshold. Noticing that a second candle has been lit beside the first is a change from a stimulus already present, so it is a difference threshold, the JND.
Apply Weber's law. Because the JND is a constant proportion of the original stimulus, doubling the original stimulus doubles the change needed to notice a difference. A whisper you would notice in a quiet library goes unnoticed at a loud concert.

In words: the candle at about 30 miles and the clock at about 20 feet mark the first flicker of sensing, and a change is noticed only when it is large compared with what was already there.

Here is a kitchen version of Weber's law. One pinch of salt changes the taste of a small cup of soup. The same pinch dropped into a large pot of the same soup is hard to taste. The pinch did not change. The amount you started with did.

An honesty note about the numbers: Galanter's figures are illustrations, written with "about," not records from one experiment. Real thresholds differ from person to person and from moment to moment. That is exactly why the definition uses 50 percent of many tries.

Misconception check. Students often mix up the absolute threshold and the difference threshold. The mix-up is tempting because both are "the smallest amount you can notice" and both use 50 percent. The cure is one question: did the stimulus start at nothing? If you are detecting something where there was nothing, it is the absolute threshold. If you are detecting a change in something already there, it is the difference threshold. One more trap: Week 3's neuron "threshold" is a different idea. That threshold is the input a single neuron needs before it fires; this week's thresholds describe what a whole person reports.

3. Notice what fades: sensory adaptation

You move into an apartment beside a shop with a flashing sign. The first night, the light through your window is all you notice. A week later, you do not see it at all unless a visitor points it out. Or you jump into a cold pool at a community center. It is shocking for a moment, and a minute later the water feels fine. The sign did not dim. The water did not warm up.

Sensory adaptation is the decline in sensitivity to a stimulus that does not change, so a steady sight, sound, smell, or touch becomes less noticeable over time.

Adaptation is useful. A sensory system that kept shouting about the same unchanging hum would leave little room to notice what is new. A change, like the sign suddenly going dark, grabs your attention again.

Worked example — three moments at the pool. A swimmer has three experiences in one afternoon. Sort each one.

The first plunge feels icy, and a minute later the water feels fine. The water stayed the same, and sensitivity to it fell. That is sensory adaptation.
A lifeguard's faint whistle, far across a quiet deck, is heard on some tries and missed on others. That is the edge between nothing and something, so it belongs with the absolute threshold.
The swimmer notices that the heater has made the water a little warmer. That is a change in a stimulus already present, so it belongs with the difference threshold.

In words: adaptation is about a stimulus that stays the same, while both thresholds are about detecting something new.

Misconception check. Students often think adaptation means the stimulus got weaker, or that adapting to the sign means you are now below your absolute threshold for it. Neither is right. The sign is exactly as bright as the first night. Your sensitivity to the unchanging light fell. The cure: ask whether the stimulus changed. If it did not, and noticing faded anyway, you are looking at adaptation.

Pause and think. Choose one of these two prompts; they carry equal weight.

(a) Describe a steady sound, smell, or touch in your own home, job, or commute that you no longer notice, and say what would make you notice it again.
(b) Describe a time a change caught your attention after a long steady stretch, such as a fan clicking off or a song ending, and explain it with adaptation.

4. Follow light and sound inward: the eye, the ear, rods and cones

Every sense works the same basic way. Energy arrives, a structure gathers and focuses it, and receptor cells transduce it. Vision and hearing show the path well.

Light enters the eye through a series of structures, each with one job.

The cornea is the clear, curved outer layer at the front of the eye that light passes through first and that begins to bend the light inward.
The pupil is the adjustable opening in the center of the iris through which light enters the eye.
The iris is the colored ring of muscle around the pupil that widens or narrows the pupil to control how much light enters the eye.
The lens is the clear structure behind the pupil that changes shape to focus incoming light onto the retina.
The retina is the light-sensitive layer at the back of the eye where rods and cones transduce light into neural signals.

The retina holds two kinds of receptor cells, and they split the work.

Rods are receptor cells in the retina that respond in dim light and dominate the edges of the retina but carry no color information.
Cones are receptor cells in the retina that need brighter light and provide color vision and fine detail.

From the retina, the neural signals travel to the back of the brain. The occipital lobe processes vision, as you saw in Week 3.

Hearing follows the same plan with different energy. Sound waves enter the outer ear and set the eardrum vibrating. Three tiny bones in the middle ear pass the vibration inward. In the fluid-filled inner ear, the vibration moves the fluid, and the receptor cells there bend.

Hair cells are receptor cells in the inner ear that bend as sound waves move the fluid around them and transduce that motion into neural signals.

Worked example — a walk at dusk. A runner heads out as the sun sets and keeps going into the dark.

Early, in daylight. The runner sees the red of a stop sign and reads a street name. Color and fine detail mean the cones are doing the work.
Dusk. Colors fade toward gray, and small print gets hard to read. The light is dropping below what the cones need.
Dark. The runner can still see shapes and movement, especially off to the side. The rods respond in dim light and dominate the edges of the retina, but they carry no color.
A prediction you can test. Because rods dominate the edges of the retina, a very faint star may be easier to catch if you look slightly to one side of it rather than straight at it.

In words: color and detail belong to the cones in good light, and the rods take over in the dark at the cost of color.

Misconception check. Students often say that rods see color. It is tempting because "rods and cones" are always taught as a pair, and the names give no hint. The cure is the dusk walk: as the light fails, color fails first, because cones need brighter light. Rods keep working in dim light, and they carry no color information.

5. Group and place what you see: Gestalt principles and depth cues

Look at a crowded bus stop. You do not see a jumble of colored patches. You see people, a bench, a sign, and a road behind them. Your brain sorts the patches into objects and sets them at distances. Psychologists who studied this grouping called the rules Gestalt principles; Gestalt is a German word for a whole form.

The Gestalt principles are the rules by which the brain groups sensations into whole, organized patterns, including figure-ground, proximity, similarity, closure, and continuity.
Figure-ground is the Gestalt principle by which the brain separates a scene into a figure that stands out and a background, or ground, behind it.
Proximity is the Gestalt principle by which things that are close together are seen as belonging to one group.
Similarity is the Gestalt principle by which things that look alike in color, shape, or size are seen as belonging to one group.
Closure is the Gestalt principle by which the brain fills in gaps to see a complete, whole object instead of separate pieces.
Continuity is the Gestalt principle by which the brain sees lines and patterns as continuing smoothly in one direction rather than changing direction abruptly.

Worked example — a stadium crowd. Fans fill the seats at a soccer match.

Figure-ground. The players on the field stand out as figures; the green field is the ground behind them.
Similarity. Fans wearing the home team's shirts are seen as one group, and fans in the visitors' shirts as another.
Proximity. Two friends sitting close together look like a pair, even in a sea of strangers.
Closure. A team crest painted with gaps in its outline still reads as a whole circle.
Continuity. When two painted lines on the field cross, you see two straight lines passing through each other, not four short lines meeting at a point.

Grouping is half the job. The other half is distance. Each retina is a curved surface, so the image on it has no depth of its own. The brain adds distance from clues.

Depth cues are pieces of information in what you sense that let the brain judge how far away things are and see the world in three dimensions.
Binocular depth cues are depth cues that need both eyes, such as the slightly different view each eye gets of the same object.
Monocular depth cues are depth cues that work with one eye alone, such as one object partly blocking another, nearer objects looking larger, and parallel lines appearing to meet in the distance.

Try the binocular cue now. Hold up one thumb at arm's length. Close one eye, then switch eyes. Your thumb seems to jump, because each eye sees it from a slightly different spot. Your brain uses that difference to judge distance. Now keep one eye closed and look around the room. The world still has depth, because the monocular cues are still working: the chair in front of the desk blocks part of it, and the far end of a hallway looks narrower.

Misconception check. Students often think you need two eyes to see depth at all. It is tempting because binocular cues are powerful up close. The cure is the one-eye test: close an eye and the room does not go flat. Overlap, relative size, and lines that meet in the distance all work with one eye, which is why a painting or a photograph can look deep.

6. Catch perception in the act: constancies and illusions

Watch a friend walk away from you down a long hallway. The image of your friend on your retina shrinks to a fraction of its first size. Yet you never think your friend is shrinking. Open a door toward you, and its outline on your retina turns from a rectangle into a narrow slanted shape. You still see a rectangular door.

Perceptual constancy is the tendency to see an object as keeping the same size, shape, and color even as the image it makes on the retina changes with distance, angle, or lighting.

Constancy is the brain correcting the raw image with what it knows about distance, angle, and light. A white shirt in deep shade reflects less light than a gray one in the sun, yet you still see it as white. That is not in the light. It is the brain's best guess about the object.

The same rules can misfire. When they do, you get an illusion.

An illusion is a perception that does not match the physical stimulus, produced when the brain's usual rules for organizing what you sense lead to a mistaken result.

Worked example — two equal lines. Draw two horizontal lines of exactly the same length. Finish one with an arrowhead at each end, like a two-headed arrow. Finish the other with an open V at each end that flares away from the line, like the tail feathers of an arrow.

What the stimulus is. Two lines of equal length. A ruler confirms it.
What most people perceive. The line with the flared tails looks longer than the line with the arrowheads.
Why. One common explanation is that the ends act like depth cues, similar to the corners of a room or a building. The brain may adjust for a distance it thinks is there, the same kind of correction that keeps your friend full-sized in the hallway.
What it proves. If perception were a copy of the stimulus, the lines would look equal. They do not, so perception is built.

A second example you can check on a clear evening: the moon low on the horizon often looks larger than the moon high overhead, though its image on the retina is about the same size. Constancy and illusion are two sides of one fact. Your brain does not record the world. It interprets it, and nearly always gets it right.

Misconception check. Students often believe that we see the world as it is, like a camera. It is tempting because perception feels effortless and direct. The cure is the evidence: constancy shows the brain correcting a changing image, and illusions show the same corrections producing a mistake. Perception is constructed. A limitation worth naming: "constructed" does not mean "unreliable." Most of the time, the construction is accurate enough to cross a street safely.

Pause and think. Choose one of these two prompts; they carry equal weight.

(a) Describe an illusion or a moment of misperception you have experienced, such as mistaking a coat on a chair for a person, and explain which rule of perception produced it.
(b) Describe a constancy at work in your day, such as recognizing a friend across a parking lot, and explain what the brain had to correct.

Key terms

Sensation — The process by which sensory receptors detect physical energy, such as light, sound waves, pressure, or chemicals, and turn it into neural signals. (first used: 1. Separate sensation from perception: what gets in versus what you make of it)
Transduction — The conversion of one form of physical energy, such as light or sound waves, into the neural signals that the nervous system can carry; it happens in the sensory receptors. (first used: 1. Separate sensation from perception: what gets in versus what you make of it)
Perception — The process by which the brain organizes and interprets the neural signals from the senses into meaningful experience, such as recognizing a face or hearing a word. (first used: 1. Separate sensation from perception: what gets in versus what you make of it)
Bottom-up processing — Perception that builds a whole from the features of the stimulus itself, starting with the raw sensory details and working up to recognition. (first used: 1. Separate sensation from perception: what gets in versus what you make of it)
Top-down processing — Perception that is shaped by what you already know, expect, or see around the stimulus, such as reading a smudged word correctly because the rest of the sentence makes it clear. (first used: 1. Separate sensation from perception: what gets in versus what you make of it)
Psychophysics — The study of how the physical properties of a stimulus, such as its intensity, relate to what a person actually senses and reports. (first used: 2. Measure the edge of sensing: absolute and difference thresholds)
Absolute threshold — The weakest amount of a stimulus that a person can detect 50 percent of the time; it marks the boundary between sensing nothing and sensing something. (first used: 2. Measure the edge of sensing: absolute and difference thresholds)
Difference threshold — The smallest change in a stimulus that a person can detect 50 percent of the time; it is also called the just noticeable difference (JND). (first used: 2. Measure the edge of sensing: absolute and difference thresholds)
Weber's law — The principle that the difference threshold is a constant proportion of the original stimulus rather than a fixed amount, so a stronger stimulus needs a larger change before the change is noticed. (first used: 2. Measure the edge of sensing: absolute and difference thresholds)
Sensory adaptation — The decline in sensitivity to a stimulus that does not change, so a steady sight, sound, smell, or touch becomes less noticeable over time. (first used: 3. Notice what fades: sensory adaptation)
Cornea — The clear, curved outer layer at the front of the eye that light passes through first and that begins to bend the light inward. (first used: 4. Follow light and sound inward: the eye, the ear, rods and cones)
Pupil — The adjustable opening in the center of the iris through which light enters the eye. (first used: 4. Follow light and sound inward: the eye, the ear, rods and cones)
Iris — The colored ring of muscle around the pupil that widens or narrows the pupil to control how much light enters the eye. (first used: 4. Follow light and sound inward: the eye, the ear, rods and cones)
Lens — The clear structure behind the pupil that changes shape to focus incoming light onto the retina. (first used: 4. Follow light and sound inward: the eye, the ear, rods and cones)
Retina — The light-sensitive layer at the back of the eye where rods and cones transduce light into neural signals. (first used: 4. Follow light and sound inward: the eye, the ear, rods and cones)
Rods — Receptor cells in the retina that respond in dim light and dominate the edges of the retina but carry no color information. (first used: 4. Follow light and sound inward: the eye, the ear, rods and cones)
Cones — Receptor cells in the retina that need brighter light and provide color vision and fine detail. (first used: 4. Follow light and sound inward: the eye, the ear, rods and cones)
Hair cells — Receptor cells in the inner ear that bend as sound waves move the fluid around them and transduce that motion into neural signals. (first used: 4. Follow light and sound inward: the eye, the ear, rods and cones)
Gestalt principles — The rules by which the brain groups sensations into whole, organized patterns, including figure-ground, proximity, similarity, closure, and continuity. (first used: 5. Group and place what you see: Gestalt principles and depth cues)
Figure-ground — The Gestalt principle by which the brain separates a scene into a figure that stands out and a background, or ground, behind it. (first used: 5. Group and place what you see: Gestalt principles and depth cues)
Proximity — The Gestalt principle by which things that are close together are seen as belonging to one group. (first used: 5. Group and place what you see: Gestalt principles and depth cues)
Similarity — The Gestalt principle by which things that look alike in color, shape, or size are seen as belonging to one group. (first used: 5. Group and place what you see: Gestalt principles and depth cues)
Closure — The Gestalt principle by which the brain fills in gaps to see a complete, whole object instead of separate pieces. (first used: 5. Group and place what you see: Gestalt principles and depth cues)
Continuity — The Gestalt principle by which the brain sees lines and patterns as continuing smoothly in one direction rather than changing direction abruptly. (first used: 5. Group and place what you see: Gestalt principles and depth cues)
Depth cues — Pieces of information in what you sense that let the brain judge how far away things are and see the world in three dimensions. (first used: 5. Group and place what you see: Gestalt principles and depth cues)
Binocular depth cues — Depth cues that need both eyes, such as the slightly different view each eye gets of the same object. (first used: 5. Group and place what you see: Gestalt principles and depth cues)
Monocular depth cues — Depth cues that work with one eye alone, such as one object partly blocking another, nearer objects looking larger, and parallel lines appearing to meet in the distance. (first used: 5. Group and place what you see: Gestalt principles and depth cues)
Perceptual constancy — The tendency to see an object as keeping the same size, shape, and color even as the image it makes on the retina changes with distance, angle, or lighting. (first used: 6. Catch perception in the act: constancies and illusions)
Illusion — A perception that does not match the physical stimulus, produced when the brain's usual rules for organizing what you sense lead to a mistaken result. (first used: 6. Catch perception in the act: constancies and illusions)

Summary

Sensation detects energy and transduces it; perception organizes and interprets it.
Bottom-up processing starts from the stimulus; top-down processing starts from what you know.
The absolute threshold is the weakest stimulus detected 50 percent of the time.
The difference threshold, or JND, is the smallest change detected 50 percent of the time.
Weber's law: the change you need grows in proportion to what you started with.
Sensory adaptation is fading sensitivity to a stimulus that does not change.
Cones give color and fine detail in good light; rods work in dim light without color.
Gestalt principles group what you sense, and depth cues place it at a distance.
Constancies and illusions show that perception is constructed, not copied.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

A barista starts a shift and the smell of roasting coffee is strong. Two hours later the barista barely notices it, though a customer walking in says the shop smells wonderful. Which concept explains the barista's experience, and why is it not a threshold?
A sound technician adds one quiet acoustic guitar to a duet, then later adds the same guitar to a full orchestra. In which setting is the added guitar more likely to be noticed, and which law predicts that?
On a camping trip, a hiker can read the colors on a trail map at noon but cannot tell the red trail from the green trail by moonlight, though the map's shapes are still visible. Explain both observations with rods and cones.
On a dim hiking trail, a person jumps back from a coiled shape that turns out to be a dropped rope. Which kind of processing produced the scare, and what does the moment show about perception?

Answers (worked reasoning — read after attempting)

Sensory adaptation. The smell did not change over the two hours, yet the barista's sensitivity to it fell, which is the definition of adaptation. The customer is new to the smell, so it still registers strongly. It is not an absolute threshold, because the smell never dropped toward nothing, and it is not a difference threshold, because nothing about the smell changed.
In the duet. Weber's law says the difference threshold is a constant proportion of the original stimulus. A duet is a small starting sound, so one more guitar is a large change compared with it. A full orchestra is a large starting sound, so the same guitar is a small change compared with it and is easy to miss.
At noon there is plenty of light, so the cones work and give color and fine detail: the hiker can tell red from green. By moonlight the light is too dim for the cones. The rods respond in dim light, so the hiker still sees shapes, but rods carry no color information, so red and green look alike.
Top-down processing. The person's knowledge and expectation, that a trail can have snakes, shaped what the coiled shape was perceived to be. The sensation was the same lines and curves either way. The moment shows that perception is constructed: the brain interpreted the input and, this time, guessed wrong.

Next up in this module: the Readings & Resources page, the lecture tutorial, the practice set, and the quiz — and next week the mechanism of the circadian rhythm shows how your brain keeps time.

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