Lecture: Sensation & Perception
Lecture script · about 15–20 minutes spoken · Week 4
Opening
A hearing-clinic technician hunts for the softest tone a person can hear. A delivery driver judges the gap to the car ahead as the light fades at dusk. A store designer picks the one sign shoppers will actually notice in a busy aisle. Each of them is asking two questions. What gets in through the senses? And what does the brain make of it?
Those are the two questions of this lecture. In Week 1, you met Wundt's laboratory, where instruments timed how fast a person responded to a signal. In Week 2, you learned that a fuzzy idea becomes measurable only when you give it an operational definition. In Week 3, you followed a message through neurons and met the occipital lobe, the brain's vision region. This week starts one step earlier, at the cells that begin that message. Next week, in Week 5, the course turns to consciousness and sleep.
Here is the plan: sensation versus perception, the edge of sensing, a steady stimulus that fades, the eye and the ear, grouping and depth, and finally the evidence that perception is built. If you remember one sentence, make it this one: your senses detect, and your brain decides.
1. Separate sensation from perception: what gets in versus what you make of it
Picture a phone camera and a person standing side by side at a concert. The same light from the stage reaches both. The camera stores a grid of colored dots. The person sees a singer, knows the song, and spots a friend three rows ahead. The camera senses. Only the person perceives.
Psychologists split that 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. The turning is its own idea. 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. Light is not a neural signal, and neither is a sound wave. A receptor cell translates, and that is where the Week 3 message begins.
Then the brain takes over. 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.
Perception runs in two directions. 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.
Here's an example. A flyer on a campus bulletin board got wet, and one letter in "library" washed out. Your eyes take in the shapes that are left. That's bottom-up. You know English words, and you know tutoring often happens in libraries, so you read "library" without a pause. That's top-down. Both worked at once.
People often use "sensation" and "perception" as if they mean the same thing. They don't. Sensation ends at the neural signal. Perception ends in meaning.
Pause and try this. You hear a noise in the kitchen and know at once that it's the kettle. Which part of that moment is sensation, and which part is perception?
Take a breath and decide before you read on.
Here's the answer. The sound waves reaching your ears and becoming neural signals is sensation; knowing "that's the kettle" is perception, because recognizing what a sound means happens in the brain.
2. Measure the edge of sensing: absolute and difference thresholds
Somewhere between "nothing" and "impossible to miss," every sense has an edge. Can it be measured? Yes, and the measuring has rules. 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.
The first rule is that the edge is fuzzy. Play the same faint tone many times, and you'll 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 does. "The faintest thing you can sense" becomes a procedure: present a stimulus many times at different strengths, and find the strength a person reports on half of the tries. That's an operational definition, exactly the Week 2 kind.
How sensitive are we? Galanter, in 1962, gave two famous illustrations. 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. Notice the conditions, though. A clear, dark night and a quiet room remove the background light and sound that would hide a faint stimulus. Against a lit sky, that same flame would be harder to catch.
An honesty note: those figures are illustrations, written with "about." Real thresholds differ from person to person and from moment to moment, which is exactly why the definition uses half of many tries.
The second kind of threshold is about change. The difference threshold is the smallest change in a stimulus that a person can detect 50 percent of the time; psychologists shorten it to JND. Noticing that a second candle has been lit beside the first is a difference threshold, because the light was already there.
How big must a change be? The answer, credited to Weber, surprises people. 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. Double the original stimulus, and you double the change needed to notice a difference. A whisper you'd notice in a quiet library goes unnoticed at a loud concert.
One warning. In Week 3, a neuron's threshold was the input one cell needs before it fires. This week's thresholds are different. They describe what a whole person reports.
Pause and try this. Galanter's candle flame about 30 miles away and noticing a second candle lit beside the first: which one is the absolute threshold, and which one is the difference threshold?
Answer it in your head first.
Here's the answer. The candle about 30 miles away is the absolute threshold, because it goes from nothing to something; the second candle is the difference threshold, because it's a change in light that was already there.
3. Notice what fades: sensory adaptation
Imagine moving 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 don't see it unless a visitor points it out. Or you jump into a cold pool. It's a shock, and a minute later the water feels fine. The sign didn't dim. The water didn't warm.
That's 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.
Adaptation earns its keep. A sensory system that kept shouting about the same unchanging hum would leave little room for what's new. And the moment something changes, say the sign suddenly goes dark, you notice again.
A common trap is thinking adaptation means the stimulus got weaker, or that you've slipped below your absolute threshold for it. Neither is right. The sign is as bright as it was on the first night. Your sensitivity to the unchanging light is what fell. So here's the test I'd like you to carry: ask whether the stimulus changed. If it didn't, and noticing faded anyway, that's adaptation.
Pause and try this. A barista stops noticing the smell of roasting coffee partway through a shift, though a customer walking in notices it at once. Is that adaptation, an absolute threshold, or a difference threshold?
Ask whether the smell changed, then decide.
Here's the answer. It's sensory adaptation, because the smell stayed the same while the barista's sensitivity to it fell; the customer is new to it, so it still registers.
4. Follow light and sound inward: the eye, the ear, rods and cones
Every sense follows the same plan. Energy arrives, a structure gathers and focuses it, and receptor cells transduce it. Vision and hearing show the plan well enough to trace.
Light first. It passes the 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. Next comes the pupil, 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. Behind it sits the lens, the clear structure behind the pupil that changes shape to focus incoming light onto the retina. And the retina is the light-sensitive layer at the back of the eye where rods and cones transduce light into neural signals.
Those two receptors 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, signals travel to the occipital lobe, which processes vision.
Picture a runner heading out at sunset. In daylight, the red of a stop sign is vivid and street names are easy to read. That's the cones. At dusk, color fades toward gray and small print blurs, because the light is dropping below what the cones need. In the dark, the runner still sees shapes and movement, especially off to the side. That's the rods: dim light, the edges of the retina, no color.
Hearing uses 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 moving fluid bends the receptors. 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.
Here's the misconception to drop: rods do not see color. The dusk run shows why. Color fails first as light fails, because cones need brighter light.
Pause and try this. By moonlight on a camping trip, you can see the shapes on a trail map but can't tell the red trail from the green one. Which receptors are doing the work, and why is the color gone?
Think about which receptor works in dim light.
Here's the answer. The rods are doing the work, because they respond in dim light; the color is gone because rods carry no color information, and the cones need brighter light than the moon gives.
5. Group and place what you see: Gestalt principles and depth cues
Look around a crowded bus stop in your mind. You don't experience a jumble of colored patches. You see people, a bench, a sign, and a road behind them. Your brain sorts patches into objects and sets them at distances.
The sorting follows rules. The Gestalt principles are the rules by which the brain groups sensations into whole, organized patterns, including figure-ground, proximity, similarity, closure, and continuity.
Take a soccer match. The players stand out against the green field: that's figure-ground, the brain separating a figure that stands out from the background behind it. Fans in the home team's shirts look like one group: that's similarity, things that look alike being seen as one group. Two friends sitting close together read as a pair: that's proximity, things close together being seen as one group. A team crest painted with gaps in its outline still reads as a whole circle: that's closure, the brain filling in gaps. And two painted lines crossing on the field look like two straight lines, not four short ones meeting: that's continuity, seeing lines continue smoothly in one direction.
Grouping is half the job. The other half is distance. 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 need both eyes, such as the slightly different view each eye gets of the same object. Monocular depth cues 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 this with your own hand. Hold up a thumb at arm's length. Close one eye, then switch. Your thumb seems to jump, because each eye sees it from a slightly different spot. Now keep one eye closed. The room doesn't go flat. Overlap, size, and lines that meet in the distance are still working. That's why people often get this wrong: you don't need two eyes to see depth at all.
Pause and try this. A painting shows a road narrowing to a point and a tree partly hiding a barn. Are those binocular depth cues or monocular depth cues, and how do you know?
Ask how many eyes each cue needs.
Here's the answer. They're monocular depth cues, because both work with one eye: parallel lines meeting in the distance and one object blocking another; a flat painting can't give each eye a different view.
6. Catch perception in the act: constancies and illusions
Watch a friend walk away down a long hallway. The image of your friend on your retina shrinks and shrinks. Yet you never think your friend is shrinking. Open a door toward you, and its outline on your retina turns into a slanted shape. You still see a rectangle.
That's 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. A white shirt in deep shade still looks white. The brain is correcting the raw image with what it knows about distance, angle, and light.
The same rules can misfire, and then you get an 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. Picture two lines of the same length. One ends in arrowheads, like a two-headed arrow. The other ends in open V shapes flaring away, like the tail feathers of an arrow. Most people see the line with the flared tails as longer. One common explanation is that those ends act like depth cues, the corners of a room or a building, and the brain adjusts for a distance it thinks is there.
So here's the last misconception: the belief that we see the world as it is, like a camera. Constancy shows the brain correcting a changing image. Illusions show those same corrections producing a mistake. Perception is constructed. A limitation is worth saying out loud, though. Constructed doesn't mean unreliable. Most of the time the construction is good enough to cross a street safely.
Pause and try this. If perception were a perfect copy of the world, what would you expect to see when you looked at the two equal lines?
Picture the two lines before you answer.
Here's the answer. You'd see them as equal, because they are equal; seeing one as longer shows that the brain is interpreting the image rather than copying it.
Closing
Let me pull the week together in the order we built it. Sensation is detection: receptors transduce energy into neural signals. Perception is the brain's interpretation, built bottom-up from features and shaped top-down by what you know. The absolute threshold is the weakest stimulus detected 50 percent of the time, like Galanter's candle flame about 30 miles away and the clock ticking about 20 feet away. The difference threshold is the smallest change detected half the time, and Weber's law says it grows in proportion to what you started with. Sensory adaptation is fading sensitivity to a stimulus that doesn't change. Cones give color and detail in good light; rods take over in dim light without color; hair cells transduce sound. The Gestalt principles group what you sense, depth cues place it, and constancies and illusions prove the whole picture is built.
Next in this module come the slide deck, the Readings & Resources page, the lecture tutorial, the practice set, and the quiz, then the discussion, the assignment, and the Analysis Workshop. Next week, in Week 5, Consciousness, the course asks what it means to be aware of all this, and what changes when you sleep.
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