Sensation & Sensory Processing

🧠 Sensation and Sensory Processing

Part 1 of 7 — Thresholds, Weber's Law, and Adaptation

🔑 Key Concept: Every sense starts with the same job. A receptor detects a stimulus strong enough to meet a threshold and converts that physical energy into neural signals (transduction). Thresholds, Weber's law and adaptation describe how much stimulation it takes to notice something, or to notice that it changed.

Sensation is the process of detecting information from the environment that meets a certain threshold and transducing it into neural messages the brain can process. Perception is the brain's organization and interpretation of those messages; perception takes over from here (see Perception & Attention). This topic stays at the sensation end of the chain.

Transduction: the shared first step

Two points the exam likes:

• Transduction happens at the receptor, not in the cortex. A damaged receptor means no signal, however healthy the brain is.
• Every sense uses the same neural "currency" (action potentials). What makes a signal feel like light or sound is where it is sent in the brain, not the signal itself.

The absolute threshold

The absolute threshold is the minimum intensity at which a stimulus is detected at least 50% of the time. Why use 50% rather than "the first time it is noticed"? Detection is not an on/off switch. Near the threshold, the same faint stimulus is sometimes detected and sometimes not, because neural activity and attention fluctuate from moment to moment. So researchers present a faint stimulus many times at several intensities and record the percentage of "yes" responses.

Worked example (practice data). A listener hears a tone at five intensities, 40 trials each.

1. Find the lowest intensity where detection reaches 50%: intensity 3.
2. That intensity is the absolute threshold for this listener under these conditions.
3. Notice that intensity 2 was still detected on some trials. A stimulus below threshold is not "undetectable"; it is detected less than half the time.

A stimulus below the absolute threshold is sometimes called subliminal. Lab studies find that such stimuli can produce brief, small effects, but there is no good evidence that hidden messages produce lasting changes in what people buy or believe.

Concept Check — Thresholds and Transduction 🎯

⚖️ Difference Thresholds and Weber's Law

The difference threshold, or just-noticeable difference (JND), is the minimum difference between two stimuli that a person detects 50% of the time. The absolute threshold asks "is anything there?"; the difference threshold asks "did it change?"

Weber's law says that to be noticed, the difference between two stimuli must be a roughly constant proportion of the original stimulus, not a constant amount:

$\dfrac{\Delta I}{I} = k$

where $I$ is the starting intensity, $\Delta I$ is the JND, and $k$ is the Weber fraction, a constant for a given kind of stimulus.

Worked example (practice values). Suppose a person's Weber fraction for lifted weight is 0.02 (2%).

The same idea explains why one extra candle is obvious in a dark room lit by two candles but invisible on a cake with 60 candles, and why a 1 dollar price rise feels large on a 3 dollar coffee but trivial on a 900 dollar laptop.

How exact is it? Weber fractions differ from one sense to another and from person to person, and the law fits best in the middle range of intensities, less well at the extremes. On the exam you will be given any fraction you need; what you must know is the proportional logic.

Sensory adaptation

Sensory adaptation is diminished sensitivity to an unchanging stimulus. Receptors and early neurons fire less when stimulation stays constant, so after a while you stop noticing your watch, the hum of a refrigerator, or the smell of your own house. Adaptation frees the system to respond to change, which is usually what matters for survival.

• Vision resists adaptation because the eyes make constant tiny movements, so the image keeps shifting across receptors. In lab setups that hold an image perfectly still on the retina, the image fades within seconds.
• Adaptation is sensory: it happens in the receptors and early pathway, and you cannot will it away. Habituation (Unit 3) is a learned decrease in a behavioral response to a repeated stimulus.

Practice Calculations and Terms ✍️

Use Weber's law and the definitions above. Type a number or a single term.

🔗 Senses Working Together: Sensory Interaction and Synesthesia

The senses are not sealed off from one another. Sensory interaction is the constant process by which one sense influences another. It happens in everyone, all the time.

Synesthesia is different. In synesthesia, stimulation of one sensory system produces an automatic, consistent experience in another system. A person with grapheme–color synesthesia sees the letter A as red every time; others "hear" colors or "taste" words. It occurs in a minority of people, and each person's pairings stay stable across years.

🔑 AP Connection: If a scenario describes two senses combining to change one experience (flavor, speech), think sensory interaction. If it describes one stimulus producing a second, cross-sense experience for a particular person, think synesthesia.

Name the Concept 🔍

🧭 Keeping the Part 1 Terms Apart

🔑 AP Connection: Many wrong answers swap "absolute" and "difference" threshold. Ask whether the scenario starts from nothing (absolute) or from an existing stimulus that changes (difference).

Applied Scenarios — Detection and Change 🔬

🧠 Sensation and Sensory Processing

Part 2 of 7 — Vision: From Light to the Brain

🔑 Key Concept: The eye is a light-focusing device built around one receptor layer, the retina. Light is bent and focused onto the retina, rods and cones transduce it, and the signal travels the optic nerve to the thalamus and then to the occipital lobe.

The stimulus: light

Visible light is a narrow band of electromagnetic energy, roughly 400 to 700 nanometers in wavelength. Three physical properties map onto three experiences:

The eye, front to back

Accommodation is the lens changing shape to focus. For a near object the lens becomes rounder (thicker), bending light more; for a far object it flattens. The image that lands on the retina is upside down and left-right reversed; the brain works with that pattern without any need to "flip" it.

Order of light through the eye: cornea → pupil (sized by the iris) → lens → retina.

Concept Check — Structures of the Eye 🎯

🔬 Inside the Retina: Rods and Cones

The retina holds two kinds of photoreceptors. Each eye has roughly 120 million rods and roughly 6 million cones (common textbook estimates).

Light and dark adaptation. Walking into a dark theater, you see almost nothing at first; over the next 20–30 minutes the rods become far more sensitive (dark adaptation), and shapes emerge, though without color. Walking back into sunlight, the world is painfully bright until the system resets within a minute or so (light adaptation) and the cones take over again.

The signal's route out of the eye

1. Light passes through the front layers of the retina to reach the rods and cones at the back.
2. Rods and cones transduce light and pass the signal to bipolar cells.
3. Bipolar cells activate ganglion cells.
4. Ganglion-cell axons bundle together into the optic nerve, which exits at the blind spot.
5. At the optic chiasm, fibers from the inner half of each retina cross, so each side of the brain receives the opposite half of the visual field.
6. Signals relay through the thalamus to the primary visual cortex in the occipital lobe.

Worked example: why look slightly away from a faint star?

1. A faint star sends very little light, so only rods can respond.
2. Rods are scarce in the fovea and plentiful in the periphery.
3. Looking straight at the star puts its image on the cone-packed fovea, where it may vanish.
4. Looking a little to one side puts the image on rod-rich retina, and the star reappears.

The same logic explains why you can see shapes but not colors in a dark room: only rods are working.

Trace the Pathway 🔍

👓 When Focusing Goes Wrong

Memory hook: a nearsighted person sees near things clearly. A related change with age is presbyopia: the lens stiffens, accommodation weakens, and reading glasses become necessary for close work.

When the eye works but the brain does not

These two cases show that seeing depends on the brain, not just the eye.

Blindsight. After damage to the primary visual cortex, a person reports seeing nothing in part of the visual field. Yet when asked to guess, the person can point to or avoid objects there well above chance. The eyes and some visual pathways still carry information to other brain areas, but it does not reach conscious awareness.

Prosopagnosia ("face blindness"). A person sees faces clearly, including eyes, nose and mouth, but cannot recognize whose face it is, even a close family member's. Other objects are recognized normally. Like blindsight, it can follow damage to the brain's visual areas, mainly in the occipital lobes (including where they meet the temporal lobe on the underside of the brain). Some people acquire it after a stroke or injury; others are born with it.

Common mix-up: the blind spot is not damage. Everyone has one in each eye; you rarely notice it because the other eye covers that region and the brain fills in the gap.

Name the Structure ✍️

Type a single term.

Applied Scenarios — Vision 🔬

🧠 Sensation and Sensory Processing

Part 3 of 7 — Color Vision: Two Theories, Two Stages

🔑 Key Concept: Color vision is explained by two theories that are both right, at different stages. Trichromatic theory describes the cones in the retina. Opponent-process theory describes how neurons beyond the cones (ganglion cells, the thalamus and the visual cortex) recode the cone signals into opposing pairs.

Stage 1: Trichromatic (Young–Helmholtz) theory

Thomas Young and Hermann von Helmholtz proposed that the retina contains three kinds of color receptors. Later research confirmed three cone types, each most sensitive to a different range of wavelengths:

Cones sit mainly in the fovea, which is why color and fine detail are sharpest at the center of gaze.

Each cone type responds to a broad, overlapping range of wavelengths. No single cone "knows" the color. The brain reads hue from the ratio of activity across all three types.

Worked example: where does yellow come from?

1. A light of about 580 nm falls on the retina.
2. It excites red (long) cones strongly, green (medium) cones somewhat less, and blue (short) cones hardly at all.
3. That particular ratio is read as yellow.
4. A mix of "red" and "green" light that produces the same red-to-green cone ratio also looks yellow. This is why a phone or TV screen can show thousands of colors with only red, green and blue pixels: it reproduces cone ratios, not actual wavelengths.

Trichromatic theory explains color mixing of lights and the most common forms of color vision deficiency (a missing or altered cone type). What it cannot explain on its own are afterimages and why some color combinations never appear.

Concept Check — Trichromatic Theory 🎯

🔄 Stage 2: Opponent-Process Theory

Ewald Hering noticed two things trichromatic theory could not handle. People can picture a reddish-yellow (orange) or a bluish-green (teal), but nobody can picture a reddish-green or a bluish-yellow. And people with red–green color deficiency usually still see yellow. He proposed that color is coded in three opposing pairs:

Where does it happen? Not in the cones. After the cones, ganglion cells in the retina and neurons in the thalamus and visual cortex are excited by one member of a pair and inhibited by the other. A single opponent cell cannot signal "red" and "green" at the same moment, which is why reddish-green is never seen.

Afterimages

An afterimage is an image that persists after the stimulus is removed. The opponent theory explains its colors:

1. Stare at a red patch for about 30 seconds. The red side of the red–green channel fires steadily and adapts (fatigues).
2. Look at a white wall. White light stimulates both sides of each channel equally.
3. The fatigued red side responds weakly, so the green side wins.
4. You see a green patch on the wall.

Worked example. A flag printed in green, black and yellow stripes produces an afterimage in red, white and blue. Each stripe flips to the other member of its opponent pair.

Predict the Afterimage 🔍

Each person stares at a colored shape for 30 seconds, then looks at a plain white wall.

🧩 Putting the Two Theories Together

On the exam, the phrase "both theories are correct" is the safe synthesis: three cone types feed signals into opponent-coded neurons.

Color vision deficiency

"Color blindness" almost never means seeing no color at all. Color vision deficiency involves damage to or irregularities in one or more cone types or in the opponent ganglion cells (red/green, blue/yellow), so some colors are confused with others.

Milder forms also exist in which all three cone types work but one is shifted in sensitivity. Red–green deficiency runs in families and is far more common in males than in females.

Testing. Plates of colored dots (such as the Ishihara test) hide a number that a person with typical color vision sees and a person with red–green deficiency does not.

Linking it back. A person missing one cone type can match any color using just two lights, as trichromatic theory predicts. Yet many people with red–green deficiency still see yellow, which opponent-process theory explains: their blue–yellow channel still works.

Quick Recall — Color Vision ✍️

Type a single word or number.

Applied Scenarios — Color 🔬

🧠 Sensation and Sensory Processing

Part 4 of 7 — Hearing: From Sound Waves to Pitch

🔑 Key Concept: Hearing (audition) turns air-pressure waves into fluid waves inside the cochlea, where hair cells on a membrane inside it transduce the motion into neural signals. Pitch is coded two ways: by where the membrane vibrates most (place) and by how fast neurons fire (frequency and volley).

The stimulus: sound waves

Wavelength and frequency are two descriptions of the same property: when waves are packed closer together (shorter wavelength), more of them pass each second (higher frequency). Young adults typically hear frequencies from about 20 to 20,000 Hz; the upper limit drops with age. The decibel scale is logarithmic: every 10 dB step is a tenfold increase in sound intensity. Normal conversation is around 60 dB, and long or repeated exposure above about 85 dB can damage hair cells.

The ear in three stages

From the auditory nerve, signals relay through the thalamus to the auditory cortex in the temporal lobe. The three stages matter mostly for one exam skill: deciding whether a hearing loss is mechanical (outer and middle ear) or neural (cochlea and nerve).

Concept Check — Sound and the Ear 🎯

🎵 How Is Pitch Coded? Three Theories

Place details. High frequencies produce peak vibration near the base of the cochlea (the end nearest the middle ear); low frequencies spread toward the far end, the apex. That is why noise damage, which tends to harm hair cells near the base, often robs people of high-pitched sounds first.

Volley details. Picture a squad of soldiers firing in turns. No soldier fires faster than he can reload, but the squad together produces a rapid stream of shots. Neurons sharing the load can mark a 3,000 Hz tone even though no single neuron fires 3,000 times a second.

Worked example: which theory fits?

The modern view combines them: frequency and volley coding for lower pitches, place coding for higher pitches, with overlap in the middle.

Locating a sound

With two ears, a sound off to one side reaches the nearer ear slightly sooner and slightly louder (the head blocks some of the sound to the far ear). The brain uses those small time and intensity differences to locate the source. A sound directly in front, behind or overhead reaches both ears at the same time and intensity, so its location is harder to judge; tilting the head breaks the tie.

Quick Recall — Hearing ✍️

Type a single term.

🦻 Hearing Loss: Where Is the Break?

A cochlear implant uses a microphone and a small processor to turn sound into electrical signals that stimulate the auditory nerve directly, standing in for damaged hair cells. It works only if the auditory nerve itself is intact.

Why hair-cell damage matters: in humans, hair cells that die are not replaced. Noise damage builds up over years, which is why the 85 dB guideline focuses on duration as well as volume. Aging produces the same kind of gradual sensorineural loss, usually affecting high pitches first, which is why older adults often struggle with high voices and consonant sounds.

Common mix-ups

🔑 AP Connection: For a hearing-loss item, locate the break first. Before the cochlea means conduction; at the hair cells or nerve means sensorineural.

Applied Scenarios — Hearing 🔬

🧠 Sensation and Sensory Processing

Part 5 of 7 — Chemical and Body Senses

🔑 Key Concept: Smell and taste are chemical senses: receptors respond to molecules, not energy waves. The body senses (touch, pain, balance and body position) keep track of the skin, the tissues and where the body is in space. Two facts anchor this part: smell is the only sense not processed first in the thalamus, and pain is processed in both the body and the brain.

Smell (olfaction)

1. Airborne molecules dissolve in the mucus at the top of the nasal cavity.
2. Olfactory receptor cells there transduce them. Humans have several hundred receptor types, and each odor activates a particular combination of them, much as letters combine into words.
3. The receptor cells send signals to the olfactory bulb at the base of the brain.
4. From the bulb, signals go directly to smell areas of the cortex and to limbic structures such as the amygdala and hippocampus. Every other sense relays through the thalamus first; smell's main route does not.

That direct line to emotion and memory areas helps explain why a smell, such as a grandparent's kitchen or a school hallway, can bring back a vivid, emotional memory in an instant.

Pheromones are chemical signals released by an animal that are detected through the olfactory system and affect the behavior or physiology of others of the same species. In many animals they shape mating, alarm and territory (moths can find mates by pheromone trails). In humans, the evidence for pheromones that steer behavior is limited and debated.

Smell adapts quickly. Within minutes you stop noticing the smell of your own home or perfume, an example of the sensory adaptation from Part 1.

Concept Check — Smell 🎯

👅 Taste (gustation)

Gustation is the sense of taste. Chemicals dissolved in saliva reach taste receptor cells clustered in taste buds, most of them on the small bumps (papillae) of the tongue, with others in the mouth and throat. Taste receptor cells are replaced every week or two. Signals travel through the thalamus to taste areas of the cortex.

The six basic tastes in the CED:

Myth check: the old "tongue map" (sweet only at the tip, bitter only at the back) is wrong. Taste buds across the tongue respond to all basic tastes.

Supertasters, medium tasters and nontasters

People differ in the number of taste receptors (taste buds) on the tongue, and that number is related to how intensely they experience tastes.

Taste needs smell

The chemical senses work together to create what we call taste. Much of flavor comes from odor molecules that rise from the mouth to the nasal cavity as you chew. Without smell, taste is muted or lost: with a cold, food seems bland, and with a pinched nose, cherry and grape candies can be hard to tell apart. This is the sensory interaction from Part 1, at work in every meal.

Classify the Taste Experience 🔍

✋ Touch

The skin holds separate receptors for pressure, warmth, cold and pain. Their signals travel up the spinal cord, through the thalamus, to the somatosensory cortex in the parietal lobe.

Where does "hot" come from? There is no dedicated "hot" receptor. The sensation of hot is produced when warm and cold receptors are activated together. In a classic demonstration, a person grips a grill of alternating lukewarm and cool bars. Neither bar alone is hot, yet the grill feels burning hot, because warm and cold receptors fire at the same time.

🩹 Pain

Pain is processed both in the body and in the brain. Receptors in skin and tissue (nociceptors) respond to damaging stimuli, but how much pain a person feels also depends on attention, mood, expectations and context.

Gate control theory (Ronald Melzack and Patrick Wall, 1965) proposes that the spinal cord contains a neurological "gate" that can block pain signals or let them pass to the brain.

Phantom limb sensation occurs when people who have lost a limb still report sensation or pain where the limb used to be. No receptors exist there anymore, so the experience must be generated by the brain. Phantom limbs are strong evidence that pain is not only a message from the body; the brain can produce it on its own.

🧭 Balance and body position

When the vestibular sense and vision disagree, as when reading in a moving car, the conflict can produce motion sickness, another case of senses interacting.

Quick Recall — Chemical and Body Senses ✍️

Type a single term.

Applied Scenarios — Chemical and Body Senses 🔬

🧠 Sensation and Sensory Processing

Part 6 of 7 — Data Analysis and AP Exam Practice

🔑 Key Concept: Sensation is measured, not guessed. Exam items hand you detection percentages, JND values or ratings over time and ask what they show. The two free-response questions, the Article Analysis Question (AAQ) and the Evidence-Based Question (EBQ), ask you to read study summaries and reason from them.

Data skill 1: reading thresholds from detection data

Practice study (hypothetical). Researchers played a high-pitched tone at five intensities to 30 adults aged 18–25 and 30 adults aged 65–75. Each person heard each intensity 20 times; the table shows the mean percent of trials detected.

1. Absolute threshold = lowest intensity detected at least 50% of the time.
2. Younger group: 20 dB. Older group: 40 dB.
3. A higher threshold means lower sensitivity: the older group needs a louder tone.
4. Link to content: age-related sensorineural loss affects high pitches first, so this pattern is what Part 4 predicts.

Data skill 2: testing Weber's law

Practice data. One participant's JND for lifted weights at four starting weights:

The JND grows, but the ratio stays constant, which is exactly what Weber's law predicts. If the JND had stayed at 2 g for every starting weight, the data would contradict Weber's law.

Data skill 3: spotting adaptation

Practice data. Volunteers rated the strength of a constant room odor (0 = none, 10 = very strong): 9 at the start, 6 after 2 minutes, 3 after 5 minutes, 2 after 10 minutes. The odor never changed, but ratings fell: a textbook sensory adaptation curve.

Data Check — Thresholds, JND and Adaptation 📊

Use the practice data above.

📝 AAQ Walk-Through (7 points, 6 parts)

Practice study summary (hypothetical, written for this lesson). Researchers tested whether distraction increases tolerance for pain. They recruited 80 undergraduate volunteers, aged 18–22, from one university's introductory psychology course in exchange for course credit. Participants were randomly assigned to one of two conditions. In the distraction condition, participants played a demanding video game; in the control condition, they sat quietly. All participants placed one hand in ice water (about 3 °C). Pain tolerance was measured as the number of seconds before the participant removed the hand, with a maximum of 180 seconds. Before starting, all participants signed informed consent forms explaining the procedure, and they were told they could remove their hand and stop at any time. Everyone was debriefed afterward. Results: the distraction group kept their hands in the water for a mean of 112 seconds, compared with 68 seconds for the control group, and the difference was statistically significant. The researchers concluded that the results are consistent with gate control theory, which proposes that messages from the brain can close a spinal "gate" to pain.

Scoring habits that earn the points:

• Part A: name the method and point to the feature that makes it so (manipulation plus random assignment).
• Part B: give a measurable definition with units, not "how much pain they could take."
• Part E: use a specific detail from the summary (age range, one university, course credit), not "the sample was small."
• Part F: state the direction (supports or refutes) and connect the specific number to the hypothesis.

AAQ Practice — Pain Tolerance Study 🎯

Refer to the practice study summary above.

📚 EBQ Mini-Set (7 points, 3 parts)

Prompt: Develop and justify an argument about whether what we sense depends on the brain and not only on the sense organs.

Source 1 (practice summary, hypothetical). In a survey of 50 adults who had lost an arm or a leg, 40 reported feeling sensations, including pain, in the missing limb at least once a week a year after surgery.

Source 2 (practice case, hypothetical). A patient with damage to her left occipital lobe reports seeing nothing in her right visual field. When asked to guess where a light appears in that field, she points to the correct location on 80% of trials, far above the chance rate.

Source 3 (practice study, hypothetical). Among 120 adults, the number of taste buds counted on the tongue correlated positively with ratings of bitterness for black coffee (r = +0.58).

Notes on the mini-set:

• B and C must use different sources and different concepts. Using phantom limb twice would lose the C(ii) points.
• Source 3 fits a qualified claim: receptor differences (supertasters) also shape sensation. A strong writer can say the brain matters in addition to, not instead of, the sense organs.
• Cite by source number, as in "(Source 1)" or "According to Source 2."

Build the EBQ Response 🔍

EBQ Practice — Judge the Response 🎯

🧠 Sensation and Sensory Processing

Part 7 of 7 — Synthesis: One Model for Every Sense

🔑 Key Concept: Every sense follows the same chain: stimulus → receptor → transduction → pathway → brain area. Most exam items about sensation ask you to locate a scenario on that chain. Knowing where something happens (receptor, nerve, thalamus or cortex) answers questions about theories, disorders and data alike.

Concept map: the whole topic in one table

Rules that cut across every sense (Part 1)

Integrated Check — Locate It on the Chain 🎯

⚖️ The Pairs the Exam Loves to Swap

Worked example: one scenario, three concepts

Practice scenario. A concert drummer, age 58, notices that after a loud show, quiet conversations sound muffled for hours, and over the years he has had more trouble hearing high voices. His audiologist says the cells in his cochlea that code high frequencies are damaged.

1. Loudness: a loud show means high amplitude (many decibels).
2. Type of loss: damage to cochlear receptor cells is sensorineural loss, made worse by aging.
3. Pitch theory: losing high frequencies specifically, from damage at particular places in the cochlea, fits place theory.

Which Theory or Concept Explains It? 🔍

🚫 Misconceptions Across the Topic

🔑 AP Connection: In free response, name the specific concept ("opponent-process theory," "sensorineural hearing loss") and tie it to a detail of the scenario or source. A general phrase such as "the senses were affected" does not earn credit.

Final Recall ✍️

Type a single term.

Integrated Scenarios — Final Review 🔬