UNIT 1.6 · BIOLOGICAL BASES OF BEHAVIOR
Sensation — AP Psychology Study Notes
Understand sensory thresholds and transduction, work through Weber’s law, and compare vision, hearing and the other senses.
Topic overview
Sensation is the biological process by which our sensory receptors (eyes, ears, nose, skin, tongue) detect physical stimuli from the environment and convert them into neural signals that the brain can understand.
This process is called transduction — converting physical energy (light, sound waves, chemicals, pressure) into electrochemical signals that neurons can transmit.
Understanding sensation helps explain how we gather information about the world, which is the first step before our brains interpret and give meaning to that information through perception.
Sensation vs. Perception
Sensation
The detection of physical stimuli from the environment through sensory receptors.
Process:
- Light hits retina → electrical signal
- Sound waves vibrate cochlea → neural impulse
- Chemical molecules bind to receptors → signal
Perception
The brain's interpretation and organization of sensory information to give it meaning.
Process:
- Recognizing a face from light patterns
- Understanding words from sound waves
- Identifying the smell of coffee
Remember: Sensation detects and encodes stimuli; perception organizes and interprets them. Bottom-up information and top-down expectations can interact during perception, so the two processes are not perfectly separate stages.
Transduction: Converting Energy
From stimulus to sensory experience
- Stimulus energy
Light, pressure changes, chemicals or temperature - Receptor transduction
Stimulus changes receptor activity - Neural pathways
Signals are relayed and processed - Perception in context
Networks integrate input with attention and prior experience
What is Transduction?
Transduction converts stimulus energy into changes in receptor activity that can influence neural signaling. Not every receptor directly produces action potentials: rods and cones produce graded responses and communicate through retinal circuits before ganglion cells send spikes along the optic nerve.
Examples of Transduction by Sense:
- Vision: Light waves → electrical signals in retinal photoreceptors (rods and cones)
- Hearing: Sound waves → vibrations → electrical signals in cochlear hair cells
- Touch: Pressure → electrical signals in mechanoreceptors
- Taste: Chemical molecules → electrical signals in taste receptor cells
- Smell: Airborne chemicals → electrical signals in olfactory receptor neurons
Key Point: Without transduction, sensory information could not reach the brain because neurons only communicate through electrical and chemical signals.
Sensory Thresholds
Thresholds determine the minimum intensity needed to detect a stimulus and the smallest change we can notice.
Absolute Threshold
The absolute threshold is the minimum stimulus intensity required for detection 50% of the time.
Why 50%? Because detection isn't perfect—it varies due to neural noise, attention, and individual differences. The 50% cutoff provides a consistent, measurable point.
Historical threshold illustrations
The examples below illustrate high sensitivity under favorable conditions. They are not universal physiological limits; stimulus properties, conditions, methods and individuals affect estimates.
- Vision: A candle flame seen from 30 miles away on a dark, clear night
- Hearing: A watch ticking in a quiet room from 20 feet away
- Touch: A bee's wing falling on your cheek from 1 centimeter
- Smell: One drop of perfume diffused in a three-room apartment
- Taste: One teaspoon of sugar in 2 gallons of water
Difference Threshold (Just Noticeable Difference - JND)
The difference threshold or JND is the smallest detectable change in stimulus intensity that can be noticed 50% of the time.
Example: The smallest increase in volume you can detect on your phone, or the minimum weight difference you can feel when holding two objects.
The difference threshold changes depending on stimulus intensity — it's easier to notice a 1-pound difference between 1 lb and 2 lbs than between 50 lbs and 51 lbs.
Weber's Law
Weber’s law approximates the JND as a constant proportion of the starting intensity over a useful range of conditions. The proportion differs by sensory task and does not hold exactly at every intensity.
Weber's Law Formula:
- = Change in intensity (JND)
- = Original stimulus intensity
- = Weber's constant (varies by sense)
Example Explained:
If Weber's constant for weight is (2%), you need a 2% change to notice a difference:
- 10 pounds: Need 0.2 lb change ()
- 100 pounds: Need 2 lb change ()
- Result: Larger stimuli require larger absolute changes to be noticed
Memory Tip: Weber's Law means "the bigger the stimulus, the bigger the change needed to notice it".
Signal Detection Theory
Signal detection theory proposes that stimulus detection depends on both the stimulus intensity and the individual's physical and psychological state (attention, expectation, motivation).
Key Concepts:
- Sensitivity: Your ability to detect the stimulus when present
- Response bias: Your tendency to say "yes" or "no" regardless of whether the stimulus is present
- Context matters: You'll notice a dim light better in a dark room when you're alert and expecting it than in a bright room when you're distracted
Example: A radiologist looking for cancer in an X-ray — detection depends on image quality AND the doctor's experience, fatigue, and expectations.
Worked detection rate
If a learner reports a faint signal on 12 of 24 signal-present trials, the observed hit rate is . This single rate does not establish sensitivity or a reliable absolute threshold: false alarms, stimulus intensities, trial counts and response criterion also matter.
Sensory Adaptation
What is Sensory Adaptation?
Sensory adaptation is the process by which our sensory receptors become less sensitive to constant, unchanging stimuli over time.
Purpose: Adaptation helps the brain focus on new or changing stimuli by tuning out constant background information that isn't immediately important.
Examples of Sensory Adaptation:
- Touch: You stop noticing your watch or bracelet after wearing it for a few minutes
- Smell: You stop smelling your own perfume or the scent of your home
- Hearing: You tune out the constant hum of an air conditioner or refrigerator
- Temperature: Water in a pool feels cold at first but normal after swimming for a while
Vision adapts too
Light and dark adaptation and color aftereffects show that visual sensitivity changes with experience. Small eye movements help prevent a stationary retinal image from fading, but they do not eliminate adaptation.
Distinguish sensory adaptation, a change in sensitivity, from habituation, reduced responding or attention to a repeated, unimportant event. Tuning out an air-conditioner hum can involve both.
Vision: The Visual System
Vision is our dominant sense, providing detailed information about the environment through the detection and processing of light.
Eye Anatomy and Light Path
Light Path Through the Eye:
- Cornea: Clear outer covering that protects the eye and bends (refracts) light
- Pupil: Opening that controls how much light enters the eye
- Iris: Colored muscle that adjusts pupil size (dilates in darkness, constricts in brightness)
- Lens: Flexible structure that focuses light onto the retina through accommodation (changing shape)
- Retina: Light-sensitive layer at the back of the eye containing photoreceptors
- Optic nerve: Transmits visual information from retina to the brain
Accommodation
Accommodation is the process by which the lens changes shape to focus images clearly on the retina.
- Near objects: Lens becomes thicker (rounder) to bend light more
- Far objects: Lens becomes thinner (flatter) to bend light less
Vision Problems:
- Myopia (Nearsightedness): Eyeball too long or lens too thick → light focuses in front of retina → distant objects blurry
- Hyperopia (Farsightedness): Eyeball too short or lens too thin → light focuses behind retina → close objects blurry
The Retina: Where Transduction Occurs
The retina is the light-sensitive layer at the back of the eye that contains photoreceptors (rods and cones) that transduce light into neural signals.
Key Retinal Structures:
- Fovea: Central area of the retina with the highest concentration of cones; provides sharpest, most detailed vision
- Blind spot (optic disc): Where the optic nerve exits the eye — no photoreceptors here, so there's a gap in vision
- Brain fills in the blind spot using surrounding visual information, so we don't normally notice it
Photoreceptors: Rods and Cones
| Feature | Rods | Cones |
|---|---|---|
| Location | Peripheral retina | Concentrated in fovea (center) |
| Number | ~120 million per eye | ~6 million per eye |
| Sensitivity | Very sensitive to light (low light/night vision) | Less sensitive (need bright light) |
| Color Vision | No (black and white only) | Yes (detect color) |
| Detail | Low detail | High detail (sharp vision) |
| Function | Dark adaptation, peripheral vision, motion detection | Color vision, fine detail, daytime vision |
Memory Tip: "Rods for low-light vision" (peripheral vision for safety), "Cones for Color" (center of vision).
Theories of Color Vision
Two complementary theories explain how we perceive color at different stages of visual processing.
1. Trichromatic Theory (Young-Helmholtz)
Explains color detection at the retina level (how cones work).
Key Ideas:
- Three types of cones detect different wavelengths:
- Short wavelength (S cones): Blue light
- Medium wavelength (M cones): Green light
- Long wavelength (L cones): Red light
- Color perception results from the pattern of activity across all three cone types
- Explains how we see millions of colors by combining signals from three cone types
2. Opponent-Process Theory (Hering)
Explains color processing after the retina (in ganglion cells and brain).
Key Ideas:
- Colors are processed in opposing pairs:
- Red vs. Green
- Blue vs. Yellow
- Black vs. White (brightness)
- Explains afterimages — stare at red, see green when you look away
- Explains why you can't see "reddish-green" or "bluish-yellow"
Both theories are correct: Trichromatic explains cones, opponent-process explains neural processing afterward.
Hearing: The Auditory System
Hearing enables us to detect and interpret sound waves, providing crucial information about our environment.
Properties of Sound Waves
Frequency → Pitch
Frequency: Number of sound waves per second, measured in Hertz (Hz).
Pitch: How high or low we perceive a sound to be — high frequency = high pitch, low frequency = low pitch.
Amplitude → Loudness
Amplitude: The magnitude of a sound wave’s pressure variation. Sound-pressure level is commonly expressed in decibels (dB), a logarithmic level relative to a reference. Loudness is a perceptual experience, influenced by frequency and context as well as physical level.
Loudness: How intense or loud we perceive a sound — greater amplitude = louder sound.
Ear Anatomy and Sound Path
Sound Path Through the Ear:
- Outer ear (pinna): Collects and funnels sound waves into the ear canal
- Eardrum (tympanic membrane): Vibrates when sound waves hit it
- Middle ear (ossicles): Three tiny bones (hammer, anvil, stirrup) amplify vibrations
- Oval window: Membrane that transfers vibrations to the inner ear
- Cochlea: Fluid-filled, snail-shaped structure in the inner ear where transduction occurs
- Basilar membrane: Vibrates in response to sound, bending hair cells
- Hair cells: Receptor cells that transduce mechanical vibrations into neural signals
- Auditory nerve: Carries signals to the brain
Theories of Pitch Perception
Three theories explain how we perceive different pitches; each works best for different frequency ranges.
1. Place Theory (High Frequencies)
Different frequencies vibrate different parts of the basilar membrane.
- High frequencies: Vibrate the base (near oval window)
- Low frequencies: Vibrate the apex (far end)
- Brain determines pitch by which part of the membrane is stimulated
2. Frequency Theory (Low Frequencies)
The firing rate of auditory neurons matches the frequency of the sound wave.
- For a low-frequency tone, neural firing can be phase-locked to the waveform. A 100 Hz tone does not require every auditory neuron to fire exactly 100 times each second
- Works best for frequencies below 1000 Hz
- Limitation: Neurons can't fire faster than ~1000 Hz
3. Volley Theory (Mid-Range Frequencies)
Groups of neurons fire in alternating, staggered patterns to represent higher frequencies.
- Different neurons can fire on different cycles while preserving timing information over a range of frequencies; boundaries are approximate
- Bridges the gap between frequency and place theories
- Combines aspects of both approaches
Types of Hearing Loss
Conduction Deafness
Damage to the outer or middle ear that prevents sound waves from reaching the cochlea.
- Causes: Earwax buildup, ruptured eardrum, damaged ossicles
- Often treatable medically or with hearing aids
Sensorineural Deafness (Nerve Deafness)
Damage to the cochlea's hair cells or auditory nerve.
- Causes: Aging, prolonged loud noise exposure, disease, certain medications
- Usually permanent because hair cells don't regenerate
- Treatment: Hearing aids or cochlear implants
Other Senses
Taste (Gustation)
Gustation is the chemical sense that detects dissolved substances in the mouth through taste receptor cells in taste buds.
Basic Taste Qualities:
- Sweet — sugars and some proteins
- Sour — acids
- Salty — sodium and minerals
- Bitter — potentially toxic compounds
- Umami — savory, protein-rich foods (glutamate)
Note: Most of what we call "taste" is actually flavor, which combines taste + smell + texture + temperature.
Smell (Olfaction)
Olfaction detects airborne chemical molecules through olfactory receptor neurons in the nasal epithelium.
Unique Features:
- Olfactory receptor signals reach the olfactory bulb and then primary olfactory cortex without an obligatory initial thalamic relay. Later olfactory processing can involve the thalamus, and connections with emotion and memory networks help explain odor associations.
- Direct connections to emotion and memory centers (amygdala, hippocampus)
- Explains why smells trigger powerful emotional memories
Sensory Interaction: Taste and smell work together to create flavor perception through retronasal olfaction (smell from inside the mouth).
Touch and Pain (Somatosensation)
The skin contains various specialized receptors that detect pressure, temperature, and pain.
Types of Touch Receptors:
- Mechanoreceptors: Detect pressure and vibration
- Thermoreceptors: Detect temperature (hot and cold)
- Nociceptors: Respond to potentially tissue-damaging stimuli; pain is an experience produced by nervous-system processing, rather than a property directly measured by one receptor.
Gate Control Theory of Pain
Pain signals can be blocked by a neural "gate" in the spinal cord. Rubbing an injury activates touch fibers that close the gate, reducing pain signals reaching the brain.
Kinesthesis (Body Position Sense)
Kinesthesis is the sense that provides information about body position and movement through receptors in muscles, tendons, and joints.
Enables coordinated movement without looking — typing, walking, catching a ball all rely on kinesthetic feedback.
Vestibular Sense (Balance)
The vestibular system monitors head position and movement, providing information about balance and spatial orientation.
Key Structures in the Inner Ear:
- Semicircular canals: Three fluid-filled tubes that detect rotational head movements
- Otolith organs (utricle and saccule): Detect linear acceleration and head tilt
Function: Works with vision and kinesthesis to maintain balance and coordinate movement.
Sensory Interaction
Senses Work Together
Sensory interaction occurs when different senses combine to create a unified perceptual experience.
Examples:
- Taste + Smell = Flavor: Holding your nose makes food taste bland
- Vision + Hearing: Watching lips move helps understand speech (McGurk effect)
- Vision + Vestibular + Kinesthesis: Work together to maintain balance
Synesthesia
Synesthesia is a rare condition where stimulation of one sense automatically triggers another sensory experience.
Examples: Seeing colors when hearing music, tasting shapes, associating numbers with colors. It involves stable additional experiences, such as colors associated with letters; its mechanisms are more complex than one simple cross-activation rule.
AP Exam Strategy
Multiple Choice Tips
- Know the difference between sensation (detection) and perception (interpretation)
- Master thresholds: absolute threshold, JND, Weber's Law formula
- Understand transduction — how each sense converts stimuli to neural signals
- Vision: Rods vs. cones, trichromatic vs. opponent-process theories, myopia vs. hyperopia
- Hearing: Place, frequency, and volley theories; conduction vs. sensorineural deafness
- Know which sense bypasses the thalamus (olfaction)
Free Response Question (FRQ) Tips
- Use precise terminology: transduction, photoreceptors, basilar membrane, olfactory bulb
- Explain pathways: trace the path of stimuli through sensory organs to the brain
- Apply Weber's Law: calculate or explain JND in context
- Compare theories: trichromatic vs. opponent-process, place vs. frequency
- Link to behavior: explain how sensory deficits affect daily functioning
Quick Review Summary
The Big Picture
Sensation is the detection of physical stimuli through sensory receptors and conversion into neural signals (transduction). Sensory thresholds define detection limits (absolute threshold) and noticeable differences (JND/Weber's Law). Vision uses rods (peripheral, low light) and cones (fovea, color, detail) explained by trichromatic and opponent-process theories. Hearing processes sound waves through the cochlea with place, frequency, and volley theories explaining pitch. Other senses include taste, smell (bypasses thalamus), touch/pain, kinesthesis, and vestibular sense. Sensory interaction combines multiple senses for unified perception.
Essential Concepts
- Sensation vs. perception
- Transduction
- Absolute threshold
- Difference threshold (JND)
- Weber's Law:
- Signal detection theory
- Sensory adaptation
- Rods and cones
- Fovea and blind spot
- Accommodation
- Myopia vs. hyperopia
- Trichromatic theory
- Opponent-process theory
- Frequency and amplitude
- Cochlea and hair cells
- Place theory
- Frequency theory
- Volley theory
- Conduction vs. sensorineural deafness
- Five basic tastes
- Olfaction differs in its initial thalamic relay; later processing can involve the thalamus
- Gate control theory
- Kinesthesis
- Vestibular sense
- Synesthesia
Sensation FAQs
What is the difference between an absolute threshold and a JND?
An absolute threshold concerns detecting a stimulus, conventionally 50% of the time. A JND concerns detecting a difference between stimuli; Weber’s law approximates that difference as a proportion of starting intensity.
Does vision experience sensory adaptation?
Yes. Light and dark adaptation and color aftereffects demonstrate visual adaptation. Small eye movements reduce fading of stationary retinal images but do not eliminate adaptation.