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AP® Psychology
Every unit of the redesigned AP® Psychology, biological, cognitive, developmental, social, and clinical, built for the new Article Analysis & Evidence-Based FRQs.
Start Unit 1 free. Unit 1: Biological Bases of Behavior is open to everyone, no account needed. Other topics are locked.
Unit 1: Biological Bases of Behavior
THE BIG PICTURE. Unit 1 builds the biological foundation for everything else in AP Psychology: neurons, the nervous and endocrine systems, the brain, sleep and consciousness, sensation, and perception. The unit weighs 15–25% of the AP exam (each of the 5 units is weighted equally). Mastery of brain anatomy, neurotransmitters, and sensory systems sets up Units 2 and 3 (cognition, learning).
THE NEURON & ACTION POTENTIAL
Structure of a Neuron Dendrites receive signals; the cell body integrates them; the axon (insulated by myelin) propagates the action potential to terminal buttons that release neurotransmitters across the synaptic cleft. Multiple sclerosis is a demyelination disease: signal conduction slows.
A NEURON is the basic cell of the nervous system. Parts: DENDRITES (receive input), CELL BODY (soma), AXON (carries signal away), MYELIN SHEATH (fatty insulation that speeds conduction), TERMINAL BUTTONS (release neurotransmitters), SYNAPSE / SYNAPTIC GAP (the space between neurons).
ACTION POTENTIAL, an electrical signal
- At rest, the neuron's interior is negative (~−70mV), the resting potential.
- Stimulation opens sodium (Na⁺) channels → Na⁺ rushes in → cell becomes positive → DEPOLARIZATION.
- If the THRESHOLD is crossed (~−55mV), an ALL-OR-NONE action potential fires down the axon.
- POTASSIUM (K⁺) then exits → REPOLARIZATION → brief REFRACTORY PERIOD before another firing.
- Speed depends on myelin (myelinated axons fire much faster: saltatory conduction jumping between NODES OF RANVIER).
The Action Potential Sodium ions rushing in depolarize the neuron; once threshold is crossed, a full spike fires (all-or-none). Potassium ions flowing out repolarize it, and a brief refractory period follows before it can fire again.
Beyond the single neuron (CED 1.3.A, 1.3.B): the nervous system has two main cell types. NEURONS transmit information; GLIAL CELLS give structure, insulate (myelin), help communication, and remove waste. When transmission breaks down, behavior changes: in MULTIPLE SCLEROSIS the immune system damages myelin, so signals slow or misfire (weakness, numbness, vision problems); in MYASTHENIA GRAVIS the immune system disrupts acetylcholine signaling at the muscles, causing muscle weakness that worsens with use.
WORKED EXAMPLE: THRESHOLD AND THE ALL-OR-NONE PRINCIPLE
A researcher stimulates one neuron three times. The weak stimulus moves the membrane from mV to mV, the medium one to mV, and the strong one to mV. Which stimuli produce an action potential, and which one produces the biggest spike?
- Step 1, compare with threshold (about mV). The weak and medium stimuli stay below threshold, so the membrane drifts back to resting potential and nothing fires.
- Step 2, the strong stimulus crosses threshold, so the neuron fires a full action potential.
- Step 3, apply all-or-none. Every action potential this neuron fires reaches the same peak. A stronger stimulus is coded by more frequent firing and by more neurons firing, not by a bigger spike.
- Trap answer: "The strong stimulus produced a larger action potential." That contradicts the all-or-none principle.
NEUROTRANSMITTERS
Chemical messengers released at synapses. Key ones:
- ACETYLCHOLINE (ACh): muscle movement, memory, attention. Loss in Alzheimer's.
- DOPAMINE: reward, motivation, voluntary movement. Excess linked to schizophrenia; loss to Parkinson's; underlies addiction.
- SEROTONIN: mood, sleep, appetite. Low levels linked to depression; SSRIs target it.
- NOREPINEPHRINE: alertness, arousal. Low levels linked to depression.
- GABA: major inhibitory neurotransmitter. Low levels linked to seizures, anxiety.
- GLUTAMATE: major excitatory neurotransmitter. Excess linked to migraines, seizures.
- ENDORPHINS: natural painkillers; "runner's high"; opioid drugs mimic.
- SUBSTANCE P: pain perception.
Neurotransmitters bind postsynaptic receptors. AGONISTS mimic or boost a transmitter; ANTAGONISTS block it. After release, transmitters are removed by REUPTAKE, enzymatic breakdown, or diffusion.
Agonists, Antagonists, and Reuptake Inhibitors Agonists mimic or boost a neurotransmitter, antagonists block its receptors, and reuptake inhibitors keep it in the synapse longer (CED 1.3.C.1).
WORKED EXAMPLE: AGONIST, ANTAGONIST, OR REUPTAKE INHIBITOR?
Classify each drug action, then state the effect on neural firing (CED 1.3.C.1).
- A medication blocks the reabsorption of serotonin into the sending neuron. Reuptake inhibitor (an SSRI). Serotonin stays in the synapse longer, so the receiving neuron is stimulated more.
- A medication occupies dopamine receptors without activating them and reduces a patient's hallucinations. Antagonist. It reduces dopamine signaling, which fits the dopamine hypothesis of schizophrenia.
- Morphine binds to the same receptors as endorphins and relieves pain. Agonist. It mimics the body's natural painkiller.
- Alcohol strengthens the effect of GABA. It acts like a GABA agonist; because GABA is inhibitory, overall neural activity drops, which is why alcohol is classed as a depressant.
PRACTICE: NAME THE NEUROTRANSMITTER
| Scenario | Neurotransmitter | Why |
|---|---|---|
| A person with Parkinson's disease has tremors and trouble starting movements | Dopamine | Loss of dopamine-producing neurons disrupts voluntary movement |
| A marathoner feels less pain in the final miles | Endorphins | The body's natural opioids reduce pain |
| A paper cut sends a sharp pain signal to the brain | Substance P | Carries pain messages |
| A seizure involves runaway neural excitation | Glutamate (too much) or GABA (too little) | Excitatory vs inhibitory balance is off |
| Muscles weaken with use in myasthenia gravis | Acetylcholine | ACh signaling at the muscles is disrupted |
| A student feels alert and keyed up before a speech | Norepinephrine | Arousal and alertness |
NERVOUS SYSTEM ORGANIZATION
- CENTRAL NERVOUS SYSTEM (CNS): brain + spinal cord.
- PERIPHERAL NERVOUS SYSTEM (PNS): all other nerves.
- SOMATIC: voluntary skeletal muscle control; sensory input.
- AUTONOMIC: involuntary (organs, glands).
- SYMPATHETIC: "fight or flight": pupils dilate, heart races, digestion slows, adrenaline surges.
- PARASYMPATHETIC: "rest and digest": pupils constrict, heart slows, digestion resumes.
- REFLEX ARC: sensory neuron → spinal cord (interneuron) → motor neuron, bypassing brain for fast response (knee jerk).
THE ENDOCRINE SYSTEM
Slower, longer-lasting communication via HORMONES in bloodstream. The redesigned AP Psychology CED narrows endocrine content to a short list of behaviorally relevant hormones: you do NOT need to memorize a full gland chart. The pituitary appears only as it relates to the brain/limbic system. Hormones to know:
- ADRENALINE (epinephrine): released by adrenal glands during stress; drives sympathetic "fight or flight" (racing heart, energy mobilization).
- LEPTIN: signals SATIETY (fullness) from fat cells; suppresses appetite.
- GHRELIN: signals HUNGER from the stomach; stimulates appetite. Leptin and ghrelin work in opposition to regulate eating.
- MELATONIN: released by the pineal gland; regulates the CIRCADIAN RHYTHM and sleep onset.
- OXYTOCIN: released by the pituitary; involved in bonding, trust, childbirth, lactation.
- PITUITARY ("master gland"): referenced for its links to the hypothalamus and limbic system; releases oxytocin among others.
THE BRAIN
The brain is organized from bottom (older, more automatic structures) to top (newer, more complex ones):
HINDBRAIN / BRAINSTEM:
- MEDULLA: heartbeat, breathing.
- PONS: sleep, arousal, coordination.
- CEREBELLUM: coordination, balance, procedural memory.
- RETICULAR FORMATION (running through brainstem): arousal, alertness.
LIMBIC SYSTEM (subcortical structures for emotion and memory)
- THALAMUS: sensory relay (all senses except smell route through here).
- HYPOTHALAMUS: homeostasis, drives, links to endocrine system.
- HIPPOCAMPUS: formation of new explicit memories. Damage → anterograde amnesia (H.M.).
- AMYGDALA: fear, aggression, emotional memory.
The Four Lobes of the Cerebral Cortex Frontal (planning, judgement, motor control, Broca's area), parietal (somatosensory cortex, spatial processing), temporal (hearing, Wernicke's area, memory), occipital (visual processing). Exam questions often require linking damage to a specific lobe to a specific function lost.
Phineas Gage (1848) A 25-year-old railroad foreman survived a 3-foot iron rod passing through his left frontal lobe. His personality, judgement, and impulse control changed dramatically: one of the earliest well-documented cases linking the frontal lobe to personality and executive function.
FOREBRAIN / CEREBRAL CORTEX:
- FRONTAL LOBE : motor cortex, executive function, planning, personality, Broca's area (speech production: damage causes Broca's aphasia, halting speech).
- PARIETAL LOBE: somatosensory cortex (touch, body awareness).
- TEMPORAL LOBE: auditory processing, Wernicke's area (language comprehension: damage causes fluent but nonsensical speech).
- OCCIPITAL LOBE: visual processing.
HEMISPHERES & LATERALIZATION:
- LEFT HEMISPHERE: language (most people), logical/sequential thinking.
- RIGHT HEMISPHERE: spatial reasoning, faces, music, holistic processing.
- CORPUS CALLOSUM: connects hemispheres. SPLIT-BRAIN STUDIES (Sperry, Gazzaniga) show each hemisphere can act independently when severed.
- CONTRALATERAL CONTROL: left hemisphere controls right body, and vice versa.
- BRAIN PLASTICITY: ability to reorganize after damage; greatest in young brains.
RESEARCH METHODS for brain study: lesions (Phineas Gage: frontal lobe damage altered personality), EEG (electrical), CT/MRI (structure), PET/fMRI (function: what's active during a task).
Testing a Split-Brain Patient Because each visual field projects to the opposite hemisphere and the corpus callosum is cut, the speaking left hemisphere never receives the word, but the right hemisphere can still guide the left hand.
CASE STUDY: SPLIT-BRAIN PATIENTS (SPERRY AND GAZZANIGA, 1960s)
Aim. To find out what each cerebral hemisphere can do on its own.
Method. Roger Sperry and his student Michael Gazzaniga tested patients whose corpus callosum had been cut to control severe epilepsy. Words or pictures were flashed briefly to one side of a fixation point, so the information reached only the opposite hemisphere. Patients responded by speaking or by using one hand to find an object by touch.
Findings. Information sent to the left hemisphere could be named aloud. Information sent only to the right hemisphere usually could not be named, yet the left hand could pick out the matching object. Sperry shared the 1981 Nobel Prize in Physiology or Medicine for discoveries about the functional specialization of the hemispheres.
Evaluation.
- Strength: tight control over which hemisphere receives the input makes the comparison clear.
- Limitation: a small number of patients whose brains had been shaped by years of seizures, so generalizing to typical brains needs caution.
- Myth alert: the research does not support "left-brained" and "right-brained" personality types. In an intact brain the hemispheres constantly share information.
PRACTICE: WHICH RESEARCH TOOL OR DESIGN? (CED 1.1.A.3, 1.4.A.7)
| Research question | Best tool or design | Why |
|---|---|---|
| Is a sleeping volunteer in REM right now? | EEG | Records electrical activity moment by moment; REM waves look like waking waves |
| Which areas become more active when people view faces? | fMRI | Tracks changes in blood oxygen that follow neural activity |
| What happens to a rat's eating when part of its hypothalamus is destroyed? | Lesioning | Damaging a structure tests what it does (animal research) |
| How did an iron rod through the frontal lobe change one man's personality? | Case study | In-depth study of one person (Phineas Gage) |
| Is a trait partly heritable? | Twin, family, or adoption study | Compare identical vs fraternal twins, or adopted children with biological vs adoptive parents |
SLEEP & CONSCIOUSNESS
Consciousness = awareness of self and environment. Sleep follows ~90-minute cycles through stages:
- NREM 1: light sleep; hypnagogic sensations; alpha → theta waves.
- NREM 2: sleep spindles, K-complexes; ~50% of total sleep.
- NREM 3: deep slow-wave sleep; delta waves; physical restoration; sleepwalking, night terrors.
- REM: rapid eye movement; vivid dreams; voluntary muscles paralyzed (atonia); brain activity ~waking.
A Night of Sleep Cycles Sleepers pass through NREM 1, 2, and 3 and then REM about every 90 minutes. Deep NREM 3 dominates early cycles; REM periods get longer toward morning.
Timing matters (CED 1.5.A): NREM 3 is longest early in the night and shrinks later, while REM periods lengthen toward morning. REM REBOUND is extra REM after a person has been deprived of it. JET LAG and SHIFT WORK push the circadian rhythm out of step with the clock, which hurts alertness and performance.
PRACTICE: WHICH SLEEP DISORDER? (the CED's list)
| Description | Disorder |
|---|---|
| Falls asleep suddenly during a class, sometimes entering REM within minutes | Narcolepsy |
| Snores loudly, stops breathing many times a night, wakes up exhausted | Sleep apnea |
| Kicks and punches while acting out a dream | REM sleep behavior disorder (the normal REM muscle paralysis fails) |
| Walks around the house during deep NREM 3 sleep and remembers nothing | Somnambulism (sleepwalking) |
| Has trouble falling or staying asleep for months | Insomnia |
SLEEP DISORDERS: insomnia, narcolepsy (sudden sleep attacks), sleep apnea (breathing stops), night terrors (beyond the AP exam), REM sleep behavior disorder.
THEORIES OF DREAMS (AP-testable)
- ACTIVATION-SYNTHESIS (Hobson): brain stem fires randomly during REM; cortex weaves the signals into a story.
- INFORMATION PROCESSING / CONSOLIDATION: dreams help consolidate the day's memories.
- Freud's psychoanalytic theory of dreams (wish fulfillment; manifest vs latent content) is historically important but is outside the scope of the current AP Psychology Exam: know it as context, don't cite it as the answer.
CIRCADIAN RHYTHMS: ~24-hour cycle regulated by suprachiasmatic nucleus in hypothalamus; melatonin from pineal gland.
WORKED EXAMPLE: ARTICLE ANALYSIS, SLEEP AND MEMORY
Practice summary (invented for this guide). Researchers recruited 60 college students through a campus email. At 9 p.m. everyone learned 40 word pairs. Participants were randomly assigned either to sleep normally in the lab (n = 30) or to stay awake all night under supervision (n = 30). Both groups slept normally the next night; two days after learning, everyone was tested. The sleep group recalled a mean of 31 pairs (SD = 4); the awake group recalled 25 (SD = 6). The difference was statistically significant (p < .01) with an effect size of d = 1.2. Students gave informed consent, were told they could withdraw at any time, and were debriefed.
- Research method: experiment. The researchers manipulated sleep and used random assignment.
- IV: sleep vs no sleep on the night after learning. DV: number of word pairs recalled out of 40 (the operational definition of memory).
- Statistics: p < .01 means the difference is unlikely to be due to chance. An effect size of 1.2 is large (the CED treats 0.8 or more as large). The larger SD in the awake group means its scores were more spread out.
- Ethics: informed consent, right to withdraw, supervision during sleep loss to protect participants from harm, and debriefing.
- Generalizability: a convenience sample of college volunteers may not represent older adults or people with sleep disorders.
- Concept link: the results support the consolidation theory of sleep (sleep helps organize and store memories).
PSYCHOACTIVE DRUGS alter consciousness
- DEPRESSANTS (alcohol, barbiturates, benzodiazepines): slow neural activity; relax muscles.
- STIMULANTS (caffeine, nicotine, cocaine, amphetamines, methamphetamine): speed up neural activity; increase dopamine.
- HALLUCINOGENS (LSD, psilocybin, marijuana): distort perception.
- OPIOIDS (heroin, morphine, oxycodone): bind endorphin receptors; powerful pain relief; highly addictive.
TOLERANCE = needing more for same effect. WITHDRAWAL = unpleasant symptoms when stopping. PHYSICAL vs PSYCHOLOGICAL DEPENDENCE.
SENSATION
SENSATION = the bottom-up process of detecting physical stimuli with sensory receptors. (In the redesigned CED, PERCEPTION: Gestalt, depth, constancies, perceptual set: lives in Unit 2: Cognition, NOT here.)
THRESHOLDS:
- ABSOLUTE THRESHOLD: minimum intensity detected 50% of the time.
- DIFFERENCE THRESHOLD / JND (just-noticeable difference): smallest detectable change.
- WEBER'S LAW: JND is a constant proportion of the stimulus (2% for weight; 8% for light/brightness).
- SIGNAL DETECTION THEORY: detection depends on intensity AND psychological state (expectations, motivation).
- SENSORY ADAPTATION: diminished sensitivity with constant stimulation.
WORKED EXAMPLE: WEBER'S LAW
Suppose the Weber fraction for lifted weight is about 2%. A hiker carries a 50-pound pack. How much weight must be added before the hiker notices the change? What about a 100-pound pack?
- Step 1: JND = 2% of 50 pounds = 1 pound.
- Step 2: JND = 2% of 100 pounds = 2 pounds. Adding 1.5 pounds to the heavier pack goes unnoticed.
- Conclusion: the just-noticeable difference is a constant proportion of the original stimulus, not a constant amount.
VISION: light enters cornea → pupil → lens (focuses) → retina (rods/cones) → optic nerve → thalamus → occipital lobe.
- RODS: peripheral, dim light, no color, very sensitive.
- CONES: center (fovea), color, fine detail, need bright light.
- BLIND SPOT: where optic nerve exits retina (no receptors).
- TRICHROMATIC THEORY (Young-Helmholtz): three cone types: red, green, blue.
- OPPONENT-PROCESS THEORY (Hering): color processed in opposing pairs (red/green, blue/yellow, black/white). Explains afterimages.
- Both theories work: trichromatic at the cone level, opponent-process beyond.
HEARING: sound waves → outer ear → eardrum → middle ear (hammer/anvil/stirrup) → cochlea (basilar membrane with hair cells) → auditory nerve → temporal lobe.
- PLACE THEORY: pitch perception based on WHERE on the basilar membrane is stimulated (high frequencies near base).
- FREQUENCY THEORY: pitch matches the frequency of nerve impulses (better for low frequencies).
- VOLLEY THEORY: groups of neurons fire in rapid alternating volleys to encode mid-range pitches beyond a single neuron's firing rate.
- Most pitch perception combines these mechanisms.
OTHER SENSES: taste (sweet, sour, salty, bitter, umami, and oleogustus/fat); smell (only sense not routed through thalamus; tied to limbic system, hence emotional triggers); touch (pressure, warmth, cold, pain: gate-control theory of pain); kinesthesis (body part position); vestibular sense (balance, in inner ear).
PRACTICE: WHICH SENSORY CONCEPT? (CED 1.6)
| Scenario | Concept | Why |
|---|---|---|
| After an infection damages the tiny bones of the middle ear, sounds seem muffled | Conduction deafness | Sound cannot be carried mechanically to the cochlea |
| Years of loud concerts damage hair cells in the cochlea | Sensorineural deafness | Damage to receptor cells or the auditory nerve |
| A person recognizes a friend's voice but not the friend's face | Prosopagnosia | Damage to face-processing areas of the visual brain |
| After occipital damage, a patient says they see nothing but points to objects far better than chance | Blindsight | Some visual information is processed without awareness |
| After staring at a red square, a person sees a green square on a white wall | Opponent-process theory | Red/green ganglion cells rebound |
| Food tastes bland during a head cold | Sensory interaction | Smell contributes to taste |
| A musician sees colors when hearing certain notes | Synesthesia | One sense is experienced through another |
| An amputee feels itching in a missing hand | Phantom limb sensation | The brain still produces sensations for the lost limb |
| Distant objects look blurry because the lens focuses images in front of the retina | Nearsightedness | Faulty accommodation |
| Someone finds coffee and broccoli intensely bitter and has many taste receptors | Supertaster | More receptors, stronger taste sensitivity |
EXAM CONNECTIONS. Expect questions that ask you to apply biological concepts to a scenario (e.g., "explain how a neuron fires when a person touches a hot stove": should reference receptors, action potential, neurotransmitters, brain region, motor response). Distinguish brain regions by function (medulla = breathing; hippocampus = memory; amygdala = fear). Match neurotransmitters to disorders (low serotonin/depression; excess dopamine/schizophrenia; low ACh/Alzheimer's). Identify research findings of classic studies (Phineas Gage, split brain, Sperry). Distinguish bottom-up (sensation-driven) from top-down (knowledge-driven) processing: the latter is technically a Unit 2 topic.
Key Terms
Neuron
Basic cell of the nervous system. Receives input via dendrites; sends action potential down axon; releases neurotransmitters at synapse.
Action Potential
Electrical signal that travels down a neuron's axon. All-or-none: fires fully if threshold (~−55mV) reached, otherwise not at all. Sodium ions rush in (depolarization), then potassium ions exit (repolarization).
Synapse
Junction between two neurons; the gap (synaptic cleft) where neurotransmitters are released from one neuron's terminal buttons and bind to the next neuron's receptors.
Neurotransmitters
Chemical messengers crossing the synapse. Examples: dopamine (reward, movement), serotonin (mood), acetylcholine (memory, muscles), GABA (inhibitory), glutamate (excitatory), endorphins (pain relief).
Sympathetic vs Parasympathetic Nervous System
Two branches of the autonomic nervous system. Sympathetic: "fight or flight": pupils dilate, heart races, adrenaline surges. Parasympathetic: "rest and digest": pupils constrict, heart slows, digestion resumes.
Endocrine System
Network of glands releasing hormones into bloodstream. Slower than nervous system but longer-lasting. The pituitary (the "master gland," part of the limbic system in the CED) is the only gland the exam covers. CED hormones: adrenaline (fight or flight), leptin (satiety), ghrelin (hunger), melatonin (sleep), oxytocin (bonding).
Cerebral Cortex Lobes
Frontal (motor, executive function, Broca's area for speech production); Parietal (touch, body sense); Temporal (hearing, Wernicke's area for language comprehension); Occipital (vision).
Hippocampus / Amygdala
Hippocampus: forms new explicit (declarative) memories: damage causes anterograde amnesia (H.M.). Amygdala: fear, aggression, emotional memory.
Split-Brain Research
Sperry and Gazzaniga studies of patients with severed corpus callosum. Showed each hemisphere can process information independently: left typically handles language, right handles spatial/holistic tasks.
Sleep Stages (NREM 1-3, REM)
NREM 1: light sleep, hypnagogic. NREM 2: spindles, K-complexes. NREM 3: deep, delta waves, restoration. REM: rapid eye movement, dreams, voluntary muscles paralyzed. Cycles repeat every ~90 min.
Trichromatic vs Opponent-Process Theory
Trichromatic (Young-Helmholtz): three cone types: red, green, blue. Opponent-process (Hering): color processed in opposing pairs (red/green, blue/yellow, black/white). Both true at different stages of vision.
Sensory Thresholds
Absolute threshold = minimum intensity detected 50% of time. Difference threshold (JND) = smallest detectable change. Weber's Law: JND is constant proportion of stimulus.
Exam Tips
- Memorize the lobes of the cerebral cortex (frontal/parietal/temporal/occipital) AND their functions. Brain-structure questions frequently hinge on them.
- Match each major neurotransmitter to a function AND a disorder (low serotonin/depression; excess dopamine/schizophrenia; low ACh/Alzheimer's).
- For sleep, know all four stages and which behaviors occur in each (sleepwalking = NREM 3; vivid dreams = REM).
- When asked about brain damage, identify the lobe AND the specific function lost (frontal damage → personality, planning; temporal → hearing, comprehension).
- For hormones, focus on the five CED-named hormones, adrenaline, leptin, ghrelin, melatonin, oxytocin, and skip the broader endocrine chart.
- For dream theories, cite activation-synthesis or information-processing/consolidation. Freud's psychoanalytic dream theory is out of scope on the current exam.
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