Inside This Unit: The Full Breakdown
Biological Bases of Behavior examines how the nervous system, brain, endocrine system, genetics, and the senses give rise to behavior and mental processes — plus sleep, consciousness, and the effects of drugs. In the redesigned course, sensation, perception, and sleep all live here rather than in separate units.
Why it matters
Biological psychology is one of the most heavily tested areas on the AP exam, and the research-methods skills you build here (experiments, correlation vs. causation, twin/adoption designs) recur on every AAQ and EBQ for the rest of the course.
Key concepts
- Neurons fire in an all-or-none action potential; communication WITHIN a neuron is electrical, communication BETWEEN neurons is chemical (neurotransmitters crossing the synapse).
- Each major neurotransmitter has signature roles — dopamine (reward, movement), serotonin (mood, sleep), acetylcholine (memory, muscle), GABA (inhibition), glutamate (excitation), endorphins (pain relief).
- Brain regions are specialized: brainstem (survival functions), cerebellum (coordination), limbic system (emotion + memory), and four cortical lobes (frontal, parietal, temporal, occipital).
- Nature and nurture interact: heritability estimates describe variation within a group, not a single person’s destiny — twin and adoption studies separate genetic from environmental influence.
- Sensation is detecting physical energy (transduction at receptors); perception is the brain organizing and interpreting it (top-down expectations shape what we perceive).
- Sleep follows ~90-minute cycles through NREM stages and REM; theories explain sleep’s role in restoration, memory consolidation, and protection.
Neural Communication
A neuron receives signals at its dendrites, integrates them in the cell body (soma), and — if it reaches threshold — fires an action potential down the axon in an all-or-none fashion. The myelin sheath insulates the axon and speeds transmission via saltatory conduction (loss of myelin underlies multiple sclerosis). At the synapse, the impulse triggers vesicles to release neurotransmitters into the synaptic gap; they bind to receptors on the next neuron, either exciting or inhibiting it, and are then cleared by reuptake or enzymes. Drugs work on this system: agonists mimic or boost a neurotransmitter, antagonists block it. SSRIs, for example, block serotonin reuptake, leaving more serotonin in the synapse. Knowing the neuron’s parts and the excitatory/inhibitory logic of the synapse is a perennial exam staple.
The Brain and Endocrine System
The brainstem (medulla, pons) runs automatic survival functions; the cerebellum coordinates movement, balance, and some procedural learning. The limbic system links emotion and memory: the amygdala flags threat and fear, the hippocampus forms new explicit memories, and the hypothalamus governs drives (hunger, thirst, temperature) and the endocrine system. The cerebral cortex’s four lobes specialize — frontal (executive function, personality, Broca’s area for speech production), parietal (touch and spatial processing), temporal (hearing, Wernicke’s area for language comprehension), and occipital (vision). The endocrine system uses hormones released into the bloodstream for slower, longer-lasting effects: the pituitary "master gland," directed by the hypothalamus, triggers other glands (e.g., the adrenal glands’ fight-or-flight adrenaline). Researchers map function with lesion studies and imaging (EEG, fMRI, PET).
Sensation, Perception, and Consciousness
Transduction converts physical energy into neural signals: light becomes vision (rods for dim light/black-and-white, cones for color and detail; the optic nerve creates the blind spot), and sound-wave frequency and amplitude become pitch and loudness. The absolute threshold is the minimum stimulus detectable 50% of the time; the difference threshold (just-noticeable difference) follows Weber’s law (a constant proportion). Perception then organizes sensation top-down: Gestalt principles (closure, proximity, figure-ground), depth cues, and perceptual set (expectations) all shape experience. Consciousness varies across sleep’s cycle of NREM stages and REM (vivid dreaming with near-paralysis); circadian rhythms time alertness. Psychoactive drugs alter consciousness — depressants (alcohol) slow neural activity, stimulants (caffeine, cocaine) speed it, and hallucinogens (LSD) distort perception — and tolerance and withdrawal mark physical dependence.
Genetics and the Nature–Nurture Question
Behavior genetics asks how much of the variation in a trait reflects genes versus environment. Twin studies compare identical (monozygotic) and fraternal (dizygotic) twins; adoption studies compare children with their biological versus adoptive families. Heritability is the proportion of variation WITHIN a group attributable to genes — it is a group statistic, never a verdict about one individual, and it shifts as environments change. The modern view is interactionist: genes set a range of possibilities (a predisposition), and experience, epigenetics, and environment determine the outcome within that range. Evolutionary psychology adds that some tendencies (taste preferences, fears, mate selection) were shaped by natural selection because they aided survival and reproduction.
AP exam tip
When a stimulus question describes a study, name the design before analyzing it: experiment (manipulated IV, measured DV, random assignment → causation) vs. correlational (relationship only, no causation) vs. twin/adoption (heritability). Stating "this is correlational, so we cannot conclude X caused Y" earns easy points and is the most common trap.
Connections to other units
- Unit 2 (Cognition): The hippocampus and neural plasticity underlie how memories are formed and stored.
- Unit 3 (Development and Learning): Brain maturation and reinforcement (dopamine) connect biology to learning and lifespan change.
- Unit 5 (Mental and Physical Health): Neurotransmitter imbalances and brain abnormalities are central to disorders and their biomedical treatment.
