AP Psychology Unit 1: Biological Bases of Behavior

Behavior develops through interacting inherited predispositions and experiences. A genetic influence is not a fixed destiny: opportunities, stress, learning, and culture can change how a predisposition is expressed.

• Twin and adoption studies compare patterns of similarity to investigate genetic and environmental influences. A correlation cannot by itself establish a cause. Heritability describes variation within a particular population and environment, not the genetic percentage of one person’s trait.
• Evolutionary explanations consider how inherited tendencies could affect survival and reproduction across generations. Avoid assuming that a trait is adaptive merely because it exists. Practice separating an observed association from a causal explanation.

The central nervous system comprises the brain and spinal cord. Peripheral nerves connect it with the body. Sensory neurons carry information toward the CNS; motor neurons carry commands to muscles and glands; interneurons connect neurons within the CNS.

• The somatic system supports skeletal muscle movement. The autonomic system regulates internal organs: sympathetic activation mobilizes energy, while parasympathetic activity supports recovery and digestion. These branches coordinate rather than simply switching the entire body on or off.
• In a spinal withdrawal reflex, sensory input reaches spinal interneurons and motor output can begin before conscious awareness of pain. The brain still receives information. Endocrine glands also coordinate behavior through hormones carried in blood.

Dendrites receive inputs, the cell body integrates them, and an axon carries an action potential when threshold is reached. Depolarization reverses the membrane’s charge locally; repolarization and a refractory period help reset it. Myelin speeds conduction along the axon.

• Action potentials are all-or-none: a stronger stimulus does not make each spike taller. Intensity can be represented by firing rate and the number of active neurons. At a synapse, released neurotransmitters bind receptors; reuptake, breakdown, or diffusion ends their action.
• Agonists increase or mimic a transmitter’s effects; antagonists block or reduce them. Dopamine, serotonin, acetylcholine, GABA, glutamate, and endorphins have multiple functions—not one emotion each. Psychoactive drugs can alter signaling; tolerance and withdrawal are different processes.

The brainstem supports vital functions; the cerebellum contributes to coordination. The thalamus relays most sensory information. The hypothalamus helps regulate drives and endocrine activity; the hippocampus is important for forming new explicit memories, and the amygdala participates in emotional processing.

• Frontal regions contribute to planning and movement, parietal regions to bodily sensation, occipital regions to vision, and temporal regions to hearing and aspects of language and memory. Functions depend on connected networks; simple left-brained/right-brained personality labels are misleading.
• The corpus callosum connects the cerebral hemispheres. Plasticity is the capacity for neural organization to change with experience or injury, not unlimited recovery. EEG records electrical activity; fMRI tracks blood-oxygen-related changes as an indirect activity measure. Association in a scan alone does not prove causation.

Circadian rhythms organize approximately 24-hour changes in alertness and sleep. Light helps synchronize the body clock; jet lag and shift work can disrupt the alignment between internal timing and local schedules.

• NREM sleep includes N1, N2, and N3. N2 features sleep spindles and K-complexes; N3 is associated with slow delta waves. REM combines wake-like brain activity and rapid eye movements with markedly reduced skeletal muscle tone. Dreaming can also occur outside REM.
• Sleep supports restoration and memory consolidation; dream theories include activation-synthesis and consolidation accounts. Insomnia concerns difficulty sleeping; sleep apnea involves repeated breathing interruptions; narcolepsy involves daytime sleep attacks. Sleepwalking usually occurs in deep NREM, while REM sleep behavior disorder involves loss of normal REM muscle inhibition.

Transduction converts stimulus energy into neural signals. An absolute threshold is the intensity detected on 50% of trials; a difference threshold concerns detecting a change. Weber’s law relates the noticeable difference to the starting intensity. Sensory adaptation reduces responsiveness to sustained stimulation.

• In vision, rods support dim-light sensitivity and cones support color and detail. Trichromatic and opponent-process theories explain different levels of color processing. In hearing, cochlear hair cells transduce vibration; place, frequency, and volley accounts help explain pitch. Damage can cause conductive or sensorineural hearing loss.
• Taste and smell interact in flavor. Touch and pain involve receptors and central processing; gate-control theory describes modulation of pain signals. Vestibular input supports balance, while kinesthetic information concerns body position and movement. Sensation supplies information; perception interprets it.