Biological Bases of Behavior

Psychology studies behavior 行为 (what a person does) and mental processes 心理过程 (what a person thinks and feels). Unit 1 asks where these come from in the body.

Heredity and environmental factors interact to shape behavior and mental processes. Heredity 遗传 is what a person inherits from their parents. Environment 环境 is everything that happens around them. Neither one acts alone.

• Nature and nurture 先天与后天 interact. For example, inherited tendencies and nutrition can both influence height.
• The evolutionary perspective 进化论视角 asks how inherited tendencies may relate to survival and reproductive success. A plausible story about an advantage is not, by itself, proof of a particular behavior's origin.
• Twin and adoption studies 双生子与收养研究 compare patterns of similarity across relatedness and upbringing. Similarity between identical twins raised apart can support an inherited contribution, but it does not cleanly separate all inherited and environmental influences.

Worked interaction. In an illustrative case, assume a student inherits a tendency that helps distinguish musical sounds. Lessons, practice and access to an instrument help develop the ability. Performance improves after the school provides instruments. The inherited tendency is the stated heredity contribution; teaching, practice and access are environmental contributions. The opportunity helps develop the tendency. Improvement with access does not prove that all differences between students are environmental, or that inheritance fixes final performance. Do not infer inherited ability merely because someone performs well.

Evaluate a twin-study conclusion. Identical twins raised apart show similar scores on a behavioral trait. The similarity is consistent with an inherited contribution, not proof that the trait is entirely inherited. They share prenatal conditions and may encounter similar environments; placement into families is not a random experiment. Comparisons with other relatives and adoptive families add evidence about patterns, rather than assigning one person's behavior wholly to one cause.

Scope note: detailed genetics, DNA and chromosome mechanisms are outside this course framework. The relationship between heredity and environment is assessed.

The nervous system 神经系统 carries information around the body. It has two main parts.

The central nervous system (CNS) 中枢神经系统 is the brain and the spinal cord 脊髓. It interacts with every process in the body.

The peripheral nervous system (PNS) 周围神经系统 relays messages between the CNS and the rest of the body. It divides again:

• The somatic nervous system 躯体神经系统 controls voluntary 随意的 movement, such as raising your hand.
• The autonomic nervous system 自主神经系统 controls involuntary 不随意的 processes, such as heartbeat and digestion. It has two branches that work against each other: the sympathetic nervous system 交感神经系统 prepares the body for action (the "fight or flight" response), and the parasympathetic nervous system 副交感神经系统 calms the body down afterwards.

Worked pathway choice. A student deliberately raises a hand, experiences a faster heartbeat before a presentation, and later notices a slowing heart rate during recovery. The deliberate skeletal movement involves the somatic division carrying motor commands. Heartbeat is an autonomic process: the sympathetic branch supports arousal and the parasympathetic branch supports recovery. These are PNS pathways interacting with the CNS. Classify the specific response, not the whole event as only voluntary or automatic. A racing heart alone does not establish one particular emotion.

Two kinds of cell do the work. Neurons 神经元 transmit information. Glial cells 神经胶质细胞 provide structure, insulation, communication, and waste transport. Together they are the building blocks of all behavior and mental processes.

A neuron has dendrites 树突 that receive signals, a cell body 细胞体, and an axon 轴突 that carries the signal away. Many axons are wrapped in a myelin sheath 髓鞘, which speeds the signal up.

Neural transmission follows a set order:

• At rest the neuron holds a resting potential 静息电位, ready but not firing.
• If incoming signals pass the threshold 阈限, the neuron depolarizes 去极化 and fires.
• Firing follows the all-or-none principle 全或无原则: the neuron fires at full strength or not at all. A stronger stimulus makes it fire more often, not harder.
• Straight after firing there is a refractory period 不应期 when the neuron cannot fire again.
• At the synapse 突触, the gap between neurons, chemicals cross to the next cell. Leftover chemicals are taken back in a process called reuptake 再摄取.

Damage to this process can cause disorders such as multiple sclerosis 多发性硬化 or myasthenia gravis 重症肌无力.

In the spinal cord, the reflex arc 反射弧 shows the system working as a unit. Sensory neurons 感觉神经元 carry information in, interneurons 中间神经元 connect inside the cord, and motor neurons 运动神经元 carry the command out - a withdrawal can be initiated through spinal circuits before conscious awareness, while signals also travel toward the brain.

Neurotransmitters 神经递质 are the chemicals that cross the synapse. Each has its own job, and the effect can depend on where in the nervous system it acts. A message is either excitatory 兴奋性的 (making firing more likely) or inhibitory 抑制性的 (making firing less likely).

Only these are examinable: dopamine 多巴胺 (reward and movement), serotonin 血清素 (mood and sleep), norepinephrine 去甲肾上腺素 (alertness), glutamate 谷氨酸 (the main excitatory transmitter), GABA γ-氨基丁酸 (the main inhibitory transmitter), endorphins 内啡肽 (pain relief), substance P P物质 (pain signalling), and acetylcholine 乙酰胆碱 (muscle movement and memory).

Hormones 激素 travel through the blood and can influence the brain as well as other organs. Their route differs from communication across a synapse; blood-borne does not mean unable to affect behavior. Again only five are examinable: adrenaline 肾上腺素, leptin 瘦素 (reduces hunger), ghrelin 胃饥饿素 (increases hunger), melatonin 褪黑素 (sleep), and oxytocin 催产素 (bonding).

Psychoactive drugs 精神药物 can change behavior by acting on neurotransmission. Three mechanisms taught in the course are:

• An agonist 激动剂 activates or supports a transmitter's effect. In an introductory excitatory-signal example, this encourages firing.
• An antagonist 拮抗剂 blocks a transmitter's action. With that excitatory signal, blocking it reduces its contribution to firing.
• A reuptake inhibitor 再摄取抑制剂 blocks reabsorption, leaving more transmitter in the synapse.

Worked synapse comparison. Assume a transmitter normally activates a receiving receptor and makes firing more likely. Hypothetical substance X activates the same receptor: it acts as an agonist in this case. Y occupies that receptor without activating it and prevents the transmitter acting: it is an antagonist. Z stops reabsorption into the sending cell: it is a reuptake inhibitor, leaving more transmitter available in the gap without directly copying it at the receiving receptor. State the affected step before predicting its consequence.

The example explicitly uses an excitatory signal. Supporting an inhibitory signal can strengthen inhibition; blocking it can remove inhibition. An agonist is not a universal synonym for stimulation of the whole nervous system. Receptor, transmitter and location matter, and the three taught mechanisms are examples rather than all possible drug mechanisms.

Drug categories: stimulants 兴奋剂 (caffeine, cocaine) increase neural activity; depressants 抑制剂 (alcohol) decrease it; hallucinogens 致幻剂 (marijuana) distort perception; opioids 阿片类药物 (heroin) relieve pain. Repeated use can lead to tolerance 耐受性, where more is needed for the same effect, and to addiction 成瘾, with withdrawal 戒断反应 symptoms when use stops.

The brain is studied from the bottom up, because the lower structures handle the most basic jobs.

• The brain stem 脑干, including the medulla 延髓, controls breathing and heart rate.
• The reticular activating system 网状激活系统 controls alertness and arousal.
• The cerebellum 小脑 controls coordination, balance, and some procedural learning 程序性学习.
• The limbic system 边缘系统 sits beneath the cerebral cortex and includes the thalamus 丘脑 (a relay station for the senses), the hypothalamus 下丘脑 (hunger, thirst, temperature), the pituitary gland 脑垂体, the hippocampus 海马体 (forming new memories), and the amygdala 杏仁核 (fear and emotion).

The cerebral cortex 大脑皮层 is divided into two hemispheres 半球 and four lobes: the occipital lobes 枕叶 process vision at the rear; the temporal lobes 颞叶 process hearing and language at the sides; the parietal lobes 顶叶 hold the somatosensory cortex 体感皮层 for touch; and the frontal lobes 额叶 handle higher-order thinking and executive function 执行功能.

Language sits mainly in the left hemisphere: Broca's area 布罗卡区 for speech production and Wernicke's area 韦尼克区 for comprehension.

Split-brain research 裂脑研究, done on patients whose corpus callosum 胼胝体 was cut to treat severe epilepsy, shows the two hemispheres can work independently. Researchers present information to one visual field 视野 at a time, because each field is processed by the opposite hemisphere.

Brain plasticity 大脑可塑性 is the brain's ability to rewire itself and form new connections, which supports recovery after damage. Research methods include EEG 脑电图 and fMRI 功能性磁共振成像 scans, case studies, and lesioning 损毁法.

Worked method choice. To identify rapid electrical patterns distinguishing sleep stages, choose EEG: scalp electrodes record electrical patterns with fine timing, although locating a precise source is more limited. To compare where blood-oxygen signals change during a memory task and a comparison task, choose fMRI: it commonly tracks regional blood-oxygen changes associated with activity. It gives useful spatial detail but measures an indirect, slower-changing signal rather than each neuron's firing. Both methods can track changes over time; that phrase alone does not distinguish them. A regional change during a task does not, by itself, establish that the region is necessary for the task or reveal exact thoughts. Match the research aim to the signal and state a limitation.

Consciousness 意识 is awareness of internal and external events, and it varies in level.

The sleep/wake cycle is a circadian rhythm 昼夜节律 of about 24 hours. Jet lag 时差反应 and shift work disrupt it.

Sleep stages are identified by EEG patterns. NREM sleep 非快速眼动睡眠 covers stages 1 to 3; hypnagogic sensations 入睡幻觉 occur as you fall asleep. REM sleep 快速眼动睡眠 is called paradoxical sleep 异相睡眠 because the brain looks awake while the body is most relaxed, and most dreaming happens here.

Two examinable theories explain dreams: activation-synthesis theory 激活-综合理论 (the brain interprets random activity) and consolidation theory 巩固理论 (dreams reflect memory processing). Sleep itself is explained by memory consolidation 记忆巩固 and restoration 恢复 theories.

Note: the psychoanalytic theory of dreams is outside the exam's scope.

Sleep disorders disrupt waking behavior and health: insomnia 失眠, narcolepsy 发作性睡病, sleep apnea 睡眠呼吸暂停, somnambulism 梦游 (sleepwalking), and REM sleep behavior disorder 快速眼动睡眠行为障碍.

Sensation 感觉 is detecting information from the environment and transducing 转导 it into neural signals the brain can use. Detection needs a certain absolute threshold 绝对阈限.

• The just-noticeable difference 最小可觉差 is the smallest change a person can detect.
• Sensory adaptation 感觉适应 is reduced sensitivity to an unchanging stimulus - you stop noticing a smell.
• Sensory interaction 感觉相互作用 means the senses work together. Synesthesia 联觉 is when one sense triggers another.

Vision. Light is focused onto the retina 视网膜 by the lens 晶状体 changing shape, a process called visual accommodation 视觉调节; when this fails the result is nearsightedness 近视 or farsightedness 远视. Rods 视杆细胞 lie in the periphery and detect shape and movement in low light. Cones 视锥细胞 sit in the fovea 中央凹 and detect colour and detail. Colour vision needs two theories: trichromatic theory 三色理论 (blue, green and red cones) and opponent-process theory 对立过程理论, which explains afterimages 后像. Damage to the fusiform face area, on the underside of the brain where the occipital and temporal lobes meet, can cause prosopagnosia 面孔失认症, or face blindness.

Hearing. Sound is air movement; wavelength gives pitch 音高 and amplitude gives loudness 响度. Pitch perception is explained by place theory 位置理论, volley theory 齐射理论, and frequency theory 频率理论. Sound localization 声源定位 tells us where a sound comes from. Hearing loss may be conduction deafness 传导性耳聋 or sensorineural deafness 感音神经性耳聋.

The chemical senses. Olfaction 嗅觉 is the only sense not relayed first through the thalamus. Gustation 味觉 covers sweet, sour, salty, bitter, umami 鲜味, and oleogustus 脂味. The two interact: without smell, taste is muted.

Touch, pain, and body position. Skin receptors transduce touch; "hot" comes from warm and cold receptors firing together. Pain is processed in body and brain, and gate control theory 闸门控制理论 explains why attention changes how much it hurts; phantom limb 幻肢 sensations show the brain's role. The vestibular sense 前庭觉 controls balance through the semicircular canals 半规管, and kinesthesis 动觉 is the sense of body movement.

Worked evidence-based generalization (2025 AP Psychology Set 1 Article Analysis). The study recruited 127 students at one large university for course credit. They were aged 18–39, with a mean age of 21.21; 69% were women and 31% men. Consider how far the findings can be extended to adults aged 65 and over.

One supported response is: the findings may not extend to adults aged 65 and over because all participants were university students aged 18–39. The study provides no direct participant evidence about that older population. This is a limit on the evidence, not proof that older adults would behave differently. A sample's size alone does not show that it represents every population. Random assignment to misinformation conditions helps compare the experimental groups; it does not turn a university sample into a representative sample of all adults.

The official Part E criterion requires a generalization claim supported by specific, relevant participant evidence. Name the population, select a participant feature and explain the link. An unexplained label such as “biased” or a statement about sample size is insufficient. This is an authored model response grounded in the source and scoring criterion, not a quotation of a scored student answer.

The article-analysis question gives a short study summary and asks about its method, its variables and what it can conclude. The studies below are invented for practice.

Researchers randomly assigned 60 adult volunteers to drink either a caffeinated or a decaffeinated coffee, without telling them which. Thirty minutes later, each person's reaction time was measured as the milliseconds taken to press a button when a light appeared. The caffeine group responded 25 ms faster on average (p = 0.02).

(a) Identify the research method, and explain how you know.

(b) Identify the operational definition of the dependent variable.

(c) Explain ONE feature of the design that supports a cause-and-effect conclusion.

(d) Explain how caffeine's effect relates to neurotransmission.

Model answer.

• (a) An experiment: the researchers manipulated the independent variable (caffeine or not) and randomly assigned participants.
• (b) Reaction time: the milliseconds taken to press the button after the light appeared.
• (c) Random assignment spread other differences, such as sleep or habits, evenly across groups, and keeping participants unaware of their condition (single-blind) reduced expectation effects.
• (d) Caffeine blocks receptors for adenosine, a neurotransmitter that promotes drowsiness, so neural activity stays higher and responses are faster.

• Learn only the listed neurotransmitters and hormones; the exam will not ask about others, and the endocrine glands are out of scope apart from the pituitary.
• Keep sensation separate from perception: sensation is detecting and transducing, perception is interpreting (Unit 2).
• The all-or-none principle means intensity is coded by how often a neuron fires, never by how strongly.
• Match structure to job before you write: hippocampus forms memories, amygdala handles fear, cerebellum coordinates movement.
• Sympathetic arouses, parasympathetic calms. If a question describes a racing heart, name the sympathetic branch.
• Say what a scan can and cannot show. EEG records rapid electrical patterns; fMRI commonly tracks regional blood-oxygen changes associated with activity.

Vocabulary card meanings

mental processes: Activities of the mind such as perceiving, remembering, and deciding. Nature and nurture: The joint influence of inherited traits and of experience. Twin and adoption studies: Research that compares relatives reared together or apart to separate heredity from environment. peripheral nervous system (PNS): The nerves outside the brain and spinal cord. somatic nervous system: The division that carries commands to skeletal muscles and also brings sensory signals in from the body. voluntary: Done under conscious command, as with moving an arm. autonomic nervous system: The division that runs organs without a person deciding each action. involuntary: Happening without a conscious decision, as with heartbeat. sympathetic nervous system: The division that prepares the body for urgent action. parasympathetic nervous system: The division that calms the body and restores energy. Neurons: Nerve cells that carry signals through the nervous system. dendrites: Branching fibres that receive signals into a nerve cell. resting potential: The voltage difference across a nerve-cell membrane while the cell is not firing. depolarizes: Makes the inside of the membrane less negative, which lets a nerve impulse start. all-or-none principle: A nerve cell either fires a full impulse or does not fire at all. refractory period: A short interval after one firing when another firing is impossible, or possible only with a stronger stimulus. multiple sclerosis: A disease in which the insulating sheath around nerve fibres is damaged. myasthenia gravis: A disease in which signals fail to make muscles contract with normal strength. Sensory neurons: Nerve cells that carry information in from the sense organs. interneurons: Nerve cells that connect other nerve cells, especially in the spinal cord and brain. motor neurons: Nerve cells that carry commands out to muscles and glands. excitatory: Making the next nerve cell more likely to fire. inhibitory: Making the next nerve cell less likely to fire. dopamine: A chemical messenger linked with reward, movement, and attention. serotonin: A chemical messenger linked with mood, sleep, and appetite. norepinephrine: A chemical messenger that raises alertness and helps the stress response. glutamate: The main chemical messenger that excites nerve cells in the brain. GABA: The main chemical messenger that quiets nerve signals in the brain. endorphins: Natural chemicals that reduce pain and can produce a feeling of well-being. substance P: A chemical messenger involved in sending pain signals. leptin: A hormone from fat tissue that signals fullness to the brain. ghrelin: A hormone from the stomach that signals hunger. melatonin: A hormone that helps set the daily sleep-wake rhythm. oxytocin: A hormone linked with bonding, birth, and trust. agonist: A drug that mimics a messenger and turns its receptor on. antagonist: A drug that blocks a receptor so the usual messenger cannot act. reuptake inhibitor: A drug that keeps a messenger in the gap between nerve cells for longer. stimulants: Drugs that speed up neural activity and bodily arousal. depressants: Drugs that slow neural activity and bodily arousal. hallucinogens: Drugs that distort perception and can produce sensations with no outside source. opioids: Drugs, related to opium, that relieve pain and can be highly addictive. occipital lobes: The rear parts of the brain that first process vision. temporal lobes: The side parts of the brain involved in hearing and in some memory. parietal lobes: The upper side parts of the brain that process touch and spatial layout. somatosensory cortex: The strip of brain that maps touch from different body parts. frontal lobes: The front parts of the brain used for planning, speech, and self-control. executive function: Mental skills for planning, switching tasks, and holding back impulses. Broca's area: A region, usually on the left, needed to produce fluent speech. Wernicke's area: A region, usually on the left, needed to understand speech. visual field: The whole area a person can see at one moment without moving the eyes. lesioning: Destroying or damaging a small brain area in order to study its job. NREM sleep: Stages of sleep in which the eyes do not move rapidly, during which dreams can still occur. hypnagogic sensations: Brief vivid feelings, such as falling, as a person is drifting into sleep. paradoxical sleep: A sleep stage in which the brain is active but most muscles are paralysed. activation-synthesis theory: The idea that dreams are the brain's attempt to make a story from random activity. memory consolidation: The stabilising of a new memory so that it lasts. restoration: The idea that sleep repairs the body and replenishes energy. insomnia: Ongoing difficulty falling asleep or staying asleep. narcolepsy: A disorder of sudden, uncontrollable sleep attacks. sleep apnea: A disorder in which breathing repeatedly stops during sleep. somnambulism: Walking about while still asleep. REM sleep behavior disorder: A condition in which the usual muscle paralysis of dreaming sleep fails. transducing: Converting one form of energy, such as light, into nerve signals. absolute threshold: The smallest stimulus detected about half the time. Sensory adaptation: Reduced sensitivity after constant, unchanging stimulation. Synesthesia: A condition in which one sense triggers an experience in another, such as seeing sound. lens: The clear structure in the eye that bends incoming light. nearsightedness: Seeing nearby objects more clearly than distant ones. farsightedness: Seeing distant objects more clearly than nearby ones. fovea: The small central pit of the retina where vision is sharpest. afterimages: Sensations that linger after the original stimulus has gone. prosopagnosia: An inability to recognise familiar faces. place theory: The idea that pitch depends on which part of the inner ear is stimulated. volley theory: The idea that groups of auditory nerve cells fire in rotation to code pitch. frequency theory: The idea that pitch depends on how fast auditory nerve cells fire. Sound localization: Judging where a sound comes from, using differences between the two ears. conduction deafness: Hearing loss from a problem in the outer or middle ear's mechanical path. sensorineural deafness: Hearing loss from damage to the inner ear or the auditory nerve. Gustation: The sense of taste. umami: The savoury taste associated with glutamate in foods. oleogustus: A proposed taste quality for fatty acids. gate control theory: The idea that signals from the spinal cord can open or close the path for pain. phantom limb: The feeling that a missing limb is still present, sometimes with pain. semicircular canals: Fluid-filled loops in the inner ear that sense rotation of the head.