Biological Bases of Behavior
Unit 1 of AP Psychology, worth 15–25% of the exam. 14 questions below, each with the working. Every answer was checked by a second pass before it was published.
Heredity and environment, the nervous system and neurons, neurotransmitters and drugs, the brain, sleep and dreaming, sensation.
How this unit is tested
Biological Bases of Behavior is largely a structure-function unit: for every part you learn (a neuron component, a brain structure, a neurotransmitter, a stage of sleep, a sense organ), you need to know what it does and what happens when it is damaged, blocked, or overstimulated. The AP exam rarely asks you to just define a term—it gives you a scenario (a patient with damage, a drug's effect, a sleep-deprived subject) and asks you to identify or explain the underlying mechanism. Practice translating symptoms into structures and drug effects into neurotransmitter systems.
Start by building a clean map of levels of organization: genes and heredity, single neurons, neurotransmitter systems, the nervous system's major divisions, specific brain structures, then the emergent behaviors (sleep, sensation). Each level explains the one above it, so if you are fuzzy on neuron firing you will struggle with neurotransmitter questions, and if you are fuzzy on neurotransmitters you will struggle with drug and disorder questions. Work bottom-up rather than memorizing structures as an isolated list.
For sleep and sensation, pay special attention to distinguishing features that are easy to confuse under time pressure: REM versus NREM (brain activity versus muscle tone), and absolute threshold versus difference threshold (can you detect it at all versus can you detect a change). For heredity, remember the AP exam wants you to reason about gene-environment interaction and heritability as a population-level statistic, not to state that traits are 'genetic' or 'environmental' in an all-or-nothing way. Practice writing one or two sentences explaining mechanism for every fact you memorize; that is the skill the exam actually tests.
What you have to know
All-or-none principle
Once a stimulus depolarizes a neuron past its threshold, the neuron fires an action potential at full strength; the action potential does not vary in size with stimulus intensity. Stronger stimuli increase firing rate and recruit more neurons, not the size of any single impulse.
Weber's Law
The just noticeable difference (jnd) between two stimuli is a constant proportion of the original stimulus intensity, not a fixed amount: $\Delta I / I = k$, where k is a constant that differs by sense.
Agonist / Antagonist
An agonist is a molecule that binds to a receptor and mimics or amplifies a neurotransmitter's normal effect. An antagonist binds to a receptor and blocks or reduces a neurotransmitter's normal effect without activating it.
Reuptake
Reuptake is the process by which a presynaptic neuron reabsorbs excess neurotransmitter from the synaptic cleft after signaling, ending the signal. Reuptake inhibitor drugs (e.g., SSRIs) block this process, leaving more neurotransmitter in the synapse for longer.
Opponent-Process Theory
Color vision is controlled by opposing retinal-neural pairs (red-green, blue-yellow, black-white); stimulation of one member of a pair inhibits the other, which explains afterimages of complementary colors.
14 practice questions
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Identical (monozygotic) twins raised apart show a correlation of .72 for IQ, while fraternal (dizygotic) twins raised together show a correlation of .40. What is the best conclusion?
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Answer. Genetic factors contribute substantially to individual differences in intelligence in this population, but environment still plays a meaningful role.
Because identical twins raised apart (sharing genes but not environment) are more similar than fraternal twins raised together (sharing environment but fewer genes), the data point to a genetic contribution. Heritability describes population variance, not a fixed percentage for any one person. -
Increasing the intensity of a stimulus on a neuron above its threshold will most likely...
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Answer. Increase the rate of firing and recruit more neurons, not change the size of any single action potential.
The all-or-none principle states a single action potential fires at full strength or not at all once threshold is reached. Stronger stimuli are coded by firing frequency and by activating more neurons, not by larger individual impulses. -
During a job interview, a student's heart rate increases, pupils dilate, and digestion slows. Which branch of the nervous system is responsible, and what is its general function?
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Answer. The sympathetic nervous system; it mobilizes the body's resources for a fight-or-flight response to stress or threat.
Sympathetic activation redirects energy toward systems needed for action (heart, lungs, pupils) and away from systems not immediately needed (digestion). Its counterpart, the parasympathetic system, calms the body afterward. -
A drug blocks acetylcholine receptors at the neuromuscular junction. What is the most likely effect?
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Answer. Muscle paralysis.
Acetylcholine is the neurotransmitter that triggers voluntary muscle contraction at the neuromuscular junction. An antagonist that blocks its receptors prevents the signal from reaching the muscle, causing paralysis rather than increased contraction. -
Explain the difference between an agonist and an antagonist, giving one specific drug example for each.
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Answer. An agonist mimics or amplifies a neurotransmitter's effect (e.g., morphine acts as an agonist at opioid receptors, mimicking endorphins); an antagonist blocks a neurotransmitter's effect (e.g., naloxone acts as an antagonist at opioid receptors, blocking and reversing an overdose).
Both act at the receptor site, but agonists activate the receptor the way the natural neurotransmitter would, while antagonists occupy the receptor without activating it, preventing the natural neurotransmitter from binding. -
SSRIs (selective serotonin reuptake inhibitors) treat depression primarily by...
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Answer. Blocking the reabsorption of serotonin into the sending neuron, prolonging its effect in the synapse.
SSRIs do not create new serotonin; they interfere with the reuptake process so existing serotonin remains active in the synaptic cleft longer, increasing stimulation of postsynaptic receptors. -
A patient with damage to a particular brain structure can no longer form new explicit long-term memories, though procedural (motor skill) memory remains intact. Which structure was most likely damaged?
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Answer. The hippocampus.
The hippocampus is critical for consolidating new explicit (declarative) memories into long-term storage. Damage to it (as in the classic case of patient H.M.) leaves procedural memory, which relies more on structures like the cerebellum and basal ganglia, largely intact. -
In a split-brain patient, a picture of a spoon is flashed only to the left visual field. The patient says they see nothing but correctly picks out a spoon with their left hand from a group of hidden objects. Explain this result.
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Answer. Information from the left visual field goes to the right hemisphere, which also controls the left hand, so the right hemisphere can identify the spoon by touch. But because the corpus callosum is severed, that information cannot reach the left hemisphere, which controls speech, so the patient cannot verbally report seeing it.
Split-brain studies reveal that the two hemispheres can process information independently when the corpus callosum is cut, and that language production is typically localized to the left hemisphere while spatial/tactile processing can occur in the right. -
Which brain-imaging technique would best show which brain areas are most active while a participant solves a math problem?
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Answer. fMRI (functional MRI).
fMRI detects changes in blood flow and oxygen use, allowing researchers to localize brain activity associated with a specific ongoing task with good spatial resolution, unlike CT or standard MRI, which show structure but not real-time activity. -
During which stage of sleep does the EEG most resemble that of an awake, alert person, even though the sleeper is difficult to wake?
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Answer. REM sleep.
REM sleep is called 'paradoxical sleep' because brain wave activity is fast and desynchronized, similar to wakefulness, while the body is in muscle atonia (temporary paralysis) and vivid dreaming occurs. -
According to the activation-synthesis theory of dreaming, what causes dreams?
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Answer. Random neural activity generated in the brainstem during REM sleep is synthesized and interpreted by the cortex, creating a dream narrative from otherwise meaningless signals.
This theory contrasts with Freud's idea that dreams disguise unconscious wishes; instead it proposes dreams are the brain's attempt to make sense of spontaneous neural firing. -
A student flies from New York to Tokyo and experiences fatigue and difficulty sleeping at appropriate local times for several days. This is best explained by disruption of...
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Answer. The circadian rhythm.
Rapid travel across time zones desynchronizes the body's internal 24-hour clock (regulated by the suprachiasmatic nucleus) from the new local light-dark cycle, producing jet lag symptoms. -
According to Weber's Law, if the just noticeable difference (jnd) for weight is a 5% change in stimulus intensity, how much weight must be added to a 200-gram object for a person to notice a difference?
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Answer. 10 grams.
Weber's Law states the jnd is a constant proportion of the original stimulus, so multiply 200 grams by 0.05 to get 10 grams; a fixed number of grams would be wrong because the required change scales with the starting intensity. -
A tired radiologist fails to notice a tumor that is clearly visible on an X-ray. In signal detection theory terms, this is an example of a...
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Answer. Miss.
Signal detection theory classifies responses to a stimulus as a hit (correctly detecting a present signal), miss (failing to detect a present signal), false alarm (reporting a signal that isn't there), or correct rejection (correctly reporting no signal). Failing to notice a real tumor is a miss.
What people get wrong
- Treating heritability as a statement about one individual ('this person's IQ is 60% genetic'). Instead, remember heritability is a population statistic describing the proportion of variance in a trait across a group that is due to genetic differences.
- Confusing agonist and antagonist direction under time pressure. Instead, anchor each term to a concrete drug you know cold (e.g., nicotine as an acetylcholine agonist, naloxone as an opioid antagonist) and reason from there.
- Mixing up sympathetic and parasympathetic effects (e.g., saying the parasympathetic system increases heart rate). Instead, memorize them as a pair: sympathetic arouses/mobilizes ('fight or flight'), parasympathetic calms/restores ('rest and digest').
- Assuming REM sleep is the 'deepest' sleep because it is hardest to wake someone from. Instead, remember REM is called paradoxical sleep: brain activity resembles wakefulness and muscles are paralyzed, while NREM stage 3 (slow-wave sleep) is the stage with the slowest, most synchronized brain waves.
- Misapplying Weber's Law by adding a fixed amount instead of a proportion. Instead, always multiply the original stimulus intensity by the constant proportion to find the jnd for that specific stimulus.
Drill this unit until it sticks
These questions come back on a schedule built from what you get wrong, alongside the rest of AP Psychology. Free, and no account needed to start.