- Unconditioned stimulus (UCS) and response (UCR)
- The UCS is a stimulus that automatically and naturally produces a response without any learning — food, pain, a loud noise. The UCR is the natural, unlearned response to the UCS — salivation, withdrawal, startle. These form the biological foundation onto which conditioning grafts new learned associations.
- Conditioned stimulus (CS) and response (CR)
- After conditioning, the previously neutral stimulus becomes a CS that elicits the conditioned response (CR) on its own. The CR resembles the UCR but is typically weaker and may differ in some respects — it is a learned anticipatory response shaped by the organism's prediction of the UCS.
- Stimulus generalisation
- The tendency for stimuli similar to the CS to also elicit the CR, even if they were not part of the original conditioning. The more similar the new stimulus to the original CS, the stronger the CR it elicits. Generalisation is adaptive — if a predator's growl is dangerous, similar growls should also trigger escape — but underlies the spread of phobias and anxieties to related stimuli.
- Stimulus discrimination
- The opposite of generalisation: learning to respond to the specific CS but not to similar stimuli. Achieved by pairing only the target CS with the UCS and presenting other similar stimuli without the UCS. Discrimination produces more precise, targeted responses and is the basis of many therapeutic applications.
- Higher-order conditioning
- Once a CS has been established, it can itself serve as a UCS to condition a response to a new neutral stimulus. A bell (CS1) that reliably predicts food can be paired with a light (NS) to make the light a conditioned stimulus (CS2) — even though the light was never paired with food. Higher-order conditioning extends the reach of conditioning into more complex chains of association.
- Preparedness and biological constraints
- Organisms are not equally conditionable to all stimuli. Garcia and Koelling's taste aversion studies showed that rats readily associate flavour with illness (even after a single trial and a long delay) but cannot associate a light-and-tone cue with illness in the same way. Biological preparedness — the evolutionary predisposition to form certain associations more easily — constrains learning in ways that pure stimulus-response accounts cannot explain.
- Conditioned emotional responses
- Watson and Rayner's Little Albert experiment (1920) demonstrated that emotional responses — in this case, fear — could be classically conditioned in humans. By pairing a white rat (NS) with a loud noise (UCS), they induced fear of the rat (CR) in an 11-month-old infant. The experiment established classical conditioning as a mechanism of phobia development, though it is now considered deeply unethical.