Classical Conditioning: Basic Concepts and Principles

Foundations of Learning and Behavioral Adaptation

• Definition and Scope of Learning:

  • Learning is the process of acquiring through experience new and relatively enduring information or behaviors.

  • Learning provides organisms with adaptability—nature's most critical evolutionary gift across the lifespan—enabling individuals to acquire new actions to cope with dynamically changing environments.

  • Human adaptability is demonstrated by the capacity to construct grass huts, snow shelters, submarines, or space stations, thriving in virtually every terrestrial and extraterrestrial climate.

  • The principle that behavior is learnable provides a foundation for education, coaching, and parenting; conversely, the realization that learned behaviors can be modified through new learning forms the theoretical basis for stress management, counseling, and psychotherapeutic interventions.

  • Learning operates across diverse psychological domains: infant and childhood cognition, language acquisition, assimilation of culture, internalization of social norms and gender roles, development of visual perceptions, anticipated drug responses, motivations, emotional regulation, personality structure, and attitude formation.

• Associative Learning and Habit Formation:

  • Associative learning occurs when an organism connects events that occur in sequence.

  • Natural sequences condition future expectations:

  • Olfactory and visual cues of freshly baked bread followed by consumption and subsequent satiety create an anticipation that encountering the aroma in the future will be satisfying.

  • Audio cues preceding a frightening event induce anticipatory fear. For instance, a 4-year-old child who witnessed a television character get mugged noted: "If I had heard that music, I wouldn't have gone around the corner!" (Wells, 1981).

  • Learned associations govern habitual behaviors (Urcelay & Jonkman, 2019; Wood, 2017).

  • Habits develop when specific actions are repeatedly paired with a fixed context—such as assuming a consistent sleeping posture, biting fingernails during examinations, or toggling a light switch when exiting a room.

  • Subsequent re-entry into the context automatically elicits the habitual response; individuals rely most heavily on established habits when experiencing mental fatigue (Neal et al., 2013).

  • Contextual conditioning governs both positive habits (such as eating fresh fruit) and detrimental habits (such as overconsuming potato chips) (Graybiel & Smith, 2014).

  • Academic success and enhanced self-control depend directly on establishing disciplined, constructive daily habits (Fiorella, 2020).

  • In an empirical investigation tracking habit formation, a British research team instructed $96$ university students to repeat a selected healthy behavior (e.g., jogging before dinner, consuming fruit at lunch) every day for $84$ days while logging perceived automaticity ("doing something without thinking that would be difficult not to do"). Behaviors achieved automaticity after an average of $66$ days (Lally et al., 2010), demonstrating that consistent daily execution for roughly two months (or slightly longer for rigorous exercise) is necessary to solidify a new behavioral routine.

• Habituation Versus Sensory Adaptation:

  • Simpler organisms demonstrate associative learning and adaptive behavioral suppression; the sea slug Aplysia retracts its protective gills when disturbed by a water squirt, but this defensive withdrawal diminishes when squirts occur repeatedly without consequence, as observed in naturally choppy waters.

  • Habituation: A decline in psychological or behavioral responsiveness resulting from repeated exposure to a non-threatening, unchanging stimulus; habituation represents a genuine form of learning.

  • When a water squirt is repeatedly paired with an electric shock, Aplysia does not habituate; instead, its defensive gill withdrawal intensifies because it has learned an association between the tactile squirt and the impending shock.

  • Sensory Adaptation: A biological process occurring at the receptor level wherein sensory receptors cease or decrease firing in response to an unvarying, continuous physical stimulus (e.g., pool water no longer feeling cool after several minutes of immersion).

  • Habituation differs fundamentally from sensory adaptation because the sensory organs continue to register the stimulus normally; the decrease occurs in the central nervous system's behavioral response (e.g., startling significantly less when a friend yells "Boo!" a second time two minutes after the first fright).

Distinguishing Learning Modalities: Associative vs. Cognitive Learning

• Primary Branches of Conditioning:

  • Associative conditioning links two contiguous occurrences, allowing organisms to anticipate the immediate future via two distinct paradigms:

  • Classical Conditioning: The process by which an organism learns to link two or more previously unrelated stimuli and anticipate events. The organism associates environmental events over which it exercises no direct control, displaying involuntary, automatic reactions termed respondent behavior.

  • Operant Conditioning: The process by which an organism learns to link its voluntary behaviors with subsequent environmental consequences. Behaviors followed by reinforcing consequences increase in frequency, whereas behaviors followed by punishing consequences decrease, producing operant behavior (actions that operate on the environment to generate consequences).

• Interaction of Conditioning Types in Natural Settings:

  • While classical and operant paradigms are conceptually distinct, they frequently operate simultaneously in real-world contexts.

  • A case study in animal husbandry involved a Japanese cattle rancher who attached electronic pagers to the ears of his herd:

  • Classical Conditioning Component: The rancher sounded a call to the electronic pagers, pairing the auditory beep ($\text{CS}$) with the delivery of food ($\text{UCS}$). After one week of pairing, the cattle anticipated feeding upon hearing the beep.

  • Operant Conditioning Component: The cattle learned that galloping toward the feeding trough ($\text{operant behavior}$) resulted in the rewarding consumption of grain ($\text{reinforcing consequence}$).

  • The interaction of classical anticipation and operant behavioral reinforcement allowed the rancher to herd and feed livestock with minimal manual labor.

• Cognitive Learning and Observational Learning:

  • Learning extends beyond stimulus-response association; organisms also acquire mental information that guides subsequent behavior through cognitive learning.

  • Observational Learning: A major subfield of cognitive learning in which an individual observes and imitates the actions, trials, and outcomes of others without direct firsthand reinforcement.

  • Non-human primates, such as chimpanzees, demonstrate observational learning: an observing ape that watches a peer manipulate a puzzle box to acquire a food reward will solve that puzzle substantially faster than a naive peer attempting trial-and-error discovery.

  • In humans, cognitive learning through observation, language, and symbolic communication enables individuals to master complex competencies and internalize knowledge regarding events they have never personally experienced or directly witnessed.

Behaviorism and the Objective Study of Behavior

• The Rise of Behaviorism:

  • Ivan Pavlov (1849–1936), a Russian physiologist, laid the empirical foundation that inspired American psychologist John B. Watson (1913) to inaugurate the school of behaviorism.

  • Watson asserted that if psychology were to establish itself as an authentic, rigorous natural science, it had to abandon subjective introspection and cease referencing unobservable mental states, such as thoughts, feelings, intentions, and consciousness.

  • According to Watson: "Its theoretical goal is the prediction and control of behavior. Introspection forms no essential part of its methods."

  • Behaviorism was the dominant paradigm in North American psychology throughout the first half of the twentieth century.

• Core Tenets and Contemporary Revisions:

  • Pavlov and Watson shared two fundamental premises:

  • Psychology must operate as an objective, empirical discipline grounded strictly in measurable, observable responses to identifiable environmental stimuli.

  • The fundamental mechanisms and laws of learning are shared across the animal kingdom, functioning identically in dogs, pigeons, rodents, and human beings.

  • Contemporary psychology has revised both foundational assumptions:

  • Complete elimination of cognitive and mental processes is now recognized as scientifically untenable; modern cognitive neuroscience confirms that mental representations, expectations, and biological predispositions heavily mediate learning.

  • However, the behaviorist insight remains universally accepted: classical conditioning constitutes a foundational, universal adaptive mechanism shared across species.

Pavlov's Experimental Paradigm and Core Classical Conditioning Components

• Ivan Pavlov's Scientific Trajectory:

  • Pavlov originally intended to follow his father into the Russian Orthodox priesthood before abandoning theology to pursue physiological research, completing his medical degree at age 33.

  • Pavlov conducted two decades of research examining the digestive physiology of canines, for which he received Russia's first Nobel Prize in 1904.

  • The final three decades of his career were devoted entirely to novel investigations of learning and conditioning (Todes, 2014).

  • Pavlov's research team was notable for its progressive composition: more than half of his research associates and laboratory technicians were women (Hill, 2019).

• The Incidental Discovery of "Psychic Secretions":

  • Canine salivation upon contact with oral food is an involuntary, biological reflex. However, Pavlov noticed that laboratory canines began salivating prior to the physical contact of meat on the tongue.

  • Salivation occurred upon the mere visual presentation of the food, the sight of an empty feeding dish, the appearance of the technician delivering the ration, or the sound of approaching footsteps in the corridor.

  • Initially viewed by Pavlov as an unwelcome experimental nuisance disrupting digestive data collection, he quickly recognized that these "psychic secretions" represented a fundamental, unmapped form of associative learning.

• Rigorous Experimental Isolation and Methodology:

  • To examine conditioning objectively and eliminate extraneous visual, auditory, or olfactory confounding variables, Pavlov constructed an isolated laboratory environment.

  • Experimental subjects were secured within an isolation chamber using an apparatus harness.

  • A surgically attached cannula directed secretions from the dog's salivary duct into an external glass container connected to a measuring instrument calibrated to record the volume of saliva produced in cubic centimeters ($\text{cm}^3$).

  • The experimenter operated from an adjoining room, out of the animal's direct sensory field, delivering meat powder via automated sliding troughs or blowing it directly onto the canine's tongue at calibrated intervals.

• The Four Basic Components of Classical Conditioning:

  • Unconditioned Stimulus ($\text{UCS}$ or $\text{US}$): A stimulus that naturally, unconditionally, and automatically elicits a specific reflexive physiological response without prior learning (e.g., food or meat powder presented in an animal's mouth).

  • Unconditioned Response ($\text{UCR}$ or $\text{UR}$): An unlearned, naturally occurring automatic physiological reflex elicited by the unconditioned stimulus (e.g., salivation triggered directly by food in the mouth).

  • Neutral Stimulus ($\text{NS}$): An environmental stimulus that elicits no target conditioned response prior to learning (e.g., an auditory tone, a buzzer, a flashing light, a tactile touch on the leg, or the display of a geometric shape like a circle).

  • Conditioned Stimulus ($\text{CS}$): An originally irrelevant neutral stimulus that, through repeated temporal pairing with an unconditioned stimulus, acquires the capacity to trigger a learned response (e.g., the previously neutral tone after pairing).

  • Conditioned Response ($\text{CR}$): A learned physiological or behavioral reaction evoked by a previously neutral, now conditioned stimulus, contingent upon the learned association between the $\text{CS}$ and the $\text{UCS}$ (e.g., salivation elicited strictly by the auditory tone).

  • Fundamental heuristic: Conditioned equals learned; unconditioned equals unlearned.

• Human Sensory Analogues:

  • Identical classical pairing occurs in humans: when hungry adult subjects in London were shown abstract geometric visual patterns ($\text{NS}$) immediately prior to receiving exposures to appetitive aromas of peanut butter or vanilla ($\text{UCS}$), subsequent neural imaging showed that their brains responded in anticipation to the abstract images alone ($\text{CS}$) (Gottfried et al., 2003).

The Five Classical Conditioning Processes: Acquisition, Extinction, and Spontaneous Recovery

• Overview of the Five Processes:

  • Across three decades of work documented across $532$ published scientific papers on salivary conditioning (Windholz, 1997), Pavlov and his colleagues mapped five core associative mechanisms:

  1. Acquisition

  2. Extinction

  3. Spontaneous recovery

  4. Generalization

  5. Discrimination

• Process 1: Acquisition:

  • Acquisition represents the initial stage in classical conditioning during which an association between a neutral stimulus and an unconditioned stimulus is established.

  • Optimal Temporal Contiguity: Pavlov tested how much time should elapse between presenting the $\text{NS}$ and the $\text{UCS}$. In most classical conditioning paradigms, forward trace or delayed conditioning requires a short interval—typically roughly half a second ($0.5\,\text{s}$)—for optimal associative bonding.

  • Order of Presentation: The $\text{NS}$ must reliably precede the $\text{UCS}$. If the $\text{UCS}$ is presented before the $\text{NS}$ (backward conditioning), conditioning generally does not occur because the neutral stimulus fails to serve as a predictive biological signal.

  • Adaptive Value: Conditioning prepares organisms for impending biologically significant events. In wild settings, a snapping twig ($\text{CS}$) reliably precedes the arrival of a predator ($\text{UCS}$), enabling flight before physical contact.

  • Reproductive Conditioning Paradigm: Male Japanese quail exposed to an illuminated red light ($\text{CS}$) immediately preceding the entrance of a receptive female quail ($\text{UCS}$) developed conditioned sexual arousal to the red light alone (Domjan, 1992, 1994, 2005). The conditioned males developed an exclusive preference for the red-light zone of their testing enclosure, mounted the female significantly faster upon arrival, and released greater volumes of semen and sperm, directly demonstrating how classical conditioning enhances reproductive fitness (Matthews et al., 2007).

  • Conditioned Sexual Arousal in Humans: Neutral auditory, olfactory, or visual cues associated with sexual intimacy can develop into conditioned stimuli triggering physiological arousal (Byrne, 1982; Hoffmann, 2012, 2017).

  • For example, an olfactory stimulus such as onion breath ($\text{NS}$) does not naturally elicit sexual arousal. However, if a romantic partner frequently consumes onions prior to passionate kissing ($\text{UCS}$), the aroma becomes a $\text{CS}$ triggering anticipatory sexual arousal ($\text{CR}$) (Tirrell, 1990).

• Higher-Order Conditioning (Second-Order Conditioning):

  • A multi-tier learning process in which a well-established conditioned stimulus ($\text{CS}_1$) is paired with a novel neutral stimulus ($\text{NS}_2$), creating a second, often weaker conditioned stimulus ($\text{CS}_2$).

  • Procedural sequence: If an auditory tone has been conditioned to food ($\text{Tone} \rightarrow \text{Food} \rightarrow \text{Salivation}$), presenting a flashing light prior to the tone without food ($\text{Light} \rightarrow \text{Tone}$) eventually results in the light alone eliciting salivation ($\text{Light} \rightarrow \text{Salivation}$).

  • The established $\text{CS}$ functions analogously to an unconditioned stimulus during second-order conditioning.

  • Everyday human application: If a person is bitten by an aggressive dog, encountering that specific dog elicits fear; subsequently, the mere distant auditory cue of a dog barking ($\text{CS}_2$) paired with the concept or sight of a dog ($\text{CS}_1$) elicits conditioned anxiety.

• Process 2: Extinction:

  • Extinction is the diminishing and eventual cessation of a conditioned response ($\text{CR}$) when the conditioned stimulus ($\text{CS}$) is presented repeatedly in isolation, without the accompanying unconditioned stimulus ($\text{UCS}$).

  • In Pavlov's laboratory, repeatedly sounding the tone without presenting meat powder caused the volume of canine drooling to steadily decline until it dropped to baseline zero.

• Process 3: Spontaneous Recovery:

  • Spontaneous recovery is the sudden reappearance of an extinguished conditioned response following an elapsed rest period or temporal pause.

  • After extinguishing a canine's salivary reaction to a tone, Pavlov allowed several hours of rest during which no tone was presented. Upon sounding the tone again after this interval, the canine salivated.

  • The recovered $\text{CR}$ is generally weaker than the peak response observed at the conclusion of acquisition.

  • Theoretical Significance: Spontaneous recovery demonstrates that extinction does not physically erase, unlearn, or destroy the underlying associative memory trace. Rather, extinction represents active cortical inhibition or suppression of the conditioned response.

Generalization, Discrimination, and Adaptive Behavioral Plasticity

• Process 4: Stimulus Generalization:

  • Generalization is the automatic tendency, once a response has been conditioned, for stimuli resembling the conditioned stimulus to elicit similar conditioned responses.

  • Pavlov observed that canines conditioned to salivate to a specific pure tone of a given frequency salivated when exposed to tones of differing pitch.

  • In tactile conditioning trials, a canine conditioned to salivate when rubbed on the body also salivated when scratched (Windholz, 1989).

  • Topographical Mapping of Generalization: Pavlov attached miniature mechanical vibrators across multiple anatomical points on a dog's body. After establishing conditioned salivation to tactile vibration on the thigh, he activated vibrators at other locations. The volume of saliva elicited was directly proportional to the physical anatomical proximity of the stimulation to the conditioned thigh site (Pavlov, 1927):

  • Thigh (primary conditioned site): elicited maximal salivary volume ($\approx 60\,\text{drops}$).

  • Pelvis: elicited high intermediate output ($\approx 45\,\text{drops}$).

  • Trunk: elicited intermediate output ($\approx 23\,\text{drops}$).

  • Shoulder: elicited lower output ($\approx 22\,\text{drops}$).

  • Foreleg, front paw, and hind paw: elicited minimal, progressively smaller responses.

• Adaptive Function, Trauma, and Moral Generalization:

  • Survival Adaptations: Generalization allows learned behaviors to transfer to novel situations without requiring dangerous firsthand conditioning; toddlers conditioned to fear moving automobiles quickly generalize this protective avoidance to moving trucks and motorcycles.

  • Post-Collision Sensitization: Young automobile drivers involved in collisions exhibit heightened driving caution and reduced vulnerability to secondary vehicular crashes for a window of two months following an accident (O'Brien et al., 2017).

  • Trauma and Post-Traumatic Stress: Fear memories generalized across contexts can persist for years (Simon-Kutscher et al., 2019; Stout et al., 2018). An Argentine journalist imprisoned and tortured during political unrest noted that decades later, his body involuntarily flinched in terror at the mere sight of ordinary black dress shoes, which had been his first visual perspective of the torturers approaching his cell floor (Timerman, 1980).

  • Neurobiological Generalization in Abused Children: Children who have suffered physical abuse exhibit sensitized cortical processing; electroencephalogram (EEG) recordings confirm that abused children's brains produce disproportionately large neural spikes to angry facial expressions compared to non-abused peers (Pollak et al., 1998). This overgeneralized threat sensitivity contributes to an increased lifetime risk of psychological and anxiety disorders.

• Generalization of Emotional and Moral Disgust: Visual or conceptual association with noxious, contaminated, or morally transgressive objects triggers generalized revulsion (Rozin et al., 1986, 2015; Fedotova & Rozin, 2018):

  • Individuals routinely decline to eat gourmet chocolate fudge when it is molded into the realistic shape of canine feces.

  • Individuals refuse to hold or touch an otherwise clean dictionary once informed it was owned and handled by Adolf Hitler.

  • Individuals experience severe physical revulsion when asked to wrap themselves in a sanitized laundered blanket believed to have been previously owned by members of the al-Qaeda terrorist network.

  • Process 5: Stimulus Discrimination:

• Discrimination is the learned ability to distinguish between a conditioned stimulus (which reliably predicts the $\text{UCS}$) and other irrelevant, non-predictive stimuli.

• Pavlov's canines were trained to salivate exclusively to one specific tone frequency by presenting food after that tone while presenting other, slightly different frequencies without food; the canines learned to salivate to the predictive tone and withhold responses to unreinforced tones.

• Evolutionary Adaptations in Wildlife: Birds that experience gastrointestinal toxicity after consuming a poisonous butterfly species generalize their avoidance to identically colored mimics, but learn to discriminate and feed upon morphologically distinct edible butterfly species (Sims, 2018).

• Olfactory Discrimination in Elephants: African elephants in Kenya flee when detecting the distinct human scent of Maasai pastoralists, who historically spear elephants to protect cattle or settle resource competition, but show no avoidance behavior to the scent of neighboring Kamba agriculturalists, who do not hunt elephants (Rhodes, 2017).

• Contextual Human Discrimination: An individual's physiological heart rate will accelerate upon confronting an aggressive guard dog, but remain relaxed upon greeting an approaching service or guide dog.

• Humorous Failures of Discrimination: A cartoon depicting a garden snail attempting to court an office tape dispenser illustrates an evolutionary failure of discrimination, wherein structural and curved morphological similarities trigger misplaced reproductive instincts.

Pavlov's Enduring Scientific Legacy and Real-World Applications

• Universal Scope Across the Animal Kingdom:

  • Pavlov's primary contribution was demonstrating that classical conditioning is not an idiosyncrasy unique to canines, but a universal biological learning mechanism.

  • Conditioning has been confirmed across every major animal taxon examined, including single-celled protozoa, earthworms, aquatic fish, amphibians, birds, domestic dogs, non-human primates, and humans (Schwartz, 1984; Zhou et al., 2019).

  • Classical conditioning represents a foundational mechanism through which all motile organisms adaptively adjust internal physiology and outward behaviors to regularities within their ecological niches.

• Methodological Model for Psychology:

  • Pavlov provided the nascent field of psychology with an empirical, objective paradigm for measuring subjective phenomena.

  • His framework dispensed entirely with philosophical conjectures regarding the dog's internal thoughts, intentions, or introspective feelings, substituting instead precise, quantifiable physical metrics: cubic centimeters ($\text{cm}^3$) and drop counts of saliva collected in graduated glassware.

  • Pavlov demonstrated how complex adaptive behaviors can be dismantled into fundamental operational components, experimentally manipulated, and systematically quantified in controlled laboratory settings.

• Modern Neuroscience Validations:

  • Contemporary neurobiology has mapped the specific neural circuits and synaptic pathways that substantiate Pavlovian conditioning.

  • Researchers have isolated the exact neuronal architecture that binds a conditioned warning stimulus ($\text{CS}$) to an impending somatic threat or pain stimulus ($\text{UCS}$) within the amygdala and associated neural networks (Harnett et al., 2016; Yau & McNally, 2018).

• Applications in Consumer Psychology and Advertising:

  • Modern marketing exploits classical conditioning by pairing novel corporate logos, brand names, or neutral products ($\text{CS}$) with unconditioned appetitive stimuli ($\text{UCS}$), such as humor, attractive models, evocative music, or picturesque landscapes.

  • Empirical investigations demonstrate that these pairings reliably transfer conditioned positive or negative emotional evaluations to the previously neutral brand identities, determining consumer purchasing preferences (Alves et al., 2020).

Review Exercises and AP Science Practice Questions

• Check Your Understanding: Foundational Concepts:

  • Question 1: What is the formal definition of learning?

  • Answer: Learning is the process of acquiring through experience new and relatively enduring information or behaviors, allowing organisms to adapt to their dynamic environments.

  • Question 2: Explain the difference between associative learning and cognitive learning, providing an illustrative example of each.

  • Answer: Associative learning involves connecting two related events that occur in close temporal proximity. This encompasses classical conditioning (associating two environmental stimuli, such as lightning preceding thunder) and operant conditioning (associating a behavioral act with its environmental reward or punishment, such as a child saying "please" to receive candy). In contrast, cognitive learning involves acquiring mental information through verbal instruction, observation, or reading without direct physical conditioning (such as observing a sibling touch a hot stove and thereby mentally learning to avoid touching the burners yourself).

  • Question 3: Why are long-standing habits, such as consuming a sugary pastry alongside morning coffee or tea, exceptionally difficult to break?

  • Answer: Habits form as repetitive behaviors become linked to a specific physical context. Once these associations are consolidated across approximately $66$ days, entering that environment automatically triggers the conditioned behavioral response without conscious thought, particularly during states of cognitive depletion or mental fatigue.

  • Question 4: What is the name of the initial stage of classical conditioning during which a neutral stimulus ($\text{NS}$) becomes associated with an unconditioned stimulus ($\text{UCS}$) to transform into a conditioned stimulus ($\text{CS}$)?

  • Answer: Acquisition.

  • Question 5: What term describes the phenomenon wherein the $\text{UCS}$ no longer follows the $\text{CS}$, causing the $\text{CR}$ to progressively diminish and vanish?

  • Answer: Extinction.

  • Question 6: Differentiate between stimulus generalization and stimulus discrimination.

  • Answer: Stimulus generalization occurs when an organism demonstrates a conditioned response to novel stimuli that physically or conceptually resemble the original conditioned stimulus (e.g., flinching at both moving cars and moving motorcycles). Stimulus discrimination occurs when an organism learns to distinguish between the conditioned predictive stimulus and other non-predictive stimuli, producing the conditioned response only to the correct stimulus (e.g., fearing a snarling guard dog while remaining calm around a trained guide dog).

  • Question 7: Identify two major historical contributions of Ivan Pavlov's research program.

  • Answer: First, Pavlov demonstrated that classical conditioning is a universal principle of adaptation operating across virtually all animal species. Second, he established a rigorous, objective experimental methodology for psychology by demonstrating that learning could be measured quantitatively through observable physical metrics without relying on subjective introspection.

• Applied Conditioning Scenario (Eye-Blink Paradigm):

  • Scenario: An experimenter presents an auditory tone immediately prior to administering an automated puff of air directly onto a participant's cornea, causing an involuntary blink. After numerous repetitions of this pairing, the participant blinks upon hearing the tone alone.

  • Component Identification:

  • Neutral Stimulus ($\text{NS}$): The auditory tone prior to training (elicits no reflexive blink).

  • Unconditioned Stimulus ($\text{UCS}$): The physical puff of air directed at the eye.

  • Unconditioned Response ($\text{UCR}$): The automatic, biological eye blink triggered by the air puff.

  • Conditioned Stimulus ($\text{CS}$): The auditory tone following the acquisition phase.

  • Conditioned Response ($\text{CR}$): The learned eye blink elicited by the auditory tone alone.