Psychology of Learning

Module 04: Classical Conditioning 1

Part 1: Basics of Classical Conditioning

Looking Back

In Module 03, we explored unlearned adaptive behaviors and proto-learning processes—reflexes, tropisms, fixed action patterns, habituation, and sensitization. We also explored opponent-process theory, which explains how emotional responses change with repeated experiences through the interaction of a-processes and b-processes. Now we turn to our first true learning phenomenon: classical conditioning. Unlike habituation and sensitization, which simply involve response changes to repeated stimulation, classical conditioning involves learning new stimulus-response relationships through association.

Classical Conditioning in the Real World: Taste Aversions

Have you ever eaten an unusual food and later been sick to your stomach? If so, it’s likely you developed an aversion to that food—even if the food didn’t actually cause your illness. This common experience illustrates classical conditioning in action: You eat a food that initially has no negative connotations (like Hawaiian Delight Pizza). You get a stomach virus and become sick. You never eat Hawaiian Delight Pizza again!

This scenario demonstrates how neutral stimuli (the taste of pizza) can become associated with unpleasant experiences (nausea) to produce lasting behavioral changes (food avoidance). The pizza didn’t make you sick, but your nervous system associated the two events, creating a powerful learned aversion. Understanding classical conditioning explains this and countless other everyday experiences.

Pavlov’s Discovery & Its Impact

Ivan Pavlov (1849-1936), a Russian physiologist, won the 1904 Nobel Prize in Medicine and Physiology for his research on digestive processes—not for discovering classical conditioning. Before Pavlov’s work, knowledge of digestion came from dead or surgically traumatized animals. Pavlov developed an ingenious surgical technique: he prepared a channel (fistula) from a dog’s digestive organs to the outside of the body, allowing him to collect and measure digestive secretions from healthy, intact animals (Pavlov, 1927).

Beginning in 1910, the Institute of Experimental Medicine in St. Petersburg built Pavlov a state-of-the-art research facility. This “Tower of Silence” allowed precise control over experimental conditions, free from outside disturbances. Here Pavlov conducted the systematic research that would establish classical conditioning as a fundamental form of learning (Todes, 2014).

The Accidental Discovery

Pavlov’s basic procedure for studying digestion consisted of placing meat powder on the tongues of dogs. He carefully collected and measured the gastric juices and saliva produced when dogs tasted the meat powder. The taste of food results in the involuntary reflex of salivation—an unconditioned reflex that requires no learning (Pavlov, 1927).

During his research, Pavlov noticed something unexpected: other stimuli, such as the sight of the researcher or the experimental equipment, caused dogs to salivate before they received the meat powder. He called this anticipatory salivation “psychic secretions.” Because researchers at this time thought behaviors like the salivation reflex were innate and unchangeable by experience, these observations intrigued Pavlov. This observation—that dogs learned to salivate to previously neutral stimuli through experience—became the foundation for studying classical conditioning (Pavlov, 1927).

The Basic Terminology of Classical Conditioning

Classical conditioning is a form of learning in which an initially neutral stimulus is paired with an unconditioned stimulus and becomes a conditioned stimulus that elicits a conditioned response. This definition requires understanding four key terms that describe the stimuli and responses involved (Pavlov, 1927).

The unconditioned stimulus (US) is a stimulus that reflexively elicits a response without any prior learning. In Pavlov’s research, meat powder served as the US because it automatically caused dogs to salivate. The US triggers an innate, unlearned response (Pavlov, 1927).

The unconditioned response (UR) is the response that is reflexively elicited by the US. In Pavlov’s research, salivation to meat powder was the UR. The UR occurs automatically without learning—it’s part of the organism’s innate behavioral repertoire (Pavlov, 1927).

The conditioned stimulus (CS) is a stimulus that, following pairing with the US, comes to elicit a response. Initially neutral, the CS acquires the power to elicit responses through association with the US. Pavlov used various stimuli as CSs—most famously a metronome (not a bell, despite popular belief), but also tones, lights, and other detectable stimuli (Pavlov, 1927).

The conditioned response (CR) is the response that is elicited by the CS after conditioning. In Pavlov’s research, salivation to the metronome was the CR. The CR resembles the UR but typically differs in strength, timing, or other characteristics (Pavlov, 1927).

An important note about terminology: Pavlov actually used a metronome (a device that produces a steady tick-tock sound), not a bell or tuning fork. The “Pavlov’s bell” story became popularized but isn’t historically accurate. Pavlov carefully chose stimuli that could be precisely controlled and measured (Todes, 2014).

The Process of Classical Conditioning

We can diagram classical conditioning in three stages:

First: A US reflexively elicits a UR. This is the unconditioned reflex—the starting point. For example, meat powder (US) reflexively elicits salivation (UR). This relationship exists before any conditioning occurs.

Then: A CS is paired with the US several times. During acquisition (the learning phase), the neutral stimulus (which will become the CS) is presented together with or just before the US. For example, a metronome sound is presented, followed immediately by meat powder. This pairing is repeated multiple times.

Finally: The researcher presents the CS by itself. If conditioning has occurred, the CS now elicits a CR. The metronome sound alone now elicits salivation. The previously neutral stimulus has acquired the power to elicit a response through association with the US.

The crucial requirement is that the previously neutral tone must be detectable. The organism must be able to sense the stimulus for it to become associated with the US. As long as the stimulus is detectable, almost anything can serve as a CS—sounds, lights, smells, tastes, touches, or even internal physiological states.

Classical Conditioning in Everyday Life

Classical conditioning isn’t limited to laboratory dogs and metronomes. It occurs constantly in everyday life. Consider these examples:

The Kiss and Onion Breath Example: Before learning, a passionate kiss reflexively elicits sexual arousal. This is an unconditioned reflex—the kiss (US) produces arousal (UR). Initially, onion breath (neutral stimulus) does not elicit sexual arousal. But imagine repeatedly experiencing passionate kisses (US) from a partner who always has onion breath (CS). Through repeated pairings, the onion breath becomes a CS that elicits arousal (CR).

The Shower Scenario: If you’ve ever been in a shower when someone flushed a toilet, you know what happens—sudden scalding hot water. After one or two experiences, the sound of the toilet flushing (CS) elicits flinching and jumping away (CR) even before the water temperature changes. The sound becomes associated with the scalding water (US) that reflexively causes you to jump (UR).

These examples show how classical conditioning operates in real life, often without awareness. Your nervous system continuously forms associations between stimuli, creating learned responses that help you anticipate and prepare for important events.

Modern Applications of Classical Conditioning

Classical conditioning principles have proven remarkably useful in clinical settings. Exposure therapy, a first-line treatment for anxiety disorders and phobias, directly applies Pavlovian principles. Through repeated exposure to feared stimuli without the anticipated negative consequences, patients learn that the conditioned stimulus no longer predicts danger, leading to extinction of the fear response (Chowdhury & Khandoker, 2023).

Recent neuroimaging research has revealed the brain mechanisms underlying classical conditioning. Using high-resolution fMRI, researchers have found that the striatum and orbitofrontal cortex encode stimulus-stimulus associations during Pavlovian conditioning, suggesting that even this basic form of learning involves more complex cognitive processes than simple stimulus-response connections (Pauli, Gentile, Collette, Tyszka, & O’Doherty, 2019).

Acquisition: The Initial Stage of Learning

Acquisition is the initial stage of learning during which a response is established and gradually strengthened. During acquisition, the organism is learning to associate the CS with the US. The neutral stimulus is becoming a conditioned stimulus (Pavlov, 1927).

Initially, researchers believed the CR developed gradually and that it was unusual for a strong CR to appear after only one pairing of CS and US. Pavlov’s early work suggested that multiple pairings were necessary for learning. However, these assumptions have been challenged. Some types of conditioning—especially taste aversion learning—can occur after a single pairing (Garcia & Koelling, 1966).

Once the basic process was established, researchers quickly began projects to determine what factors affected classical conditioning. What makes conditioning stronger or weaker? What timing relationships work best? These questions drove decades of research.

Are the UR & CR Really the Same?

In Pavlov’s research, both the UR and CR were salivation. Does this mean they’re identical? Not really. Because the US is part of an innate reflex, the food powder US is very likely to elicit a stronger response (the UR) than the CS elicits (the CR). The UR also includes other digestive processes like stomach contractions that may not appear as strongly in the CR (Pavlov, 1927).

The UR represents the organism’s full, innate response to a biologically significant stimulus. The CR represents what the organism has learned to do in anticipation of that stimulus. The CR often resembles the UR but is typically weaker, shorter in duration, or different in quality. Sometimes the CR even opposes the UR—a phenomenon we’ll explore when discussing drug tolerance and compensatory responses (Siegel, 1975).

This distinction between UR and CR is important theoretically. It shows that conditioning doesn’t simply transfer the response from one stimulus to another. Rather, the organism learns something new about relationships between events in the environment, and this learning manifests as a conditioned response.

Looking Forward

We’ve established the basic terminology and process of classical conditioning, understanding that neutral stimuli can acquire the power to elicit responses through pairing with unconditioned stimuli. In Part 2, we’ll examine the factors that affect acquisition—how many CS-US pairings are needed, how strong the US must be, what timing relationships work best—and explore extinction, spontaneous recovery, and the distinction between appetitive and aversive conditioning.