Psychology of Learning

Module 03: Unlearned Adaptive Behaviors

Part 2: Habituation & Sensitization

Looking Back

In Part 1, we explored elicited behaviors that occur without learning—reflexes, tropisms, kineses, taxes, and fixed action patterns. These innate behaviors are triggered by specific stimuli and unfold in characteristic patterns shaped by evolution. Now we turn to phenomena that occupy a borderline position between reflexive behavior and true learning: habituation and sensitization. When stimuli are presented repeatedly, responses change—either decreasing (habituation) or increasing (sensitization). These changes result from experience, which sounds like learning, but understanding their simpler mechanisms is crucial for understanding the more complex forms of learning we’ll study in later modules.

Proto-Learning or Quasi-Learning?

When a novel stimulus is presented repeatedly, changes in behavior can occur that may not involve true learning. Consider this scenario from your own experience: Someone sneezes right behind you and you are startled. Another sneeze—you’re startled again but less so. After several more sneezes, you barely react at all. Your startle response decreased with repeated sneezes until you hardly responded. This is habituation in action.

Psychologists debate whether habituation and sensitization constitute true learning or represent simpler processes that precede learning. Some call them proto-learning (precursors to learning) or quasi-learning (learning-like but not quite learning). They meet some criteria for learning—they involve behavior change resulting from experience—but they’re simpler and shorter-lasting than the learning we’ll study in classical and operant conditioning (Thompson & Spencer, 1966).

Habituation: Decreased Response to Repeated Stimulation

Habituation occurs when repeated presentations of a stimulus result in a decrease in the strength of the elicited response. The organism becomes less responsive to the stimulus as it becomes familiar (Harris, 1943; Thompson & Spencer, 1966).

Habituation is ubiquitous—it occurs across all animal species and affects virtually all response systems. You habituate to the feeling of clothing against your skin, to background noise in a room, to the sight of familiar objects. Without habituation, you’d be constantly responding to every stimulus in your environment, unable to focus attention on novel or important events.

Habituation in Aplysia: A Model System

Researchers have studied habituation extensively in Aplysia, a marine snail (sea slug) with a relatively simple nervous system. Eric Kandel won the 2000 Nobel Prize in Physiology or Medicine for his work using Aplysia to understand the neural basis of learning and memory (Kandel, 2001).

Aplysia has a gill-withdrawal reflex: touching the siphon (a fleshy spout used for water intake) causes the siphon and gill to reflexively withdraw under the protective cover of the mantle. When researchers touch the siphon every 30 seconds, the gill-withdrawal reflex gradually declines in strength. After as few as 10 touches, habituation can last 2 to 3 hours (Pinsker, Kupfermann, Castellucci, & Kandel, 1970).

Why is Aplysia so valuable for studying habituation? The simplicity of its nervous system allows researchers to identify exactly which neurons are involved and how they change during habituation. Aplysia has only about 20,000 neurons (compared to roughly 86 billion in humans), and many are large enough to see individually and record from with electrodes (Kandel, 1976).

Neural Mechanisms of Habituation

Kandel and colleagues discovered that habituation involves changes at the synapse—the connection between neurons. Specifically, habituation involves a decrease in excitatory neurotransmitter release at the synapses between sensory neurons and interneurons (Castellucci & Kandel, 1974).

Here’s what happens: The sensory neurons remain sensitive to stimulation—they still detect the touch. The motor neurons remain capable of triggering the withdrawal response. But less neurotransmitter is released from the sensory neurons’ axon terminals. This occurs because calcium influx into the axon terminals decreases, and since calcium triggers neurotransmitter release, less neurotransmitter flows into the synapse. With less neurotransmitter, the interneurons receive weaker signals, leading to weaker activation of motor neurons and a weaker withdrawal response (Castellucci & Kandel, 1974).

This demonstrates that learning can involve local rather than global changes in the nervous system. You don’t need to rewire the entire brain to produce habituation—just change the effectiveness of specific synapses. This finding revolutionized our understanding of the neural basis of learning.

Habituation Versus Sensory Adaptation

It’s crucial to distinguish habituation from sensory adaptation. Both involve decreased responding to repeated stimulation, but they have different mechanisms and different implications.

Sensory adaptation is a decrease in sensation that results from repeated presentation of a stimulus. Unlike habituation, adaptation occurs from receptor fatigue—the sensory receptors simply cease to function in a normal, responsive manner (Dallenbach, 1939). Your clothing pressing against thousands of touch receptors on your skin all day long provides an example. The receptors initially respond to the pressure but gradually become less responsive through fatigue.

How can researchers demonstrate that decreased responding results from habituation rather than adaptation? The answer is simple but clever: change to a different stimulus. If you’ve habituated to sneezes and someone coughs, the startle response should return—habituation is stimulus-specific. But if sensory adaptation has occurred, changing from sneezes to coughs should produce minimal startle because the receptors remain fatigued regardless of the specific sound (Harris, 1943).

Dishabituation: Recovery of Response

Dishabituation is the recovery of a habituated response following presentation of a novel stimulus. A strong, unexpected stimulus can suddenly restore a response that had habituated (Thompson & Spencer, 1966).

Imagine you’ve habituated to repeated sneezes and stopped startling. Suddenly, a loud door slam occurs. This novel, intense stimulus causes sensitization (which we’ll discuss next), and your startle response to the next sneeze is restored. The habituated response has temporarily recovered—it’s been dishabituated.

Dishabituation demonstrates that habituation isn’t simply receptor fatigue or muscle fatigue. The mechanisms for responding remain intact; they’re just temporarily suppressed. A sufficiently strong stimulus can overcome this suppression and restore responding.

Sensitization: Increased Response to Repeated Stimulation

Sensitization occurs when repeated presentation of a stimulus results in an increase in the strength of subsequent elicited responses. The organism becomes more responsive, showing heightened reactivity to stimuli (Groves & Thompson, 1970).

Sensitization is essentially the opposite of habituation. While habituation involves decreased responding to familiar stimuli, sensitization involves increased responding. If you experience a threatening or intense stimulus, you often become more reactive to other stimuli for a period afterward.

In Aplysia, if an experimenter administers a mild electric shock to the tail, the sensitivity of the gill-withdrawal response increases. Subsequently, lighter-than-normal touches to the siphon produce the withdrawal response. Depending on the number of shocks administered, this sensitization can last from a few minutes (single shock) to several days (series of shocks) (Pinsker et al., 1970).

The neural mechanism involves facilitating interneurons that enhance neurotransmitter release from sensory neurons. Sensitization involves different neural circuits than habituation—circuits that amplify responses rather than diminish them (Kandel & Schwartz, 1982).

General Characteristics of Habituation & Sensitization

Both habituation and sensitization help organisms adapt to their environment, but in different ways:

Adaptive Value: If something occurs repeatedly and causes no harm, it’s probably not important—you can ignore it (habituation) and save your resources for other things. But if you encounter a snake, even though it didn’t bite you, you should remain doubly alert (sensitization) for other snakes for a while. Both processes optimize resource allocation.

Duration of Effect: Both processes are relatively short-lasting compared to true learning. Habituation and sensitization typically last minutes to hours, occasionally days. In contrast, learned behaviors can last months, years, or even a lifetime. This shorter duration is why some psychologists hesitate to classify habituation and sensitization as true learning (Thompson & Spencer, 1966).

Stimulus Specificity: Habituation is very stimulus-specific. If you habituate to a particular tone, a slightly different tone will elicit responding. In contrast, sensitization generalizes widely—even across different sensory modalities. If a snake frightens you, you become more reactive not just to potential snakes but to all sorts of stimuli: sudden movements, unexpected sounds, unusual shadows (Groves & Thompson, 1970).

Stimulus Intensity: Although exceptions exist, habituation and sensitization occur most strongly with moderate stimulus intensities. Very weak stimuli may not produce enough responding to habituate. Very strong, threatening stimuli tend to produce sensitization rather than habituation—you don’t habituate to genuinely dangerous stimuli (Thompson & Spencer, 1966).

The Dual-Process Theory

We’ve established that habituation doesn’t reflect sensory adaptation or muscle fatigue. So where does habituation occur? Given that sensory neurons and motor neurons remain functional, habituation and sensitization must involve interneurons and their interfaces with sensory and motor neurons (Groves & Thompson, 1970).

The dual-process theory, proposed by Groves and Thompson (1970), states that when a stimulus is presented, the potential exists for both habituation and sensitization to occur simultaneously. A single stimulus can elicit both processes—a loud noise may elicit a startle response that decreases with repeated presentations (habituation) while also increasing the organism’s responsiveness to other stimuli (sensitization).

Which Process Dominates?

If both sensitization and habituation result from stimulus presentation, which process manifests behaviorally? The stronger process determines the behavioral outcome. We can think of this mathematically: if habituation produces a -11 effect while sensitization produces a +4 effect, the net result is -7 (decreased responding)—habituation dominates. But if the effects are equal, they cancel. If sensitization is stronger, sensitization dominates (Groves & Thompson, 1970).

The relative strengths of these processes depend on several factors: stimulus intensity, novelty, biological significance, and the organism’s current state. Weak, familiar, non-threatening stimuli favor habituation. Strong, novel, potentially threatening stimuli favor sensitization. This makes adaptive sense—ignore the unimportant, attend to the potentially important.

Experimental Evidence for Dual-Process Theory

Groves and Thompson (1970) provided compelling evidence for dual-process theory. They repeatedly presented tones to rats while measuring startle responses. With moderate-intensity tones, initial presentations produced sensitization (increased startle), but continued presentations led to habituation (decreased startle). The transition from sensitization to habituation reflects the changing balance between the two processes.

Furthermore, they showed that different neural pathways mediate the two processes. Habituation involves the direct reflex pathway (sensory → interneurons → motor neurons). Sensitization involves a separate “state” system that modulates overall arousal and reactivity. These systems operate simultaneously, and behavior reflects their combined influence (Groves & Thompson, 1970).

Why Habituation & Sensitization Matter

Understanding habituation and sensitization is important for several reasons:

Foundation for Learning: These processes provide the simplest examples of experience-dependent behavior change. Classical conditioning, which we’ll study in Module 04, builds directly on these mechanisms.

Clinical Applications: Habituation plays a role in treating phobias and anxiety through exposure therapy. Repeated, safe exposure to feared stimuli leads to habituation of fear responses. Understanding sensitization helps explain why traumatic experiences can produce lasting hypervigilance (Marks, 1987).

Attention & Perception: Habituation allows organisms to filter out irrelevant stimuli and focus attention on novel or important events. The classic cocktail party effect—your ability to focus on one conversation while ignoring others—relies partly on habituation to background noise (Cherry, 1953).

Developmental Studies: Habituation is used to study infant perception and cognition. Infants look longer at novel stimuli than familiar ones. By measuring how quickly infants habituate to stimuli, researchers can assess what infants perceive as similar versus different, providing insights into early cognitive development (Fantz, 1964).

Looking Forward

We’ve explored habituation and sensitization—the simplest forms of experience-dependent behavior change—seeing how these processes have distinct neural mechanisms, serve adaptive functions, and operate simultaneously according to dual-process theory. In Part 3, we’ll examine opponent-process theory of emotion, which extends these ideas to explain emotional experiences, drug tolerance, addiction, and why emotional reactions change with repeated experiences.