Habituation vs sensory adaptation: the difference, and the tests that tell them apart

Sensory adaptation is a decrease in the response of a sensory receptor or a sensory system to a stimulus that continues or repeats. Habituation is a decrease in a behavioural response to a repeated stimulus, and sensory adaptation and fatigue do not explain that decrease. Researchers count habituation as a simple form of learning. In sensory adaptation, the sensory system responds less. In habituation, the senses still work, and the animal or person responds less.

Most study guides stop at "receptors versus brain". That rule is a useful start. But researchers do not define the two terms that way, and a well-known textbook example does not follow the rule. Here are the definitions, the three tests that researchers use, a method to sort exam scenarios, and the places where the two terms overlap.

• The APA Dictionary of Psychology defines sensory adaptation as "reduced responsiveness in a sensory receptor or sensory system caused by prolonged or repeated stimulation".
• A 2009 consensus paper by Catharine Rankin and colleagues defines habituation as a decrease in a behavioural response that "does not involve sensory adaptation/sensory fatigue or motor fatigue".
• Three tests separate the two: dishabituation, stimulus specificity, and the speed of recovery.
• In 1970, Eric Kandel's group studied a reflex in the sea slug Aplysia. The group traced habituation of the reflex to a synapse, not to the sensory receptors.
• "Receptors versus brain" is a simplification. Sensory adaptation also occurs in the brain.

The difference in one table

Sensory adaptation: the sensory system responds less

The APA Dictionary of Psychology has no separate definition under "sensory adaptation". The entry sends you to the entry for "adaptation". The second sense there is the relevant one: "reduced responsiveness in a sensory receptor or sensory system caused by prolonged or repeated stimulation". The open textbook Psychology 2e from OpenStax describes the result for the person: "we often don't perceive stimuli that remain relatively constant over prolonged periods of time".

Receptors adapt at different speeds. The textbook Biology 2e from OpenStax describes the Pacinian corpuscles in the skin as "rapidly adapting mechanoreceptors that sense deep transient (but not prolonged) pressure and high-frequency vibration". A receptor of this type responds to a short pressure, but not to a pressure that continues. The same chapter calls another touch receptor, the Merkel's disk, "slow-adapting".

Adaptation is not limited to receptors. Michael Webster of the University of Nevada, Reno, reviewed the research on sensory adaptation in 2012. He wrote that in the visual system "multiple stages of adaptation are well-documented". Adaptation to brightness or colour "begins as early in visual coding as the photoreceptors". Adaptation to patterns, such as the direction of a movement, "reflects changes at more central levels". In humans, these levels are thought to include the cortex.

The AP Psychology course framework puts sensory adaptation in Topic 1.6, Sensation. There, it is one explanation for "diminished sensitivity to stimuli".

Habituation: the animal learns to respond less

The standard research definition comes from a 2009 paper by Catharine Rankin and 13 colleagues. They wrote that habituation is "a behavioral response decrement that results from repeated stimulation and that does not involve sensory adaptation/sensory fatigue or motor fatigue". So the definition excludes sensory adaptation. If a tired receptor or a tired muscle causes the decrease, the decrease is not habituation.

Shorter definitions say the same thing with fewer conditions. The APA Dictionary entry for habituation gives this sense: "the diminished effectiveness of a stimulus in eliciting a response, following repeated exposure to the stimulus". The AP framework lists habituation in Topic 3.7, Classical Conditioning. Its wording is that habituation "occurs when organisms grow accustomed to and exhibit a diminished response to a repeated or enduring stimulus". The topic name can mislead you. Habituation is not classical conditioning, as the next paragraph explains.

Researchers count habituation as learning. Rankin and colleagues note that it is termed "the simplest form of learning". It is non-associative: one stimulus repeats, and the animal learns nothing about a second event. That is the difference from classical and operant conditioning, where the animal learns a relation between two events. According to the same paper, habituation "allows animals to filter out irrelevant stimuli and focus selectively on important stimuli".

The list of its properties is old and stable. In 1966, Richard Thompson and William Alden Spencer described nine behavioural characteristics of habituation. In August 2007, a group of habituation researchers met to revise the list. The 2009 paper is the result of that meeting. It says that the 1966 descriptions "have held up remarkably well".

The group added a tenth characteristic, long-term habituation, which can last hours, days or weeks. They also gave a warning. In their words, "remarkably little is known about the neural mechanisms" of habituation.

The sea slug that showed where habituation happens

Classic evidence comes from the sea slug Aplysia. When something touches its skin, it withdraws its gill. When the touch repeats, the reflex becomes weaker.

In 1970, Eric Kandel's group published two papers on this reflex in Science. In the first, Kupfermann and colleagues recorded from the motor neurons of the gill while the animal habituated. They reported that peripheral changes in the sensory receptors or in the gill muscles did not cause habituation or dishabituation. The cause was a change in the excitatory synaptic potentials at the motor neurons.

In the second paper, Castellucci and colleagues recorded from a sensory neuron and a motor neuron at the same time. The synaptic potential between them showed "a profound low-frequency depression" when they elicited it repeatedly. They concluded that habituation and dishabituation "can be explained in part and perhaps entirely by changes in the efficacy of specific excitatory synapses".

So in this reflex, the receptors did not cause the decrease. The connection behind them became weaker. Do not apply this result to all habituation. Rankin and colleagues expect many different cellular mechanisms in different parts of the nervous system.

Three tests that tell them apart

A weaker response alone does not show which process occurred. Rankin and colleagues list three tests that distinguish habituation from sensory adaptation or fatigue:

1. 1.Dishabituation. Present a different stimulus. If the response to the original stimulus then increases again, the decrease was habituation.
2. 2.Stimulus specificity. Present a new stimulus in the same sense. A habituated animal still responds to it. The authors expect sensory adaptation or fatigue to "generalize across a broad range of stimuli within a sensory modality".
3. 3.Frequency-dependent spontaneous recovery. Stop the stimulus and measure the recovery. After habituation, the response recovers faster when the stimuli came at a high frequency than when they came at a low frequency.

Two old demonstrations show the logic. In his history of habituation, Thompson quotes one from a 1933 book on learning by G. Humphrey.

Someone claps their hands behind a baby every two seconds. The baby blinks at first, and the blinking usually stops by the sixth or seventh clap. Then the cradle gets a sharp blow. At the next clap, the baby blinks again. A different stimulus restored the response at once. That is dishabituation.

The second demonstration is older. Thompson reports that C. S. Sherrington analysed the "fatigue" of reflexes in dogs in a 1906 book. Sherrington stimulated the skin next to the area that he had stimulated repeatedly. The response was weaker there too, so receptor adaptation could not be the cause. The muscle also responded normally to a different reflex, so muscle fatigue was not the cause. In Thompson's summary, the fatigue was "a central phenomenon".

Be careful with the names. Dishabituation is the return of the response to the old stimulus. A full response to a new stimulus is stimulus specificity. Rankin and colleagues note that many studies, for example developmental studies of language acquisition, have "improperly termed" the second test a dishabituation test. The APA Dictionary describes dishabituation as "a useful method for investigating perception in nonverbal individuals or nonhuman animals".

Some systems show no dishabituation. The authors give that as one reason to use the other two tests.

Classifying exam scenarios

Two questions sort most scenarios:

1. 1.Does the scenario describe a receptor or a sense organ that responds less, or an animal or a person that responds less?
2. 2.Does a different stimulus, or a change of the stimulus, restore the response at once?

If a receptor responds less, the answer is sensory adaptation. If a behaviour decreases across repeated trials, or a change of stimulus restores the response, the answer is habituation.

The lemon juice scenario is a real sample question from the AAMC, the organisation behind the MCAT. In the question, salivation declines across the trials, and then the researcher changes to lime juice. The correct answer is "Habituation and dishabituation". The AAMC explanation points to two signs: reduced responding to a repeated stimulus, and then a change in the stimulus.

The question does not say which response the researcher measures after the change. By the strict definition from Rankin and colleagues, dishabituation is a stronger response to the lemon juice after the lime juice. A full response to the lime juice shows stimulus specificity. For the exam, remember the pattern: a repeated stimulus, a declining response and a change of stimulus point to habituation and dishabituation.

These answers follow from the definitions above. In a laboratory, only the three tests can show which process occurred.

Where the two terms overlap

The first problem is a textbook example. OpenStax Psychology 2e illustrates sensory adaptation with a flashing light outside a hotel window. After some time you no longer notice it. But the text also says that "the photoreceptors in your eyes still sense the light, but you no longer perceive the rapid changes in lighting conditions". In this example, the receptors still respond and the person responds less. That fits the definition of habituation at least as well as it fits "reduced responsiveness in a sensory receptor".

If your course uses this textbook, the expected exam answer is still sensory adaptation. Learn the definition that your course uses.

The second problem is the rule itself. Sensory adaptation is not only a receptor process. The APA definition says "sensory receptor or sensory system", and Webster's review describes adaptation at many levels of the visual system. Adaptation can also be specific. The APA Dictionary notes that it "may be specific, for example, to the orientation of a particular stimulus". So a full response to a new stimulus does not always prove habituation.

The third problem is that research fields use the words differently. In a 2017 review of human smell, Pellegrino and colleagues call the decreased behavioural response to an odour habituation. They use adaptation for "the neural processes that constitute this decrease". A 2023 review by Merchie and Gomot is equally careful. It describes neural adaptation as a "possible processing link to habituation".

A safe summary is this. Habituation is defined by behaviour and by the three tests. Sensory adaptation is defined by the reduced responsiveness of a sensory system. "Receptors versus brain" is a simple rule. It agrees with the classic examples, such as the reflex of Aplysia and the rapidly adapting touch receptors. It is not the definition.

Keeping the two terms apart

A memory aid: in sensory adaptation, the sense adapts. In habituation, the animal forms a habit: it does not respond. Learn the two terms together with dishabituation and sensitisation. The AAMC sample question above combines two of them.

Pairs of similar terms are hard to keep in memory if you only read about them. Our guide to memorising psychology terminology explains why a test of your own memory is better than a second reading. Spacing your reviews explains why short sessions on different days are better than one long session.

Psychology World is made for this type of practice. You look up the terms and save them to a list of your own. Then you drill the list as flashcards, as multiple-choice questions when the list has four or more terms, or by typing the answer. The home page shows the four steps of the learning loop and what each study mode does.