Habituation

Any unconditioned response that is elicited in response to an unconditioned stimulus will show habituation if the stimulus is repeated. That is, measures of the response will show orderly decreases in magnitude as the stimulus is repeatedly applied. Habituation can be seen in the withdrawal responses of protozoa to tactile stimulation, in the freezing-defensive responses of rodents to auditory stimuli, and in the orienting responses of human infants to complex auditory and visual stimuli; it is one of the most fundamental properties of behavior. More simply put, habituation is a process in which a stimulus loses novelty and in which decreases in responsiveness to the stimulus are seen when the stimulus is presented repeatedly or for an extended time period. An organism ceases to engage in attending or otherwise responding to the stimulus; the stimulus comes to be ignored.

One of the most apparent trends in recent psychology has been the attempt to redefine basic behavioral processes in terms of more complex cognitive processes; as a result, habituation used to be considered nonassociative learning because only one stimulus was involved but has been termed implicit learning by some writers, implying that “implicit memory” may be involved.

What Are Some Of The Properties Of Habituation?

The formal study of habituation goes back nearly a century, being described by writers in the early 20th century and with the basic properties being outlined in the 1930s and again in the 1960s and 1970s. Some nine basic features of habituation were formally outlined and have come to serve as a formal definition of habituation:

1. If a specific stimulus elicits a response, repeated presentations of that stimulus result in decreased response or habituation;  the  decrease  is  typically an orderly function of the number of stimulus presentations.

2. If the stimulus is subsequently withheld, the response showing habituation will tend to return or recover with the passage of time, termed spontaneous recovery.

3. If the habituation experience and subsequent opportunity for recovery are repeated in a series, then habituation will progress more rapidly.
4. With all other variables held constant, the higher the frequency of stimulation, the more rapid and pronounced is habituation.

5. With weaker stimulation, habituation will progress more rapidly and be more pronounced.

6. The effects  of  habituation  training  may  progress below baseline measures of the response.

7. If habituation occurs to a specific stimulus, then habituation to similar stimuli through stimulus generalization also occurs.

8. Presentation of  an  irrelevant  stimulus  will  result in the recovery of the habituated response, termed dishabituation.

9. With repeated presentation of the dishabituating stimulus of number 8, the degree of resultant dishabituation is decreased or shows habituation itself.

A modified list of defining features has also been put forth, with the following additional defining characteristics:

1. The process of habituation will be disrupted by any unpredictable changes in the eliciting stimulus.

2. Habituation will occur more slowly to stimuli that are presented in a varying manner relative to stimuli presented in a constant, unchanging manner.

3. Spontaneous recovery may occur faster after more rapid rates of stimulus presentation relative to slower rates of stimulus presentation.

4. Spontaneous recovery may be incomplete recovery in that some habituation will persist over a long time period.

5. Habituation will  be  observed  for  most  if  not  all animal species; habituation occurs in response to most stimuli, including events without ingestive consequences such as lights and noises. The rate at which habituation will be observed will differ as a function of the species, the stimuli employed, the responses being measured, and the individual subject.

As defining features, any behavioral stimulus-response contingency is said to display habituation if the response decrement shows these properties.

How Is Habituation Studied In Infants?

In general, studies of human infant habituation employ variations of some basic themes. In measuring the rate of a reflex such as the sucking reflex, researchers measure the initial baseline rate of sucking to a pacifier equipped with transducers to measure muscle contractions applied to it. Then an auditory stimulus is presented, such as a human voice uttering a speech phoneme such as “ba.” In response to a novel stimulus such as this, the rate of sucking will predictably increase but come to decline to baseline levels as “ba” is repeatedly presented, showing habituation. If the phoneme “pa” were to then be presented, the rate of sucking would again increase, demonstrating that the infant can discriminate between these two stimuli.

To measure more complex responses, slightly different procedures are used. When an intense or novel stimulus is presented, an orienting response is elicited, and an infant (or any person) turns to look and fix the gaze at the source of a visual stimulus or turns the head toward  the  source  of  an  auditory  stimulus,  as  well as shows changes in heart rate, electroencephalogram (EEG) patterns, and other autonomic responses. Initially, infants show this orienting (or orientation) response to a stimulus, but after repeated trials of stimulus presentations, infants no longer respond to the stimulus, showing habituation of the orienting response. Researchers in human development have used measures of the orienting response as assessment tools of complex human development, described in the next section.

Role Of Habituation In Human Development And Learning

Habituation procedures have increasingly been employed to assess the covert-behavioral abilities of preverbal infants to study the development of perception, remembering, and what is termed information processing as part of basic developmental research. In addition, the performance of infants on habituation tasks has been shown to be a reliable and independent predictor of long-term development. Indexes of the speed or efficiency with which infants show habituation have been shown to predict outcomes in behaviors such as language acquisition as well as more general behavioral outcomes such as verbal and nonverbal intelligence. Infants who show difficulty during habituation or habituate at slower than normal rates have been found to be at increased risk for a range of significant developmental delays. Populations of infants with Down syndrome, teratogen-exposed infants, malnourished infants, and premature infants have been studied.

Some  researchers  have  found  that  at  an  age  of 16 months, high-risk infants show rates of habituation comparable to newborn infants. Full-term infants have been shown to have more favorable measures of habituation indices than did preterm infants (birth weight < 1,750 grams) at 5, 7, and 12 months. Despite its widespread use, some writers have argued that the precise psychometric parameters of habituation need to be established before any definitive and significant dialogue on the predictive value of habituation can be conducted. Some such studies have also been criticized for using highly artificial events as stimuli, such as an artificial voice box, instead of a stimulus with higher ecological validity, such as recordings of actual human utterances as auditory stimuli.

Despite cross-study comparisons being obfuscated by procedural differences as well as in the response being measured, and despite that normally developing infants show a range of individual variations in habituation, acceptable measures of test-retest reliability have been obtained for some measures of habituation. Unfortunately, few such data exist for populations of high-risk infants, among whom the pertinent data would be of greater interest.

Why And How Does Habituation Occur?

The observation that habituation is so catholic points to an obvious and basic adaptive significance. Constantly responding to meaningless stimuli would be taxing and wasteful to any organism; learning to not respond to biologically irrelevant events while still responding to events that are biologically significant has clear survival value.

For some stimulus-response contingencies that show habituation, the physiological mechanisms have been identified, and although habituation has 9 (or 14) defining features, only one established physiological process has been elucidated. In studies of simple reflexes of invertebrates, such as a defensive-withdrawal reflex to tactile stimulation, the organism ceases to respond as the repeated stimulation causes a sensory neuron receptor to no longer release the necessary neurotransmitter to an effector motor neuron for the response to occur. The sensory neuron will continue to release its neurotransmitter in response to a different stimulus, and the reflex will still occur to a different stimulus but not to the stimulus to which habituation has occurred. That is,  the  observed  change  in  responsiveness  will  be stimulus specific but show stimulus generalization to physically similar stimuli.

In terms of identifying the biological bases of habituation in vertebrates, less progress has been made, but research so far points to similar neural events taking place in distinct lower brain areas depending on the sensory modality of the stimulus and the nature of the response undergoing habituation. Little if any research has been conducted into the biological basis of habituation in humans, but the infrahuman models of habituation are considered more or less applicable.

Summary

Habituation is a ubiquitous behavioral phenomenon, seen with virtually every response of a wide range of organisms to repeated stimulation, and because it is so universal, it has been argued as having adaptive value. Whether it merely reflects basic behavioral processes such as learning or more complex actions such as attention or memory is debated. Researchers in human development have studied habituation both for purposes of basic research and as a possible diagnostic mechanism to map and possibly predict developmental delays and disorders. Habituation will continue to be the subject of human development research for many years to come.

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