Maximizing exposure therapy: An inhibitory learning approach
An article by: Craske, M. G., Treanor, M., Conway, C. C., Zbozinek, T., & Vervliet, B. (2014).
Behaviour Research and Therapy, 58, 10-23

Inhibitory learning model of extinction

In a Pavlovian conditioning model, a neutral stimulus (conditional stimulus, CS) is followed by an aversive stimulus (unconditional stimulus, US). After a number of parings, the CS will come to elicit anticipatory fear reactions (conditional response, CR). The CR is presumed to depend upon the CS becoming a reliable predictor of the US. An association is posited between the memory representations of the CS and the US such that presentations of the CS will indirectly activate the memory of the US.

One powerful way to reduce conditional fear reactions is through extinction, in which the CS is repeatedly presented in the absence of the US.

Inhibitory learning is regarded as being central to extinction, although additional mechanisms, such as habituation, are likely to be involved. Inhibitory learning models mean that the original CS-US association learned during fear conditioning is not erased during extinction, but rather is left intact as new, secondary inhibitory learning about the CS-US develops, specifically that the CS no longer predicts the US.

The amygdala, which is particularly active during fear conditioning, appears to be inhibited by cortical influences identified as occurring from the medial prefrontal cortex as a result of extinction learning. After extinction, the CS possesses two meanings: 1) Its original excitatory meaning (CS-US). 2) an additional inhibitory meaning (CS-no US)

Even though fear subsides with enough trials of the CS in the absence of the US, retention of at least part of the original association can be uncovered by various procedures, which each one showing a continuing effect of the original excitatory association after extinction. 1) Conditional fear shows spontaneous recovering. 2) The strength of the CR increases in proportion to the amount of time since the end of extinction. 3) Renewal of conditional fear occurs if the surrounding context is changes between extinction and retest 4) Fear extinction appears to be specific to the context in which extinction occurs 5) Reinstatement of conditional fear occurs if unsignaled (or unpaired) US presentations occur between extinction and retest. 6) Rapid reacquisition of the CR is seen if the CS-US pairings are repeated following extinction

Deficits in inhibition and anxiety disorders

The number of individuals who fail to achieve clinically significant symptom relief from exposure-therapies or who experience a return of fear following exposure therapy may derive in part from the deficits in extinction learning. More specifically, deficits in inhibitory learning and inhibitory neural regulation during extinction. Anxious individuals show deficits in the mechanisms that are believed to be central to extinction learning.

Inhibitory learning versus habituation and behavioural testing approaches to exposure

Habituation models posit that fear reduction during an exposure trial is a necessary precursor to subsequent, longer lasting cognitive changes in the perceived harm associated with the phobic stimulus.

The strategies that derive from inhibitory learning models do not emphasize fear reduction per se during exposure trials and instead sometimes use strategies designed to maintain elevated fear throughout exposure trials. The amount by which fear has reduces at completion of extinction is not predictive of the amount of fear expressed at the follow-up extinction retest in either animals or human laboratory samples. There is divergence in response systems. Outward fear on the one hand, and conditional associations indicative of underlying learning on the other hand, may not always change in concordance.

Therapeutic strategies for enhancing inhibitory learning and its retrieval

Expectancy violation

This strategy is to design exposures that maximally violate expectancies regarding the frequency or intensity of aversive outcomes. It derives from the premise that the mismatch between expectancy and outcome is critical for new learning and for the development of inhibitory expectancies that will compete with excitatory expectancies. The more the expectancy can be violated by experience, the greater the inhibitory learning.

Clinically, it is important that the client identifies the US when predicting the expectancy to be violated. The expectancy approach ties exposure parameters directly to consciously stated expectancies for averse events.

Exposure strategies that specifically impede habituation were found to be more effective than strategies that do not.

In the expectancy violation approach, the end of an exposure trial is determined by conditions that violate expectancies and not by fear reduction. Learning is centred around whether the expected negative outcome was nor, or was ‘bad’ as expected. Following each exposure trial, the learning is consolidated by asking participants to judge what they learned regarding the non-occurrence of the feared event, discrepancies between what was predicted and what occurred, and the degree of ‘surprise’ from the exposure practice. Mental rehearsal is an important component of memory consolidation.

A key aspect of an expectancy violation model is to facilitate attention to both the CS and the non-occurrence of the US. Error-correction models posit an important role for the salience of the CS such that any change in associative strength will be directed to the cue that is most salient. In as much as extinction learning represents the formation of a non-contingent relationship between CS and US, awareness of both the CS and the non-occurrence of the US are essential.

Cognitive interventions may reduce the expectancy of a negative outcome before exposure and thereby lessen the mismatch between initial expectancy and actual outcome.

Deepened extinction

In deepened extinction either multiple fear CSs are first extinguished separately before being combined during extinction, or a previously extinguished cue is paired with a novel CS. This reduces spontaneous recovery and reinstatement of fear in animals and humans.

Whenever possible, we combine multiple cues during exposure therapy, after initially conducting some exposure to each cue in isolation. It is important that both stimuli predict the same US.

Occasional reinforced extinction

Occasional reinforced learning during extinction involves occasional CS-US pairings during extinction training. The benefits may derive from an expectancy violation effect in which the participant is less likely to expect the next CS presentation to predict the US because CS-US pairings have been associated with both further CS-US pairings and CS-no US pairings. The procedure of occasional reinforcement during extinction may enhance salience of the CS which in turn contributes to new learning about the CS. Occasional reinforced extinction sustained fear arousal during extinction but attenuated the subsequent reacquisition of fear.

The phenomenon of rapid reacquisition is most likely in the presence of repeated aversive outcomes. It may also be likely to occur in the context of dangerous environments that lead to retraumatization, although the approach of occasionally reinforced extinction is ethically prohibitive in such cases.

Removal of safety signals

The prevention or removal of ‘safety signals’ or ‘safety behaviours’. Safety signals alleviate distress in the short term, but when they are no longer present, the fear returns. This effect is believed to derive in part from interference with the development of inhibitory associations.

The ability of safety behaviours to mitigate extinction learning likely varies depending on the ratio of inhibition and excitation in a given trial. The presence of inhibitory stimuli will mitigate extinction learning inasmuch as they decrease the expectation of the US, and the discrepancy between what is predicted and what actually occurs determines the degree of associative change. The impact of inhibitory stimuli on extinction learning will depend on the number and strength of inhibitory stimuli versus the number and strength of excitatory stimuli.

The general consensus remains that safety signals or behaviours should gradually be phased out over the course of exposure therapy. Gradually phasing is recommended only in order to reduce treatment attrition. If willing, removal of safety signals is preferred.

Variability

Stimulus variability throughout exposure since varying the to-be-learned task enhances retention of learned non-emotional material. It is believed to enhance the storage capacity of newly learned information, pair the information to-be-learned with more retrieval cues, or to generate a rule that captures the invariance among tasks, which renders the information more retrievable at a later point in time.

Variability during exposure may offset context renewal effects after exposure.

In variable exposure, exposure is conducted to items from the hierarchy in random order, without regard to fear levels or fear reduction, although usually beginning with the least anxiety producing item to avoid treatment refusal.

The greater variability in fear levels throughout exposure is a positive predictor of outcomes. Variation in fear level throughout exposure will offset context renewal once exposure therapy is completed. Variability in emotional state may also enhance salience of the phobic stimulus and thereby enhance learning of inhibitory associations.

Retrieval cues

One option for enhancing retrieval of extinction learning and offsetting context renewal is to include retrieval cues (of the CS-no US association) during extinction training to be used in other context once extinction is over. One risk of retrieval cues is that they may acquire an inhibitory value and become a safety signal. Retrieval cues differ from safety signals in that they retrieve the CS-no US relationship, whereas safety signals are directly associated with the non-occurrence of the US.

The strategies are best employed as a relapse prevention skill. Using retrieval cues early in therapy, while the focus is on acquisition of extinction learning, may negatively impact progress as these cues can reduce the expectancy of the aversive event (and therefore mitigate expectancy violation effects). Any retrieval cues should be used sparingly to mitigate their likelihood of becoming a conditioned inhibitor or safety signal.

Multiple contexts

Context renewal involves the return of fear to a phobic stimulus when it is encountered in a context that differs from the context in which exposure therapy was conducted.

Reconsolidation

Retrieving already stored memories induces a process of reconsolidation. It may be possible to change memories during the reconsolidation time frame upon retrieval. Extinction during a reconsolidation window may weaken the fear memory itself.

Therapeutic strategy for enhancing inhibitory regulation

Linguistic processing or affect labelling may work to augment associative inhibitory processes within extinction or may work in an independent but complementary manner to extinction learning.

Linguistic processing activates a region of the cortex that reduces activity in the amygdala, thereby attenuating anxious responding. It appears that engaging the executive functioning cortical areas of the brain works to dampen the limbic system activity. Linguistic processing in the form of labelling can improves outcomes from exposure.