Systematic Review of Studies on Subliminal Exposure to Phobic Stimuli: Integrating Therapeutic Models for Specific Phobias

Abstract

We systematically review 26 papers investigating subjective, behavioral, and psychophysiological correlates of subliminal exposure to phobic stimuli in phobic patients. Stimulations were found to elicit: (1) cardiac defense responses, (2) specific brain activations of both subcortical (e.g., amygdala) and cortical structures, (3) skin conductance reactions, only when stimuli lasted >20 ms and were administered with intertrial interval >20 s. While not inducing the distress caused by current (supraliminal) exposure therapies, exposure to subliminal phobic stimuli still results in successful extinction of both psychophysiological and behavioral correlates: however, it hardly improves subjective fear. We integrate those results with recent bifactorial models of emotional regulation, proposing a new form of exposure therapy whose effectiveness and acceptability should be maximized by a preliminary subliminal stimulation. Systematic Review Registration: identifier [CRD42021129234].

Perceptually Visible but Emotionally Subliminal Stimuli to Improve Exposure Therapies

“…This unexpected ability to sustain a supraliminal-even if very brief-exposure to phobic stimuli could improve a patient's self-esteem, empowerment, and compliance to treatment, thus increasing the probability of a successful therapy [4]. [6] shows the impact of different levels of exposure to threats: while supraliminal and perceptually subliminal stimuli, respectively, impact the cognitive or the defensive survival circuits, emotionally-subliminal stimuli could impact on both circuits simultaneously. As a consequence, exposure therapies administering supraliminal stimuli only (tA) could leave behavioral and physiological correlates of fear dormant [23]; exposure therapies administering perceptually-subliminal stimuli only (tB) are ineffective in reducing the conscious feeling of fear; exposure therapies administering perceptually-subliminal, emotionally-subliminal and supraliminal stimuli in this order (tC) could maximize the benefit coming from each step.…”

“…Despite their efficacy, traditional exposure therapies are hardly accepted by the majority of phobic patients, and their clinical success is significantly affected by motivational factors [ 4 ]. Thus, the possibility of obtaining a comparable reduction of phobic symptoms through a safer, subliminal exposure gained a growing interest [ 5 , 6 ]. The observation that these protocols subliminally induced habituation in physiological reactions to phobic stimuli led some researchers [ 7 ] to incautiously compare their efficacy to that of traditional exposure therapy, despite the most clinically-relevant component of phobia (i.e., the subjective feeling of fear) was unaffected [ 6 ].…”

“…Thus, the possibility of obtaining a comparable reduction of phobic symptoms through a safer, subliminal exposure gained a growing interest [ 5 , 6 ]. The observation that these protocols subliminally induced habituation in physiological reactions to phobic stimuli led some researchers [ 7 ] to incautiously compare their efficacy to that of traditional exposure therapy, despite the most clinically-relevant component of phobia (i.e., the subjective feeling of fear) was unaffected [ 6 ]. The root causes of this paradox (i.e., considering a phobia successfully treated even if phobic stimuli still induce subjective fear) lie in the inappropriate overlap between subjective emotions and their physiological and/or behavioral correlates [ 8 , 9 ].…”

“…Can a phobic stimulus fail to elicit fear thanks to its reduced duration, even if consciously detected? From the systematic review of the studies on subliminal phobic stimulations [ 6 ], it is reasonable to derive this conclusion. In fact, studies adopting a trial-by-trial assessment of stimulus awareness found that stimuli supposed to be subliminal were occasionally perceived: as an example, Sebastiani and colleagues excluded 27.7% (10/36) of participants because they detected several stimuli supposed to be subliminal lasting 20 ms [ 11 ].…”

“…These numbers highlight the relevance of performing a trial-by-trial check of stimulus detection, the neuroimaging correlates of which could represent a potential marker of neuropsychiatric disorders (as already reported in response to supraliminal stimuli [ 12 ]). Nevertheless, most of the reviewed studies failed to assess the effectiveness of masking on a trial-by-trial basis, making it hard to discern whether the results reported should be ascribed to the processing of unperceived stimuli rather than to the processing of the few stimuli possibly perceived despite the masking technique [ 6 ]. Ignoring this evidence, authoritative authors [ 10 , 13 ] keep considering as “subliminal” even phobic stimuli that are merely “very briefly” exposed (i.e., lasting 30 ms, as compared to an average of 20.5 ms adopted by different authors in 17 comparable studies [ 6 ]).…”