Strange face illusions: A systematic review and quality analysis

Background: Strange face illusions describe a range of visual apparitions that occur when an observer gazes at their image reflected in a mirror or at another person’s face in a dimly lit room. The illusory effects range from mild alterations in colour, or contrast, to the perception of distorted facial features, or new strange faces. The current review critically evaluates studies investigating strange face illusions, their methodological quality, and existing interpretations. Method: Searches conducted using Scopus, PubMed, ScienceDirect and the grey literature until June 2022 identified 21 studies (N= 1132; healthy participants n= 1042; clinical participants n=90) meeting the inclusion criteria (i.e., providing new empirical evidence relating to strange face illusions). Results are reported using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The review was preregistered at the Open Science Framework (OSF: https://osf.io/ek48d). Results: Pooling data across studies, illusory new strange faces are experienced by 58% (95%CI 48 to 68) of nonclinical participants. Study quality as assessed by the Appraisal Tool for Cross-Sectional Studies (AXIS) revealed that 3/21 (14.28%) studies were rated as high, 9/21 (42.86%) as moderate and 9/21 (42.86%) as low quality. Whilst the items relating specifically to reporting quality scored quite highly, the detail relating to study design and possible biases is lower and more variable. Study quality accounted for 87% of the variance in new strange faces reported, with higher quality studies associated with lower rates of illusion being reported. The prevalence of illusions was also significantly greater in older samples, samples with higher proportions of female participants and for the interpersonal dyad (IGDT) compared to the mirror gaze paradigm (MGT). The moderating impact of study quality persisted in a multiple meta-regression involving participant age, paradigm type (IGDT vs MGT) and level of feature distortion. Our review suggests that the conditions required to induce strange face illusions appear to be reduced light levels, face stimuli and prolonged eye fixation. Conclusion: Strange face illusions commonly occur in both mirror-gazing and interpersonal gazing dyad paradigms. Further research of higher quality is required to establish the prevalence and particularly, the mechanisms underpinning strange face illusions.