Strange face illusions: A systematic review and quality analysis.

• Strange face illusions are commonly experienced when fixating on face stimuli for prolonged periods under low light levels. • Strange face illusions involve either the perception of distorting facial features on the actual face being observed or the perception of completely new strange faces. • Illusions of new faces show a mean prevalence rate of 58% in healthy participants. • Age and gender are positively related to prevalence rates, with more new faces reported by samples of older individuals and those with more female participants. • Some key areas of methodological concern were apparent in the studies, relating to small unjustified sample sizes, a lack of preregistration and providing conclusions and interpretations that do not seem to be justified by results. • Future research should aim to examine how the behavioural and environmental components of this paradigm combine to result in illusory faces and explain why illusions are more prevalent when gazing at others compared to self-reflections. 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. Searches conducted using Scopus, PubMed, ScienceDirect and the grey literature until June 2022 identified 21 studies (N = 1,132; healthy participants n = 1,042; clinical participants n = 90) meeting the inclusion criteria (i.e., providing new empirical evidence relating to strange face illusions). The total sample had a mean age of 28.3 years (SD = 10.31) and two thirds (67 %) of participants tested to date are female. 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). 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, those relating to study design and possible biases were lower and more variable. Overall, study quality accounted for 87% of the variance in reporting rates for strange faces, with higher quality being associated with lower illusion rates. The prevalence of illusions was also significantly greater in samples that were older, had 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 point to the importance of reduced light levels, face stimuli and prolonged eye fixation for strange face illusions to emerge. Strange face illusions reliably 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. [ABSTRACT FROM AUTHOR]