Strange face illusions: A systematic review and quality analysis

Abstract

Strange face illusions (SFIs) describe a range of visual apparitions that occur when an observer looks at their image reflected in a mirror (Mirror Gaze Task: MGT) or gazes at another’s face (Inter-personal Gazing Dyad Task: IGDT) 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 faces. The current review critically evaluates studies investigating SFIs, their methodological quality, and existing interpretations. Searches conducted using Scopus, PubMed, ScienceDirect and the grey literature up to June 2021 identified 20 studies (N= 874) meeting the inclusion criteria (i.e., providing new empirical evidence relating to SFIs) . 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, we report that SFIs are experienced by 67% (95% CI: 55-77) of nonclinical participants. Study quality was assessed using the Appraisal Tool for Cross-Sectional Studies (AXIS) and indicated that most studies (85%) were poor-to-fair quality. Study quality was inversely related to prevalence rates, accounting for almost half of the variance in reported SFI rates. We also report that age was positively related to prevalence rates, with more illusions reported by older samples. Our review suggests that the conditions required to induce strange face illusions appear to be reduced light levels, face stimuli and prolonged eye fixation.In conclusion, SFIs commonly occur in both the MGT and the IGDT paradigms. Current psychodynamic interpretations of SFIs as dissociative identities remain largely speculative and further research of higher quality is needed to establish the prevalence and mechanisms of SFIs more firmly.