Reducing the neural search space for hominid cognition: what distinguishes human and great ape brains from those of small apes?

Enfacement illusions: Filling a knowledge gap in eating disorder risk assessment, prevention, and intervention?

Body image disturbance (BID) is central to eating disorders (EDs), yet the role of self-face perception has received limited empirical attention despite rising sociocultural pressures emphasizing facial appearance through technologies such as social media. Emerging evidence suggests impairments in self-face recognition accuracy and distorted perceptions of facial appearance among individuals with EDs. Enfacement illusions, involving the experimental induction of perceived ownership over another's face, offer a novel paradigm to comprehensively investigate the perceptual multisensory integration processes underlying self-face perception disturbances in ED populations. Such an approach may hold promise for elucidating core pathological mechanisms contributing to BID and ED psychopathology. We discuss how rigorous investigation of self-face perception through the enfacement illusion paradigm represents an innovative direction of research and/or clinical application that may advance etiological models of EDs and possibly inform interventions targeting the potentially multidimensional nature of body and facial image disturbances characterizing EDs. PUBLIC SIGNIFICANCE STATEMENT: Body image disturbance is central to eating disorders (EDs), yet, the role of face-related disturbances remains critically under-investigated. After summarizing findings on face-related disturbances in EDs we propose how enfacement illusions (i.e., the experimental induction of ownership over another's face) may elucidate self-face perception disturbances in EDs, and their underlying mechanisms. Enfacement illusions may also offer an intervention to potentially address multifaceted face and body image disturbances characterizing EDs.

Ocular pigmentation in humans, great apes, and gibbons is not suggestive of communicative functions

Pigmentation patterns of the visible part of the eyeball, encompassing the iris and portions of the sclera, have been discussed to be linked to social cognition in primates. The cooperative eye hypothesis suggests the white sclera of humans to be a derived adaptive trait that enhances eye-mediated communication. Here, we provide a comparative analysis of ocular pigmentation patterns in 15 species of hominoids (humans, great apes & gibbons) that show marked differences in social cognition and quantify scleral exposure at the genus level. Our data reveals a continuum of eye pigmentation traits in hominoids which does not align with the complexity of gaze-mediated communication in the studied taxa. Gibbons display darker eyes than great apes and expose less sclera. Iridoscleral contrasts in orangutans and gorillas approach the human condition but differ between congeneric species. Contrary to recent discussions, we found chimpanzee eyes to exhibit a cryptic coloration scheme that resembles gibbons more than other apes. We reevaluate the evidence for links between social cognition and eye pigmentation in primates, concluding that the cooperative eye hypothesis cannot explain the patterns observed. Differences in scleral pigmentation between great apes and humans are gradual and might have arisen via genetic drift and sexual selection.

A neuroanatomical predictor of mirror self-recognition in chimpanzees

The ability to recognize one's own reflection is shared by humans and only a few other species, including chimpanzees. However, this ability is highly variable across individual chimpanzees. In humans, self-recognition involves a distributed, right-lateralized network including frontal and parietal regions involved in the production and perception of action. The superior longitudinal fasciculus (SLF) is a system of white matter tracts linking these frontal and parietal regions. The current study measured mirror self-recognition (MSR) and SLF anatomy in 60 chimpanzees using diffusion tensor imaging. Successful self-recognition was associated with greater rightward asymmetry in the white matter of SLFII and SLFIII, and in SLFIII's gray matter terminations in Broca's area. We observed a visible progression of SLFIII's prefrontal extension in apes that show negative, ambiguous, and compelling evidence of MSR. Notably, SLFIII's terminations in Broca's area are not right-lateralized or particularly pronounced at the population level in chimpanzees, as they are in humans. Thus, chimpanzees with more human-like behavior show more human-like SLFIII connectivity. These results suggest that self-recognition may have co-emerged with adaptations to frontoparietal circuitry.

Mirror self-recognition: a review and critique of attempts to promote and engineer self-recognition in primates

We review research on reactions to mirrors and self-recognition in nonhuman primates, focusing on methodological issues. Starting with the initial demonstration in chimpanzees in 1970 and subsequent attempts to extend this to other species, self-recognition in great apes is discussed with emphasis on spontaneous manifestations of mirror-guided self-exploration as well as spontaneous use of the mirror to investigate foreign marks on otherwise nonvisible body parts-the mark test. Attempts to show self-recognition in other primates are examined with particular reference to the lack of convincing examples of spontaneous mirror-guided self-exploration, and efforts to engineer positive mark test responses by modifying the test or using conditioning techniques. Despite intensive efforts to demonstrate self-recognition in other primates, we conclude that to date there is no compelling evidence that prosimians, monkeys, or lesser apes-gibbons and siamangs-are capable of mirror self-recognition.