Body-ownership and visual perception

The idea that our body plays an important role in visual perception has a long history in storytelling and philosophy. Some ideas are very intuitive. For example, few will disagree with the notion that smaller people perceive the world to be bigger, and vice versa, that larger people perceive the world to be smaller. In contrast, more controversial ideas regarding the role of our body in visual perception have been debated by philosophers. According to these philosophers, the very nature of visual perception lies in the fact that we have a body that moves in space. Since George Berkeley (1685 – 1753) first formalized such a fundamental role for our body in visual perception, different philosophical theories have branched out to account for the latest scientific findings. Although experimental psychologists and neuroscientists have long neglected these intriguing philosophical accounts, the recent development of body-ownership illusions allowed for a more rigorous investigation of the supposed link between our body and visual perception. In body-ownership illusions, research participants experience an artificial body (or body-part) to be their own. These illusions allow for the dissociation between the subjective experience that your body belongs to you (i.e. body-ownership) and the mere visual impression of your body from a first-person perspective. The studies that comprise this thesis use different ownership illusions to investigate the role of body-ownership in visual perception, and the role of visual perception in body-ownership, with visual perception being an umbrella-term for both visuospatial perception and visual awareness. In Study I and Study II we investigated the mechanisms by which body-size influences the perceived size of the world, by having participants experience ownership of different sized (and sometimes invisible) bodies. Our results show that this own-body-size effect does not rely on visual information per se, but instead, on the recalibration between visual and tactile information that updates the representation of external space. In Study III we combine the rubber hand illusion with binocular rivalry to show that body-ownership promotes visual awareness of a fake hand. And in Study IV we combine the rubber hand illusion with continuous flash suppression to show that ownership can be induced in the absence of visual awareness. Such unconscious ownership calls for a reevaluation of the standard definition of body-ownership. Taken together, these studies illuminate the intricate relationship between body-ownership and visual perception. In addition to the scientific research fields of visuospatial perception, visual awareness, and body-ownership, these results are valuable to the philosophical debate on the nature of visual perception and might provide future applications in clinical psychology

Being Barbie: The Size of One’s Own Body Determines the Perceived Size of the World

A classical question in philosophy and psychology is if the sense of one's body influences how one visually perceives the world. Several theoreticians have suggested that our own body serves as a fundamental reference in visual perception of sizes and distances, although compelling experimental evidence for this hypothesis is lacking. In contrast, modern textbooks typically explain the perception of object size and distance by the combination of information from different visual cues. Here, we describe full body illusions in which subjects experience the ownership of a doll's body (80 cm or 30 cm) and a giant's body (400 cm) and use these as tools to demonstrate that the size of one's sensed own body directly influences the perception of object size and distance. These effects were quantified in ten separate experiments with complementary verbal, questionnaire, manual, walking, and physiological measures. When participants experienced the tiny body as their own, they perceived objects to be larger and farther away, and when they experienced the large-body illusion, they perceived objects to be smaller and nearer. Importantly, despite identical retinal input, this “body size effect” was greater when the participants experienced a sense of ownership of the artificial bodies compared to a control condition in which ownership was disrupted. These findings are fundamentally important as they suggest a causal relationship between the representations of body space and external space. Thus, our own body size affects how we perceive the world

GABAA agonist reduces visual awareness: a masking-EEG experiment

Consciousness can be manipulated in many ways. Here, we seek to understand whether two such ways, visual masking and pharmacological intervention, share a common pathway in manipulating visual consciousness. We recorded EEG from human participants who performed a backward-masking task in which they had to detect a masked figure form its background (masking strength was varied across trials). In a within-subject design, participants received dextromethorphan (a N-methyl-d-aspartate receptor antagonist), lorazepam (LZP; a GABA(A) receptor agonist), scopolamine (a muscarine receptor antagonist), or placebo. The behavioral results show that detection rate decreased with increasing masking strength and that of all the drugs, only LZP induced a further decrease in detection rate. Figure-related ERP signals showed three neural events of interest: (1) an early posterior occipital and temporal generator (94-121 msec) that was not influenced by any pharmacological manipulation nor by masking, (2) a later bilateral perioccipital generator (156-211 msec) that was reduced by masking as well as LZP (but not by any other drugs), and (3) a late bilateral occipital temporal generator (293-387 msec) that was mainly affected by masking. Crucially, only the intermediate neural event correlated with detection performance. In combination with previous findings, these results suggest that LZP and masking both reduce visual awareness by means of modulating late activity in the visual cortex but leave early activation intact. These findings provide the first evidence for a common mechanism for these two distinct ways of manipulating consciousness

Evolutionary Psychology as a Metatheory for the Social Sciences: How to Gather Interdisciplinary Evidence for a Psychological Adaptation

Evolutionary psychology has been proposed as a new metatheory for the social sciences (Buss, 1995). Evolutionary psychology is an approach that emphasizes the evolutionary background of psychological phenomena (e.g., cognition, motivation, perception), with the expectation that knowledge about this background enhances our understanding of the working of the present human mind. This proposal has met with both enthusiasm and criticism. An important criticism is that it is hard, if possible at all, to find empirical evidence for a hypothesized psychological adaptation. This criticism has been addressed with the proposal to build a nomological network of evidence around a hypothesized psychological adaptation (Schmitt & Pilcher, 2004). In this article, we show that it is possible to use this nomological network of evidence to support the hypothesis that face recognition is an adaptation. We reviewed the literature on face recognition from different disciplines (psychology, medicine, neuroscience, genetics, primatology, and anthropology) and conclude that there is an extensive network of evidence for the proposed hypothesis. We argue that building a nomological network of evidence is a promising way to address several criticisms of evolutionary psychology, and that such a network can serve as a metatheoretical framework for the social sciences