You are the center of the universe!

Well, not because the world revolves around you – it doesn’t. But you’re literally an egoist.

Who are you? What is this thing you call your self (yourself)? And how does this self relate to the physical world around you? For centuries, questions like these have been subject to philosophical debate. Chances are high that you have had heated discussions about them yourself (likely on long nights with hefty doses of red wine).

Bodily self-consciousness

In order to discuss the self, it is essential that we make a distinction between at least two conceptions. Firstly, the “narrative self” is a more or less coherent self (-image) which is stable over time. We refer to this notion when talking about ourselves in past and future states. The second notion of a “minimal self” is of particular interest in philosophy of mind and the cognitive sciences. This notion tries to capture the essential features of the self, limited to what is accessible by pre-reflective consciousness, that is, by direct and immediate experience. It is important to note that self-consciousness here means being consciously aware of oneself as a subject of experience.

In recent years, bodily self-consciousness, a candidate for the minimal self, has gained the interest of converging multidisciplinary approaches. In the following, I will present some insights concerning this notion. The self, in this sense, is thought of as the subject of experience that is located within a body and therefore occupies a certain location in space. This notion of bodily self-consciousness initially included a sense of ownership (“This is my body”) and a sense of agency (“I am the initiator of the movements of this body”).

Scientists are particularly interested in studying systematic alterations of self-consciousness in order to disassemble this mystery of our selves.

Your arm is my arm!

Such alterations are found in patients suffering from neuropsychological conditions like somatoparaphrenia, which typically occurs as a result of right parietal brain damage. These patients deny ownership of a body part and often ascribe it to someone else. In “Alien Hand” syndrome, patients maintain ownership but claim to have lost volitional control over their hand, which can result in bizarre situations, like when patients get choked by their own anarchic hand. The fact that the sense of ownership and agency can be independently impaired indicates that they rely on separate neuronal mechanisms underlying bodily self-consciousness (double dissociation).

Recently, a condition that is sort of a reverse somatoparaphrenia has been described. Patients with this delusion seem to integrate body parts of another person into their bodily self. Different experiments indicate that the delusional belief of owning an alien limb is completely embedded in the patients’ sensory-motor system and, in turn, also affects their sense of agency.

This rubber hand belongs to me!

Another approach to investigate bodily self-consciousness is to induce altered states of body ownership in healthy individuals. A simple but powerful paradigm to achieve this is the “Rubber Hand Illusion” in which participants describe a sense of ownership for a fake hand. The experiment is a science classic and can easily be done at home:

Place a rubber hand (or some rubber glove) on the table, coherently in front of a friend while their real hand is hidden under the table. Now, stroke the fake hand and the real hand both synchronously with a brush. This should lead to a qualitative (experience of ownership) and quantitative (shift of perceived location) modulation of embodiment. In other words, your friend will feel as though the rubber hand is their own. The following short video clip shows a person reacting to a threat to the rubber hand as if it were his own limb. (The fact that he is not moving the right hand shows that it’s not just a bodily reaction to the sudden movement from the right side.)

Out of the body, into an avatar.

With emerging technical advances in virtual reality (VR), it is now possible to induce all sorts of illusions related to body ownership like full-body, out-of-body and body-swap illusions. In principle, they are all related to the good old rubber hand paradigm.

Using questionnaires, behavioral tasks and even physical measures like the skin conductance response, scientists show that these VR experiments can induce a variety of illusions: illusory ownership of virtual body parts, self-identification with a virtual body avatar, altered self-location and a change in perspective during out-of-body experiences. This latter illusion is described as an experience of moving out of the body to a different location (often hovering above the lying body) and observing oneself from this position.

One in ten people experience out-of-body illusions at least once in their life outside of such experimental situations. It is only in the last decade that these kinds of illusions have been investigated scientifically without being completely discredited for their reports. In 2007 it was discovered by chance that it is also possible to induce such an experience by electrically stimulating the brain (specifically, the superior temporal gyrus).

How to DIY:

If you think all this is unbelievable, why don’t you get the equipment and play around yourself!

One element of such an illusion, namely the feeling of displacement outside one’s own body, is quite easy to recreate: Film a friend from behind and let them watch themselves being filmed from behind on the VR display. When you stroke their back, they will see another body being stroked in front of them in synchrony to his feeling. This can lead to an identification with the seen body and a shift of position towards this body.

To generally play around with your self-consciousness, these recently formulated constraints have to be met:

• Proprioceptive constraint: The body (part) has to be in a coherent position to the own body.
• Visual Information constraint: The body (part) has to visually match roughly with the real part.
• Peripersonal Space constraint: The body (part) has to be inside the peripersonal space. (That’s the “personal” space of your immediate surrounding, which, when entered by other people – like when you step into a crowded elevator – leads to an uncomfortable feeling.)
• Embodiment constraint: The two bodies (or body parts) have to receive prolonged visuo-tactile stimulation (like in the rubber hand illusion).

How does this help to get a grip on our selves?

Experiments like these indicate that the self, as a bodily self, is characterized by three fundamental properties, each of which can be separately manipulated in order to create altered states of self-consciousness.

1. Self-Identification: The experience of owning a body and identifying with it as a whole
2. Self-Location: The experience of being located in a specific point in space
3. First-Person Perspective: The experience of a perceived or imagined point of origin of mental and sensory processes

This indicates that the sense of agency is not a necessary condition for the notion of a “minimal self”. As we have seen, a passive experience of “owning” another body is enough to evoke this minimal conscious selfhood. Following this line of thought, the “thing” we call a self is best understood as a representation of the entire body being available for global control. And this is maintained by an ongoing, flexible process.

So, what is going on in the brain?

A major approach to explain the phenomena at hand relies on the concept of multisensory integration – the process of combining information from different sensory modalities like sight, sound, touch, smell, taste and self-motion. Following this approach, bodily self-consciousness depends on the integration of bodily signals from different sensory modalities.

The aforementioned constraints on inducing altered states of self-consciousness can be translated into neuronal processing constraints. Multisensory integration thereby depends on:

• Inputs from proprioception (muscles and tendons) and the vestibular system (sense of balance) that signal the location of body (parts) in space.
• Visual input of shape & structure.
• Limited space surrounding the body (which can be extended!)
• Spatio-temporal coherence of bodily signals.

Under normal circumstances, these constraints are met by our own body but in some cases, it can happen that other parts of the direct surrounding are a better fit and get integrated in our sense of self.

Combining data from human behavior, human neuroimaging and animal neurophysiology, researchers are able to build computational models of and find the neuronal substrates for multisensory integration of signals underlying bodily self-consciousness. This way, neuroscientists get an empirical grip on how the brain realizes this ongoing process that leads to a sense of self. Nevertheless, we are far from calling the philosophical debate obsolete. In fact, careful philosophical analysis of empirical findings related to self-consciousness is needed. Thereby, a vivid and fruitful interdisciplinary exchange can help to develop new neuroscientific approaches and on the other hand to refine theories of the mystery of our self.