On the perception of the body from within and from the outside

Chairs:  Matthew R. Longo¹ & Manos Tsakiris²

¹ Department of Psychological Sciences, Birkbeck, University of London, UK
² Department of Psychology, Royal Holloway, University of London, UK,


Abstract:

Symposium Abstract:
We have uniquely intimate knowledge of our own body, as the seat of somatic
sensations and the constituent of our sense of self. In this sense, we perceive our
body ‘from within’, as the origin of first-person experience. We also, however,
perceive the body ‘from the outside’ as a conspicuous and ever-present object in
our perceptual field, and as a reference frame for our representation of the
external world and our interactions with it. This fundamental versatility of the body
has been reflected by largely distinct research literatures on ‘interoception’, ’body
representations’ and ‘body image’. Recent research has begun to investigate
connections between these facets of embodiment revealing increasingly
fundamental links between our intrinsically private experience of our body from
inside and our essentially public body as an object in the physical world. This
symposium will showcase these recent advances by starting from the
neurocognitive understanding of the experience of embodiment and moving on to
the mental representation of the physicality of the body, bringing together leading
researchers in the field from across Europe. While addressing a broad range of
issues, the talks in the symposium share a common focus on understanding the
reciprocal connections between the body perceived from within and from the
outside.

Talk 1:

 Multisensory Mechanisms of Owning an Entire Artificial Body

 Henrik Ehrsson
Department of Neuroscience, Karolinska Institute, Sweden


When we look down at our body we immediately experience that it belongs to
us. We do not experience our body as a set of fragmented parts, but rather as a single
entity. How does this perception of owning an entire body arise? Here we address this
question by using a ‘body-swap’ illusion where people experience an artificial body as
their own body, in combination with brain imaging and behavioral experiments. Our
behavioral and psychophysiological results suggest that the following factors are
necessary for the elicitation of the illusion: i) temporal congruency of visual and tactile
signals; ii) spatial congruency of visual and tactile signals in an external reference frame
centered on the body; iii) a humanoid body shape; (iv) a first person visual perspective.
Importantly, we further describe how ownership generalizes from the stimulated body part
to the rest of the (unstimulated) body. Our functional magnetic resonance imaging studies
revealed a tight coupling between the experience of full-body ownership and neural
responses in bilateral ventral premotor and left intraparietal cortices and the left
putamen. Importantly, activity in the ventral premotor cortex reflected the construction of
ownership of a whole body from the parts as it was present irrespectively of which body
part that was stimulated to trigger the illusion, and further, this area displayed multivoxel
patterns carrying information about full-body ownership. Taken together these results
provide a mechanistic multisensory framework to explain how we come to experience an
entire body as our own.

Talk 2:

 Just a heartbeat away from one’s body: interoceptive sensitivity and malleability of body-representations

Manos Tsakiris
Psychology, Royal Holloway, University of London, UK

Body-awareness relies on the representation of both interoceptive and
exteroceptive percepts coming from one’s body. However, the exact relationship and
possible interaction of interoceptive and exteroceptive systems for body-awareness
remain unknown. Based on recent models of self-awareness that consider the insula as a
convergence zone linked to the representation of the bodily self, we examined the
interaction between interoceptive and exteroceptive awareness of the body. Across three
experiments, we combined measures of interoceptive sensitivity with experimental
manipulations of body representations. Consistent results suggests that interoceptive
sensitivity predicts the malleability of body representations, that is, people with low
interoceptive sensitivity experience stronger illusions of embodiment (“rubber hand
illusion”) and identification (“enfacement illusion”). In one final experiment, we
manipulated interoceptive sensitivity by mirror self-observation. Overall these findings
suggest that interoceptive sensitivity modulates the integration of multisensory
information and predicts the strength and malleability of body-representations.

Talk 3:

How changes in structure and function of the physical body affect body
and space representation.

 Andrea Serino
 University of Bologna, Italy.

The brain contains multiple representations of the body and of the space
surrounding the body, i.e. peripersonal space (PPS). We asked how much such
representations are sensitive to changes in the structure and the function of the body they
represent. In order to test the effects of a change in the structure of the physical body, we
tested body and PPS representations in patients undergone to upper limb amputation and
prostheses implantation. Amputation deformed body and PPS representation, so that
patients perceived their stump as shorter and the PPS around the stump was
disorganized, as compared to the intact limb. These effects can be partially reversed by
prosthesis implantation, as just wearing the prosthesis extended the perceived length of
the amputated limb and the representation of the space around it. In a second study, in
order to test the effects of a change in the function of the physical body, we tested body
and PPS representation before and after 10 hours of immobilization of the right arm,
resulting in a parallel extraordinary use of the left arm. This procedure did not change the
implicitly perceived length of the immobilized right arm, but did increase the perceived
length of the over-used left arm. Conversely, PPS representation was reduced around the
immobilized arm, but did not change around the over-used arm. These findings show that
body and space representations are plastically shaped as a function of both structural and
functional proprieties of the physical body.

Talk 4:

 A Hierarchy of Body Representations

Matthew R. Longo

 Department of Psychological Sciences, Birkbeck, University of London, UK

We experience our body as a coherent, 3-D, volumetric object. Initial
somatotopic maps in somatosensory cortex, however, represent the body as a set of
fragmented, 2-D skin surfaces. I will discuss a series of recent experiments investigating
different classes of body representation of the hand, which suggest they fall along a
continuum from fragmented 2-D maps of individual skin surfaces to coherent 3-D maps of
the body as a whole. First, tactile localisation on the skin appears to rely on a purely 2-D
representation of skin surfaces. Localisation biases, though consistent from person-to-
person, differ qualitatively between the palm and the dorsum. Second, body
representations underlying position sense appear to rely on an intermediate
representation. Distortions of hand shape are qualitatively similar between the palm and
dorsum, suggesting that they do not rely on fully distinct 2-D representations of each
surface. However, the magnitude of distortions is reduced on the palm, inconsistent with a
representation of the hand as a fully 3-D object. Position sense may rely on a 2.5-D
representation of the body, analogous to the 2.5-D sketch proposed in vision by David
Marr. Finally, the conscious body image appears to be largely undistorted, with a clear
match between the palm and dorsum, suggesting they rely on a fully-integrated 3-D
representation of the hand as a volumetric object. Together, these findings reveal a
hierarchy of body representations effecting a coordinate transformation from fragmented
2-D maps in somatosensory cortex to a volumetric representation of our body in the
world.

Talk 5:

The Perception of Spatial Layout as a Biologically Functional Adaptation

Sally Linkenauger
Human Perception, Cognition, and Action group, Max Planck Institute for Biological Cybernetics, Germany

From a biological perspective, visual and perceptual systems evolved to promote
adaptive actions with minimal energetic cost. As a result, humans are only sensitive to
the visual information which is necessary for successful environment interaction.
Additionally, individuals perceive this information in an adaptive way which supports
successful behaviors. Information specifying the spatial layout not only allows for the
execution of visually controlled actions, but also allows perceivers to determine which
actions they can perform. In order to make decisions about possibilities for action, visual
information specifying the environment needs to be scaled to action capabilities of actors’
bodies. I will provide evidence that this rescaling provides the metric to which the optical
information specifying perceived sizes and distances are scaled. In other words,
individuals perceive sizes and distances as a proportion of the action-relevant aspect of
their body. Hence, individuals do not perceive the world, but the relationship between
their body’s action capabilities and the environment.