body illusions and the multisensory self - lnco.epfl.ch · pdf file• what brain...

Body illusions and the Multisensory self

Dr Jane Aspell

LNCO lab

1. Introduction - bodily self-consciousness - dysfunctions (OBEs)

2. Body-part illusions – the rubber hand illusion

3. Full body illusions

4. The mislocalisation of touch to a virtual body

5. Summary

Outline of lecture

Bodily self-consciousness• What brain mechanisms underlie our perception of being someone?

• The self has been defined in multiple ways across and within disciplines (psychology, neuroscience, philosophy, psychiatry…)

• Arguably the most basic foundation of the self is the representation of the body by the brain (Damasio, 2000; Gallagher, 2005, Metzinger, 2003)

• Bodily sensory input is always present and can therefore provide the necessary stability and continuity required for a sense of self

• What sort of body representations could provide the basis for the sense of self? Could some be more important than others?

• What do you identify as ‘you’? Your hand? Your head? Or your entire body as a single entity?

1. Introduction

• One of the most basic questions about your self is ‘Where am I?’– the question of self location

Self-location and self-consciousness

• The self is usually perceived to be located inside the body (embodied)

• The body is perceived at a particular spatial position. This perception arises from the integration of multisensory inputs –visual, vestibular, auditory…

• Could self location and body location become dissociated –could you ever perceive your self to be located at a different location to your body?

1. Introduction

Out-of-body experiences1. Introduction

Out-of-body experiencesIn an OBE: self-location

body

• the self is localised outside one’s body(disembodied)

• the world and one’s own body are seen from an extra-corporeal and elevated perspective

> The body and self location do not coincide

• OBEs can arise due to severe migraine, epilepsyor certain types of brain damage

• Can also occur in healthy people under some conditions e.g. anaesthesia, with drug use, sensory deprivation

1. Introduction

Out-of-body experiences

• Studies of OBEs have influenced scientific thinking on the nature of bodily self-consciousness.

• They highlight the fact that bodily self-consciousness can be broken down into several components

• Phenomenology of OBEs demonstratesthese components are dissociable- suggest they may have distinct neural bases

1. Introduction

Out-of-body experiences

• Frequently associated with abnormalfunctioning at the temporo-parietal junction, a brain area important for multisensoryintegration & mental imagery of disembodiment

• OBEs have also been induced byelectrical stimulation of the TPJ(Penfield & Erickson, 1941; Blanke et al., 2002)

Could an OBE - or a weaker alteration in bodily self-consciousness - be induced in the lab using a non-invasive method?

1. Introduction

Multisensory integration and the body• The body is distinguished from the external world by detecting specific types of multisensory perceptual correlations

• Self-attribution – identifying a body part as one’s own –depends on a match between the look and feel (e.g. the felt positionof a limb given by proprioception) of the body or body part

1. Introduction

Multisensory integration and the body

• Ownership of body parts develops very early: babies at 5 months old discriminatebetween a video that matches their own leg movement versus one that doesn’t (Barrick & Watson, 1985)

makes sense for survival as a human - or any animal - must distinguish self from other to withdraw it from harm including self-harm (e.g. eating one’s own tail)

1. Introduction

Body-part illusions& visuotactile interactions

2. Body-part illusions

Inducing body illusions in healthy humans

• Body perception relies on multisensory integration

• Dysfunction in this integration has been suggested to be the cause of OBEs (Blanke et al., 2004; Blanke & Mohr, 2005)

Thus it should be possible to disturb bodily self-consciousness by inducing multisensory conflicts in healthy people


• e.g. what would happen if a body-part were seen and felt to be in different locations?…

Using multisensory conflicts to alterbody perception:

The rubber hand illusion

The perceived location of the hand can be altered by a simple manipulation - the RHI

Botvinick & Cohen, 1998

The real hand is hidden and subjects view a rubber hand being stroked

The rubber hand is stroked at the same timeand place (synchronously) as the real hand


Botvinick & Cohen, 1998

Subjects mislocalise their real hand towards the rubber hand and they feel that the rubber hand is theirs and that the touch is felt on the rubber hand

The questions underlined showed a statistically significant tendency to evoke affirmative responses. Points indicate mean responses. Bars indicate response range.


The rubber hand illusion –fMRI study

Compared synchronous stroking, asynchronous stroking and congruent and incongruent hand conditions

Ehrsson et al. 2004

(congruent = when the rubber hand is aligned with the subject's own handIncongruent = rubber hand is not aligned but rotated so that it points toward real hand)


The rubber hand illusion –fMRI studyBilateral premotor activity reflects the rubber hand illusion

Ehrsson et al. 2004

Subjects with strongest illusion during the synchronous congruent condition relative to the control conditions also showed the strongest fMRI signal in the bilateral premotor cortex


The rubber hand illusion – fMRI study

Ehrsson et al. 2004

• Neurons in ventral premotor cortex represent both the seen and felt position of the arm

• During the RHI, there is a change in the proprioceptive and tactile representations of the hand because of dominance of visual information

• Premotor activity could reflect matching of the visual and somatic signals


Skin temperature, tactile processing, and vividness of the illusion

Moseley et al. 2008

Psychologically induced cooling of the hand during the RHI

What else may change during RHI?

The mean ± SEM skin temperature of the experimental hand was 0.27 ± 0.11°C lower during the RHI trials than during the control trials

Recent study measured temperature of the stroked hand

Skin temperature of the real hand decreased during RHI-decrease is limb-specific: did not occur in the un-stimulated hand, nor in the ipsilateral foot


The rubber hand illusion – mechanisms?

Temporal order of events: drift in felt hand position, referral of touch to the RH and ownership over RH have not yet been clarified (and there are probably feedback loops occurring)

The following points are likely important in explaining the mechanism:

• In situations of multisensory conflict, especially with a spatial discrimination task, vision often dominates

• Vision has superior spatial resolution than proprioception (especially for a stationary limb)and thus is weighted more highly in a model of hand position


The rubber hand illusion – mechanisms?

• Touch is intrinsically linked with the self (Bermudez, et al., 1995), e.g. Touch alone: when you feel a touch it must be you being touchedVision alone: when you see a hand, it’s not necessarily your hand

• When a touch is felt it must be me being touched, and when a (synchronous) touch is seen on RH and mislocalised – because of visual dominance - to the RH, that probably affects the sense of it being part of my body (Makin et al., 2008)


Visual effects on touch

Subjects detected tactile targets at different body locations (neck or face)while viewing images of hand, neck or face – visuo-tactile relationship could beneutral (e.g. viewing hand with target on face), compatible (e.g. viewing neck with target on neck), ) or incompatible (e.g. viewing face with target on neck)

Tipper et al. 2001


Visual effects on touch

Tipper et al. 2001

When attempting to detect targets on the neck, vision of the incompatible face significantly impaired performance

> Reveals that cross-modal interactions are produced at body sites that can never be directly viewed, i.e. they are sites that have no history of proprioceptive orienting of eyes and head towards them


Kennett et al. 2001

Non-informative vision and the spatial resolution of touch

Subjects report whether they are touched by one or two tactile stimulators

The spatial separation of the two stimulators is varied to find the threshold distance at which they can no longer be resolved and are perceived as a single tap


Kennett et al. 2001

Non-informative vision improves spatial resolution of touch

Viewing the arm gives bettertactile resolution thanperformance in darkness or viewing a neutral object (projected via mirrors to appear in the same location as the arm)

Magnifying the view of the arm further improves tactile resolution. The moment of tactile stimulation is never seen, preventing trivial vision of the stimulators

> This may be due to a modulation of tactile receptive fields by visual input(see Haggard et al. 2007)


How far can the study of body parts – e.g. the hand – take us in investigations of self-consciousness?

Self-consciousness, whole-body and body parts

Self-consciousness

A key feature of bodily self-consciousness is its global character, its association with a single, coherent, whole-body representation (Blanke & Metzinger, 2009; Metzinger, 2007)

The RHI manipulates the attribution and localisation of the hand relative to the self - a part-to-whole relationship - it does not manipulate selfhood per se

?Self-consciousness

OR


• Visual system forms representations of different complexity:

Holistic/Global representations in the brain

- of local features, e.g. oriented edges in area V1- of intermediately complex features e.g. concentric circles in area V4

- of complex objects such as flowers, cars, faces etc. in inferior temporal cortex


Holistic/Global representations in the brain

• Patients with prosopagnosia cannot recognise faces even though they see the components of the face –eyes, lips, nose etc. – they cannot integrate them

• Face recognition involves holistic processing –crucial aspect is the integration of the features intoa global representation that encode the spatial relations between the facial features


• Might there also be ‘global’ / whole-body representations?

Whole-body representations

• If so, these whole-body representations might be important for the representation of the self by the brain

• Given this, manipulations of global body perceptionshould inform us more about self-consciousness

• How can global body perception be manipulated in the laboratory?

• One approach is to manipulate self-location…


Manipulating self-location: ‘Where am I?’Stratton (1899) built a sophisticated portable mirror device -3 mirrors aligned in such a way that it seemed as if he viewed himself “from a point above his head”.

Stratton, 1899

-this created a multisensory conflict between the seen and the felt position of his whole body

-after several hours he reported:

It was “as if my body had been elongated”“I had the feeling that I was mentally outside my own body”

On the third day “touch sensations were not referred to aany other than their visible locality“

He concluded:

“we would feel a thing to be wherever we constantly saw it (…) and if we were always to see our bodies a hundred yards away we would probably also feel them there


• The body is arguably the foundation for self-consciousness thus manipulation of body perception should modulate bodily self-consciousness

Interim summary

• Several studies have demonstrated the role of multisensory

integration in body perception

• Body part studies may be limited in providing insight into

self-consciousness because the self is arguably associated with

the whole-body rather than with body parts

Full Body illusions

Lenggenhager et al. 2007

Stroked for 1 minute – synchronously or asynchronously

14 subjects tested

The Full Body Illusion3. Full body illusions


The Full Body Illusion

Q1 – “It seemed as if I were feeling the touch in the location where I saw the virtual body being touched”Q2 – “It seemed as though the touch I felt was caused by the stick touching the virtual body”Q3 – “It felt as if the virtual body was my body”



The Full Body Illusion3. Full body illusions

Ehrsson, 2007

A different full body illusion

Method very similar but chest wasstroked (synchronously or asynchronously) with stroking applied close to the camera

Tested 12 subjects

Stroked for 2 minutes


Ehrsson, 2007

A different full body illusion

Subjects reported (questionnaires)that during synch. stroking they felt like theywere located behind their real body

No drift measured but skin conductance response measured (an objectivemeasure of arousal, e.g. increased fear)when a hammer was swung towards the camera lens

Increased SCR during synch (illusion) –further evidence that self-location wasat the camera position


Why the differences?

Ehrsson, 2007


In study A self-location was biasedtowards the seen body for synch vs. asynch stroking

A

BIn study B self-location was biasedtowards the camera location for synch vs. asynch stroking


Ehrsson, 2007 Lenggenhager et al. 2007

Meyer 2008

Which cue is the most important for self-location?

Yellow ball is whereself-location is biasedtowards

Grey body is realbody, white bodyIs virtual body

suggested there are at least 4 important cues for self-location: (1) where the body is seen, (2) where the world is seen from (visual perspecive) (3) wherethe stroking is seen (4) where the stroking is felt



Meyer 2008

Which cue is the most important for self-location?

Yellow ball is whereself-location is biasedtowards

Grey body is realbody, white bodyIs virtual body

In neither study did self-location exactly coincide with where the touch was felt (where the realbody was); where the body was seen biased self location in Lenggenhager et al., not inEhrsson’s study

In both cases, self-location was biased towards where the touch was seen thus thisis argued to be the most important


• Congruence between tactile and visual input is an additional important factor in determining self-location in this context

Blanke et al. 2008


• It seems that when vision and touch are incongruent, the influence of the visual information about stroking’ is weaker

• Thus in the asynchronous condition, subjects’ self-location is closer to where the touch is felt (i.e. where their physical body is actually located)


Note that there was not a 100% shift in self-location in either study, only a bias, so we cannot be sure that seen touch (or visuo-tactile congruence) are stronger cues for self-location than the location of the visuo-spatial perspective

The first person perspective has been argued to be the most self-specifcaspect of consciousness – it is a property that a self cannot lack

(The perspective relates any represented object to the representing subject)

Legrand & Ruby 2009



Comparing the methods

Either (1) back was stroked and stroking seenon back

Illusory body (self location)

Real body

or(2) Chest was stroked and stroking was seen being applied just in front of camera




• Back stroking: greater self-identification with virtual body and referral of touch to the virtual body during synchronous as compared to asynchronous stroking

Illusory body (self location)

Real body

• Synchronous chest stroking led to decreased self-identification with the virtual body and the illusory touch was referred to the camera position


• Self-localization (measured with mental ball dropping measurement) lower for synchronous back stroking than synchronous chest stroking



• As before, self-location more biased towards virtual body in synchronous back stroking condition and relatively more towards location of camera (visuo-spatial perspective) in synchronous chest stroking condition


Summary

• The body is arguably the foundation for self-consciousness thus manipulationsof body perception should modulate bodily self-consciousness

• Several studies have demonstrated the visual influence on touch and on the perception of body parts using the rubber hand illusion

• Body part studies may be limited in providing insight into self-consciousness becausethe self is arguably associated with the whole-body rather than with body parts

• Whole-body perception can also be manipulated with multisensory conflicts which can lead to alterations in self-location and identification with a virtual body (c.f. ownership of rubber hand)

• In the full body illusion a mislocalisation of touch occurs so that the spatial representation of touch is biased towards the location of a virtual body seen at two metres distance

• Such methods allow scientists to systematically manipulate the cues that the brain usesto generate the perception of the self in the world and thus provide insight into the brain’s generation of self-consciousness

Considerations of self-location could lead us into the dualistic trap: the notionof the self in the body/head leading toan infinite regress of homunculi

Epilogue: Descartes’ Error?

If the self is not a ‘mini-me’inside my head then what is it? The next lecture will examine this by introducing the concept of the minimal phenomenal self

body illusions and the multisensory self - lnco.epfl.ch · pdf file• what brain...

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