The Illusion of Owning a Third Arm

[thinker_jeff]

Abstract

 

Could it be possible that, in the not-so-distant future, we will be able to reshape the human body so as to have extra limbs? A third arm helping us out with the weekly shopping in the local grocery store, or an extra artificial limb assisting a paralysed person? Here we report a perceptual illusion in which a rubber right hand, placed beside the real hand in full view of the participant, is perceived as a supernumerary limb belonging to the participant's own body. This effect was supported by questionnaire data in conjunction with physiological evidence obtained from skin conductance responses when physically threatening either the rubber hand or the real one. In four well-controlled experiments, we demonstrate the minimal required conditions for the elicitation of this “supernumerary hand illusion”. In the fifth, and final experiment, we show that the illusion reported here is qualitatively different from the traditional rubber hand illusion as it is characterised by less disownership of the real hand and a stronger feeling of having two right hands. These results suggest that the artificial hand ‘borrows’ some of the multisensory processes that represent the real hand, leading to duplication of touch and ownership of two right arms. This work represents a major advance because it challenges the traditional view of the gross morphology of the human body as a fundamental constraint on what we can come to experience as our physical self, by showing that the body representation can easily be updated to incorporate an additional limb.

 

Link: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0017208

 

 

 

[lemur]

This work represents a major advance because it challenges the traditional view of the gross morphology of the human body as a fundamental constraint on what we can come to experience as our physical self, by showing that the body representation can easily be updated to incorporate an additional limb.

What do you mean by, "experience as our physical self?" Do you mean accepting the arm as a coordinated part of your body system or actually feeling it as connected to your nervous system? If you just mean coordination, then a crane or other heavy machinery could be experienced as part of oneself, as could a Wii character.

[thinker_jeff]

What do you mean by, "experience as our physical self?" Do you mean accepting the arm as a coordinated part of your body system or actually feeling it as connected to your nervous system? If you just mean coordination, then a crane or other heavy machinery could be experienced as part of oneself, as could a Wii character.

The first, I have to be clear that is not what I mean by those words. These words are actual ABSTRACT OF THE PAPER. I have such post here for sharing and discussion, which typically has the topic I like.

To my understanding, what it means by "experience as our physical self" are both of feeling as connected your nervous system and coordination as a part of self-body.

You can verify that in the full paper which I gave the link already.

[Ringer]

The problem with having extra limbs isn't necessarily psychological. You'd have to think about what body parts would have to be added/subtracted in order to have the correct structure. It has been shown that healthy limbs can be grown where they aren't supposed to be (I know of an experiment where they had drosophila grow legs where its antenna were supposed to be), but adding to a place it usually isn't can have a negative impact overall.

 

It would be interesting to see the effects of neuronal layout/concentration on, say, hands if there were a extra one that was fully functioning.

[thinker_jeff]

The problem with having extra limbs isn't necessarily psychological.

Do you think the behavioral change unrelated with psychological change?

 

It would be interesting to see the effects of neuronal layout/concentration on, say, hands if there were a extra one that was fully functioning.

Good question. I guess that the effects of neuronal layout should be mainly in visual system and motor system.

[lemur]

The first, I have to be clear that is not what I mean by those words. These words are actual ABSTRACT OF THE PAPER. I have such post here for sharing and discussion, which typically has the topic I like.

To my understanding, what it means by "experience as our physical self" are both of feeling as connected your nervous system and coordination as a part of self-body.

You can verify that in the full paper which I gave the link already.

I figured the reason you posted a thread was to discuss your post, not promote the article.

[Ringer]

Do you think the behavioral change unrelated with psychological change?

 

No I think the problem would be anatomical, I'm sure behavior could be adapted to suite the use of extra limbs the same way it can be changed sans limbs.

 

 

Good question. I guess that the effects of neuronal layout should be mainly in visual system and motor system.

 

Well you would have to think that in development neurons 'sniff' where they are supposed to go by proteins and such released by cells, similar to how some cells differentiate. So It would be interesting to see if if the neural density in, say, the pre and post-central gyri increase substantially. Or they could just space the nerve cells that are normally in and space them out differently.

 

I don't think that the visual system would be effected very much, I don't know of any way the visual system would actually act on extra limbs and vice versa.

 

Of course I'm talking about actually incorporating organic body parts into the anatomy of people, the addition of robotic/synthetic limbs would be less of a problem. Not saying that I would be easy or anything, but I don't think Goro from Mortal Kombat would work nearly as well.

[thinker_jeff]

I don't think that the visual system would be effected very much, I don't know of any way the visual system would actually act on extra limbs and vice versa.

 

I didn't think either that the visual system would be effected that much until I read this paper.

Clearly no other sensory systems sensed the "third arm" but the visual system did; therefore, this system must involve the change of neural circuitry between input and output. Of course I don't have empirical data to support my thought; however in logic, it should be right.

[Marat]

Of course, it has been known for a very long time that 'gross morphology is not a limit on the physical sensation of the presence of the self,' since in the phantom limb phenomenon, people continue to experience themselves as present in limbs that they have lost.

[thinker_jeff]

since in the phantom limb phenomenon, people continue to experience themselves as present in limbs that they have lost.

Do you have some source about the study to this issue?

[PhDwannabe]

Do you have some source about the study to this issue?

 

The "real" source material is going to be pretty arcane to the non-specialist. What Marat's talking about is pretty well-known, however. Proprioceptive and interoceptive sense is normatively contiguous with the bounds of the physical body, but it doesn't have to be. The brain's not that hard to trick. You can screw it up in one way by chopping off a limb. You can screw it up in the opposite direction with some easy induction techniques (one fun intro psych trick: you have someone put their hand under the table and stare at a fake hand above the table. You stroke both identically with feathers for a minute or two as they stare at the fake hand. Then, you pound your fist on the fake hand. They yell, engage in a withdrawal reflex, and often wince in pain.)

 

My recommendation? Kick back and let the Indiana Jones of neuroscience, the great Vilayanur Ramachandran, explain things for you.