Chapter 20: Extended Body Hypothesis and Peripersonal Space

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We take complex, fascinating sources and strip them down to the essential, surprising core insights.

Today we are asking a question that sounds like it belongs firmly in the realm of philosophy or maybe science fiction.

What are the true lived conscious boundaries of your physical self?

Can you feel a sensation beyond your skin?

We're going to be doing a really deep dive into the heart of 4E cognition, specifically the phenomenological side of the extended body hypothesis.

So we're moving way beyond just the simple mechanics of using a tool.

We want to know not just if we can extend our body's reach, but how we consciously experience that extension.

And are there hardwired psychological limits to that feeling?

The conversation often starts, and this chapter certainly does, with the really disruptive artistic work of the Australian body architect, Stellark.

I mean, this is an artist who has spent decades intentionally redesigning, augmenting, and sometimes even digitally animating his own body.

Stellark is famous for several projects.

I think the most controversial one is probably the creation of a functional surgically implanted ear on his forearm.

He's also done a lot of work with robotics using an external controlled third arm that he operates just with muscle signals.

And his whole mission, which is really the perfect starting point for our philosophical inquiry, is summed up in this quote he has.

He says,

we can't continue designing technology for the body because that technology begins to usurp and outperform the body.

Perhaps it's now time to design the body to match its machines.

And that just immediately raises the stakes, right?

When we talk about bodily boundaries, we often think of something like an organ transplant.

But Stellark forces us to ask, what happens when we intentionally graft on new limbs or senses that have no genetic blueprint that fundamentally defy what we used to call the human form?

It's not just performance art.

It's a really radical exploration of metaphysical and psychological identity.

OK, so let's unpack that and define our terrain for today.

We're focusing purely on the phenomenological implications.

We're not talking about cognitive load or information processing.

We're talking about the conscious, lived experience of feeling like something is actually part of you.

Exactly.

So our mission is to rigorously evaluate the extended body hypothesis.

And we can break this down into two sort of testable versions.

The weak version is what most people kind of accept intuitively,

that your bodily awareness can stretch beyond your biological skin and skull, mostly through embodying objects we use, like a simple tool.

Right.

And the strong version is where it gets into that science fiction territory.

It's much more radical, much more challenging to confirm.

The strong version asks if bodily awareness can extend beyond even the apparent boundaries of the body.

Can it go into non embodied objects or even just empty space?

I mean, if the brain is plastic enough to incorporate a stick, could you theoretically feel a sensation projected far away, say onto the surface of the moon, if the sensory feedback was perfect?

That's the ultimate challenging target of this deep dive.

That is a phenomenal intellectual challenge.

So we have our roadmap ready for you.

We are going to start with the most common everyday form of extension tool use and analyze the surprising neuroscience behind it.

Then we're going to hit the hard limits because spoiler, we can't feel everything in a tool.

And finally, we'll tackle those mythical cases, those strange psychological experiments where sensations seem to hop right out of the body and see if they confirm that radical, strong hypothesis.

We don't need cybernetic graphs or performance art to study extension.

You're probably doing it right now.

If you're holding your phone or writing a note, every time you pick up a toothbrush, a knife or a pen, you are performing a bodily extension.

Right.

And the author is very clear that for this research, we need a precise definition of a tool.

It's not just any object you hold passively.

A tool is an unattached external object that you have to actively manipulate for some functional purpose.

That active relationship is the key.

And tools immediately extend three core things for us, our motor abilities, our sensory capabilities, and our spatial reach.

It's just incredible how far back this insight goes.

We tend to think of this as some 20th century neurological discovery, but Samuel Butler in his 1872 utopian satire, Erawan, was already thinking about it.

Butler's analysis was so ahead of its time.

He was talking about machines, which he called merely a supplementary limb.

But his example of the spade is perfect for what we're talking about here.

He writes,

observe a man digging with a spade.

His right forearm has become artificially lengthened and his hand has become a joint.

And for Butler, this wasn't a metaphor.

He genuinely saw the machine as being fused into the body.

And that fusion, that sense of the tool becoming an extension of the self is what modern empirical research has been confirming over and over.

Our brains process tools in a lot of significant ways as if they were actually our hands or parts of our limbs.

So to really structure this, we need to ask two core philosophical questions that will frame all the studies we're about to look at.

First, do tools only stretch our space of action, so the area where we can affect the world, or do they fundamentally stretch the space of our body itself actually changing our internal map?

That's the first hurdle, moving from just extending our actions to genuinely extending our body.

And the second question follows right from that.

If the internal body space is stretched, is this integration only happening at an unconscious sensor rotor level?

You know, your brain just figures out the movement without you realizing it, or does it actually modify your conscious bodily awareness?

Do you actually feel that your arm is functionally longer than it was five minutes ago?

That second question is the phenomenological key, but we have to start with the space of action, which immediately brings up this concept of peripersonal space.

Indeed.

It's totally undeniable that tools expand our range of motor opportunities.

I mean, that's the whole point of using a tool.

The technical term for this critical region is peripersonal space, or PPS.

And to find precisely, PPS is the area immediately around the body, within which the body can act and interact.

Think of it as your personal protective bubble.

It's the space you need to keep clear to feel safe.

The space where you can grab a coffee or fend off a threat.

And using a tool clearly pushes the edge of that bubble farther out.

Okay.

Let's dive into the neural evidence for this bodily stretching.

And we have to start with the classic research that really cemented this idea, the big monkey study by Iriki.

And it's so when the food was placed far away, outside the monkey's natural reach,

those neurons were just silent.

Exactly.

But what they found was extraordinary.

After the monkeys were trained and used the rake over and over, the visual receptive fields of those neurons suddenly started firing in response to the food at that distant location.

The neurons, which used to only care about space right near the physical hand, now cared about space near the tip of the rake.

So that's the brain literally remapping the physical boundary.

The distant space, which used to be irrelevant, was now neurally treated as close and actionable, all thanks to the tool.

And here's a crucial detail that really gets at the plasticity question.

The effect was entirely temporary.

When they stopped using the tool, the visual receptive fields of those neurons shrank right back to their original size within just a few minutes.

The brain isn't just updating, it's dynamically expanding and contracting its spatial map as needed.

But let's challenge this like the authors do.

Does extending parapersonal space automatically mean you've extended the body?

Not necessarily.

And this is a really critical distinction.

We have to differentiate between the external boundaries of PPS, how far out our actions can reach, and the internal boundaries, which is where the body ends and PPS starts.

You can extend that external boundary without changing the internal representation of the body itself.

Can you give the example of a surgeon using remote controlled devices,

like in telemedicine?

Right.

In that case, the surgeon is controlling a device that's meters away.

The space around that device is absolutely processed as peripersonal space.

The surgeon is acting and interacting there, but it's highly unlikely they are representing the entire distance between their physical body and that device is a continuous internal part of their own body.

So the PPS is stretched, but the body map probably isn't.

So we need evidence that the internal boundaries, the representation of the limb itself is pushed forward.

That would prove the tool is being incorporated into the body's schema.

And that evidence moves us into the realm of sensory motor incorporation.

So we look at follow -up research, particularly studies by Cardinali and colleagues in 2009.

They had human participants repetitively use a long mechanical grabber, a tool that required really precise motor control to grasp objects.

And the functional experience is key here.

You're not just passively holding it.

You're actively controlling the lengths and the grip.

Exactly.

So after this training, the participants were tested without the grabber, reaching for objects with their bare hand, and the experimenters carefully monitored the kinematics, you know, the specifics of their movement patterns and how they plan their trajectory.

And this is where the internal body map is revealed.

The kinematics of their unassisted movements were significantly changed.

They planned their reaches, the initial velocity, the acceleration, as if their arm were genuinely longer than it biologically was.

And this effect wasn't just limited to the task they trained on.

It generalized to other movements like simple pointing.

This is strong confirmation that the grabber was incorporated into the sensory motor representation of the body.

The brain updated its internal model of the arm's length.

So it's a functional extension, not just a spatial one.

The brain genuinely updated its internal map of the body's dimensions.

And this rapid functional updating gives us a really useful contrast with a more pathological phenomenon like phantom limbs.

We often group these concepts together because they both involve perceived body boundaries, not matching biological reality, but their mechanisms are opposites.

Phantom limbs are generally the result of body representations feeling to update after an amputation.

The map is stubborn, it stays fixed.

Whereas tool use requires the map to be fluid and hyper responsive.

It requires body representations to be incredibly plastic,

modifying and reverting hundreds of times a day, integrating a new length, and then instantly recalibrating back to the baseline.

It just shows that this rapid plasticity is a fundamental part of our body representation system.

So now we can address that second core question.

Does this sensor motor change this unconscious updating of how you reach spillover into conscious felt experience, what we're calling bodily awareness?

This is where we bridge that gap between action and perception.

The Milner and Goodale perception action model reminds us that the brain systems designed to improve action might operate completely outside of conscious perception.

So just because your motor system thinks your arm is longer, doesn't automatically mean you feel that it's longer.

So back to that grabber study from Cardinali, the motor evidence confirmed the arm was artificially lengthened, just like Butler said.

But what about the conscious perceptual evidence?

After using the grabber, participants were asked to consciously localize various body parts, their elbow, wrist, and fingertip, using a visual indicator.

And they consistently mislocalized these parts.

They perceived a significantly larger distance between their fingertip and their elbow than they did before they used the tool.

That's subjective confirmation.

They actually felt their arm was longer.

So the weak hypothesis is confirmed, bodily awareness is extended.

Now let's go one step further.

Can we feel the sensation at the tip of the tool, not just in the hand?

This brings us to the famous tactile illusion studies.

Right.

The classic example is the crossed hands effect.

If you close your eyes, cross your arms, and someone tries to touch your hands one after the other, it's hard to judge which was touched first.

It's surprisingly difficult.

And that's because the brain is experiencing a conflict.

You have the fixed internal map, the bodily frame of reference.

This is my right hand.

This is my left.

And that's conflicting with the external location, the egocentric frame.

This sensation is coming from the left side of space.

That conflict causes the confusion.

So to test tool extension, we just substitute that biological crossing with an object crossing.

Yamamoto and Kitazawa did this experiment in 2001.

Participants held two sticks that were crossed in front of them, but their actual hands remained uncrossed and still.

Then they were touched or vibrated sequentially at the tips of the sticks.

Now, if the sensation were only felt on the hands, which were uncrossed, they should have judged the order easily.

There'd be no spatial conflict.

But the stick had become the new boundary.

It had participants experience the exact same difficulty judging the order as they did with their actual hands crossed.

This implies the brain was localizing the sensation at the crossed tips of the sticks, not on the uncrossed hands, holding them.

The tool became the new sensory surface and the sensation was perceived out at that extended boundary.

This is really powerful evidence for perceptual extension.

But now we hit a bit of a philosophical wall.

Is that sensation truly felt on the stick or is it just a cognitive inference based on feeling pressure in the palm?

I mean, anatomically, the receptors are in our palm.

This is the real crux of the debate about tool use phenomenology.

If it's just an inference, what philosophers call indirect perception, then it's not a true extension of bodily awareness.

It's just clever cognition.

The author leans heavily on Dretsky's 1995 distinction to argue that this is, in fact,

direct perception.

OK, so how does Dretsky draw the line between direct and indirect perception?

Well, indirect perception, to put it simply, requires some additional knowledge or an inferential step based on past associations.

If you hear a dog barking and you know that's your neighbor's dog and you know your neighbor's dog only barks and the postman comes, you infer the postman is here.

Direct perception, on the other hand, means all the necessary information about the event is fully embedded in the immediate input.

Hearing the wind howl is direct perception of the wind.

So applied to the tool is feeling the bump at the end of the cane and inference based on the feeling in your hand and past experience.

Or is the information about the bump instantly available?

And the author uses the classic anecdote.

The blind man in his white cane.

Phenomenologically, the blind man reports feeling the resistance or the bump of the curb at the tip of the cane,

not the subtle vibration in his palm.

His palm feels something, of course, but the conscious experience is localized distally.

But does he need training to make that jump?

No.

And that's the crucial argument.

The necessary information about the obstacle is already embedded in the complex pattern of pressure and vibration traveling up the cane and into the palm.

It just needs to be extracted and structured by the brain's sensorimotor system.

The blind man doesn't have to think, ah, that specific vibration pattern in my hand must be a pothole 20 inches away.

The world is felt in a certain way at the end of his cane immediately.

I see.

So the brain extracts the relevant external information from the input in the hand and immediately structures the experience as being out there at the end of the tool.

That makes it direct.

It's not an intellectual step.

It's an instantaneous perceptual structuring.

So the conclusion here is pretty robust.

We directly feel touch on tools.

To summarize this first half, we have strong evidence that tools extend periprocessional space.

They dynamically modify our sensorimotor body representations,

and they allow for direct conscious tactile experiences, which fully confirms the weak version of the extended body hypothesis.

OK, we've established that a spoon or a pen is functionally and perceptually an extension of the arm.

But if they're so integrated, why don't they feel like true biological limbs?

And now we've reached the crucial limit of the weak hypothesis.

Despite the profound integration we've just documented, there is a fundamental and very robust restriction.

You cannot feel what are called intransitive sensations and tools.

Give us some examples of this hard limit.

What are intransitive sensations?

You can't scratch the tip of your fork no matter how much you might want to relieve a phantom itch there.

You can't feel a tickle localized in your pen.

And maybe most importantly, for body ownership, while you might feel really upset if you smash your car fender, you do not feel a sharp localized throbbing pain in the trunk.

When the tool is damaged, the feeling is protective or emotional, but it's not somatosensory pain.

And the functional consequence of that is massive.

We rely on this boundary.

We routinely use tools in situations where we would instantly and violently pull our own limbs away, stirring boiling soup, gripping a freezing cold handle, handling toxic stuff.

Right.

If the tool were fully embodied, those innate protective motivational signals tied to pain wouldn't make the tool useless or even dangerous.

This limit has to be explained.

And the explanation isn't biological or mechanical.

Why not mechanical?

I mean, the rubber hand illusion where stroking a rubber hand and a hidden real hand makes you feel the rubber hand is yours.

That works even without physical contact.

So physical discontinuity doesn't stop it.

Exactly.

And it's not a biological boundary either, because phantom limbs, which are non -biological, non -physical, can feel the most excruciating localized pain imaginable way outside the skin barrier.

So the limit has to be philosophical and functional.

So what's the fundamental difference between the touch we can feel in the cane and the pain we cannot feel in the cane?

For that, we turn to a distinction from the philosopher D .M.

Armstrong back in 1962.

The crucial difference between transitive and intransitive sensations.

This is really the linchpin of the whole argument.

OK, let's take a moment to define this clearly for everyone listening.

Transitive sensations like touch, pressure or texture.

They have two necessary dimensions, have a bodily component.

So I feel the texture on my skin.

But they also have a mandatory extraceptive dimension.

They are fundamentally about the external world.

For example, the pressure of the pen in my hand tells me about the pen's hardness, its shape.

The main function of transitive sensation is to acquire knowledge about external objects.

So it reaches out, it gathers information, it's world oriented.

Correct.

Now contrast that with intransitive sensations like pain, tickles, itches.

These sensations only have a bodily component.

When you feel an itch, the sensation is only about your body part.

My leg itches.

When you feel pain, even if it's caused by a hot stove, the experience represents the body part in pain, not the stove itself.

The pain is purely body oriented.

This type of sensation is primarily encoded in a somatotopic frame, a map of the body surface.

So the function of pain isn't to describe the world.

Its function is to prevent your body from being damaged.

Precisely.

So if the function of tools is to extend our reach into the external extroceptive world, then they are perfectly suited for experiences that have that strong extroceptive component like transitive touch.

When the blind man uses his cane, he needs the extroceptive reality of the world, the bump, the curb to be localized at the tip of the cane.

But if a painful stimulus is applied to the end of a tool, there isn't any corresponding extroceptive information for the brain to localize.

It's exactly.

Localizing a warning signal about body damage in an inanimate object would be functionally pointless.

The purpose of pain is withdrawal, protection and repair of biological tissue.

Since the tool is in biological tissue, that warning signal gets intercepted at the core body map.

This means tools are spatially and motorically embodied, but they're missing a vital ingredient for full integration.

They're not effectively embodied.

Intransitive sensations carry an intrinsic, effective dimension.

They're inherently pleasant or unpleasant, and they have a motivational role like making you withdraw or scratch.

The fact that we don't feel these effective sensations in tools confirms that the brain only manages a partial functional embodiment.

And we mentioned the huge survival benefit of this limitation.

If our tools felt pain, our ability to interact with the world safely would be seriously compromised.

Absolutely.

If early humans had fully incorporated their stone axes the first time they struck a tree trunk with force, they might have felt this massive localized pain at the axe head, triggering a withdrawal response.

The brains solve this evolutionary pressure really elegantly by keeping the effective boundary at the biological skin.

It lets us functionally extend without taking on unnecessary personal risk.

This distinction retaining that pain and effective map brings us to a fascinating point about the stability of the self.

If tools were fully embodied effectively in all, we might lose our sense of our stable core physical self, what Merleau -Ponty called the habitual body.

Right.

Given that we incorporate and discard tools hundreds of times a day, the system needs a fixed point.

If that extension were total, we'd constantly be losing track of our original biological body.

The core effective damage warning representation of the body stays untouched by the tool, and this stability serves a vital function.

It acts as the default reference standard, the factory setting for the body map.

Exactly.

This protected map ensures that the highly plastic sensor motor system, the part that learns to swing the rake, can temporarily incorporate the tool, knowing there's a reliable map to recalibrate back to the correct size of the biological limbs the moment the tool is dropped.

It provides that constant reliable reference point that William James described as the same old body always there.

And this recalibration mechanism is also intrinsically linked to the sense of ownership, which is another major limit of tool extension.

I can use a pair of tongs perfectly, but I don't feel like there are my tongs in the way I feel ownership over my hand.

That lack of ownership is commonly described by people who rely on extensions like amputees using prostheses.

They often call the prosthesis just a tool.

One patient explicitly reported using a prosthetic is not a natural thing because a prosthetic is not a substitute leg.

It is a tool.

If this really highlights the distance between functional incorporation and true subjective ownership.

So the link is clear.

Lack of ownership correlates directly with a lack of effective embodiment.

No pain felt, no ownership felt.

And this relationship is strongly confirmed by contrasting tool use with the rubber hand illusion or RHI.

RHI, unlike standard tool use, actively induces illusory ownership.

Subjects really feel the rubber hand is part of their own body.

The crucial experiment by Valenzuela Mogulansky in 2011 tested this connection directly using noxious stimuli.

They didn't just stroke the hands, they applied a painful stimulation.

Correct.

They synchronously strokes the hidden real hand and the visible rubber hand to establish that sense of ownership.

Then they applied a brief, noxious, painful stimulus to the rubber hand.

And they found a powerful correlation.

The subjects who reported the highest sense of ownership of the rubber hand also reported feeling pain localized in the rubber hand.

Wow, that is genuinely unsettling that the feeling of ownership can generate the experience of pain in something that's not flesh and blood.

It confirms that effective embodiment, the potential for pain, for motivational signaling is absolutely crucial for the experience of true bodily ownership.

So the core conclusion here is that the effective, protected body representation is immune to standard tool use,

specifically because it provides that critical reference standard for the body, the self that has to be protected above all else.

OK, since the pen can't feel pain, we know embodiment has limits.

But what if we just discard the pen and its functional use entirely?

Now we pivot to the strong version of the extended body hypothesis.

Can we stretch bodily awareness without embodiment?

Is exosomalsthesia feeling sensations in objects that bear no functional relationship to the body truly possible?

This is the ultimate test of plasticity.

Armel and Ramachandran proposed the ultimate thought experiment to frame this section.

They asked, if you look through a telescope at the moon and, using some optical trick, managed to stroke the image of the moon in perfect sync with stroking your own hand, would you project the sensation to the moon?

So the core constraint we have to test is, do these referred sensations absolutely require the object to be functionally embodied?

Or can they happen just based on external cues like synchronicity or perceived causality?

Right.

Let's examine the attempts to break this embodiment constraint, starting with objects that are clearly not body shaped.

Howie and Payton, in 2010, used the RHI setup, but suddenly swapped the rubber hand for a small, inanimate, white cardboard box.

So the experiment is basically, if I stroke your hand and I stroke a random box in sync, will you feel the touch on the box?

They did report touch on the box.

However, and this is the vital rebuttal, the illusion only worked if it was established first with the rubber hand.

They couldn't induce the illusory feeling on the box from the get go.

This proved the box wasn't a neutral canvas.

So it wasn't a clean break from the need for embodiment.

Right.

The sensation was localized, but the interpretation is tricky.

Participants either perceive the box as a visual distortion of their hand, the flexible body representation just adjusting to a strange container,

or more compellingly, they perceive their own hand as being inside the box.

Neither of those is true exosomal seizure.

The sensation is still localized on or in the embodied hand or its spatial equivalent.

What about when the sensation seems to hop along an object?

The cutaneous rabbit illusion applied to a stick.

So Miyazaki and colleagues in 2010 used the cutaneous rabbit illusion where sequential rapid touches on your arm create the feeling of the sensation hopping across the untouched skin in between.

Here, participants held the stick between their two index fingers and mechanical pulses were delivered sequentially to the index fingers through the stick and the result.

Participants reported feeling touches between the two fingers right on the stick itself.

The authors initially suggested this meant sensations could hop out of the body.

But the chapter author's rebuttal is simple.

The stick is a tool.

Precisely.

The participants were actively gripping and using the stick to interact with the stimulating device.

It's being functionally manipulated, which means it easily meets the criteria for tool embodiment we established earlier.

So this case shows embodiment isn't constrained by bodily resemblance.

It can be a simple stick, but it does not show that embodiment is unnecessary for referral.

We need a case with no functional manipulation at all to really test the strong hypothesis.

And for that, we look at some older, more unusual reports, specifically from the scientist Jorg von Bakseze back in 1959 involving a loudspeaker.

How is this set up?

The observer was near a loudspeaker and could see its membrane moving in perfect sync with faint sensations applied to their arm.

After a period of concentrated training and exposure, the observer began to project the skin sensations out into the room, localizing them on the loudspeaker's membrane.

OK, this is the strongest challenger to embodiment so far.

The observer had no control over the loudspeaker, no prior interaction.

It's a completely non -biological object.

Why can't we call this direct exosomal sthesia?

The problem is in a necessary condition.

It required learning or training over time.

That fact suggests that the observer was only indirectly localizing the sensation on the loudspeaker based on perceived causality.

Ah, so we're back to Dreske's indirect perception.

If I see the speaker move and I immediately feel a corresponding pressure, my cognitive system infers the speaker caused the touch.

And so I subsequently localized the feeling there.

It's an intellectual learned step, not a direct perceptual one.

If it were direct, the sensation should localize on the loudspeaker immediately, like the first time the blind man picks up his cane.

The fact that learning and training were required undermines the claim of direct, non -embodied referral.

So while this case proves that indirect referred sensations don't require embodiment,

it fails to support the strong hypothesis that we can directly feel non -embodied, non -functional sensations.

This brings us to the final and maybe the most mysterious challenge.

If we can't feel a sensation on the moon, what about feeling it in empty space itself?

If we can localize a touch in a specific empty region, that region can't possibly be embodied as a material object.

This is where we return to peripersonal space.

But now we're defining it as a bodily frame of reference rather than just a boundary for action.

We established earlier that the zone surrounding the body up to about 30 centimeters is unique.

That's the protective bubble.

Yes.

And it's special because it's neurally encoded relative to the body surface by those bimodal neurons we talked about in the Uriki study.

They respond to both visual stimuli, seeing something approach and tactile stimuli, feeling a touch close to the body.

This proximity dictates a shared body centered spatial frame.

And we know this from cross modal interference studies, right?

Absolutely.

Spence and his colleagues showed that visual distractors only interfere with our ability to accurately discriminate tactile information, making you slower and less accurate when those visual stimuli are close to the body within PPS.

It's compelling evidence of a common spatial frame integrating touch and vision.

We even see this relationship in pathological contexts like tactile extinction.

In patients with certain right hemisphere lesions, they might be unaware of a touch on their left hand if they're simultaneously touched on the right.

Crucially, that extinction also happens if they just see a visual stimulus near their right hand.

The visual information in that PPS bubble affects the awareness of touch on the body itself.

So the visual tactile system is constantly anticipating events in that 30 centimeter zone.

What's the function of that?

It's an early warning system.

PPS is often described as the gray zone where the body could be contacted in the near future.

The sight of objects approaching generates tactile expectations, which modify how we experience actual tactile stimuli.

The system is designed to take information from that space and map it onto the body's surface.

OK, now for the puzzling cases of sensations felt in empty space.

The invisible hand illusion.

This study from Gooder Stamm in 2013 is remarkable.

They used the RHI setup, but they synchronously stroked a participant's hidden real hand and a discreet volume of empty space above the table, which was in their direct view.

No rubber hand, just air.

And the participants reported feeling touch localized in that empty space.

They did.

They perceived an invisible hand occupying that location, and they subjectively localized the touch there.

This is essentially creating an invisible phantom limb on demand localized entirely in empty space.

There are other similar reports, too.

Von Bacchese's later work were placing two out of phase vibrators on spread fingers resulted in feeling vibration in the empty space between them.

And of course, the pathological cases of amputees reporting sensations in space distal to the phantom fingertips when their stump was touched.

These certainly seem like sensations hopping out of the body, localizing not on a tool, but on pure emptiness.

Why doesn't this confirm the strong hypothesis?

Here's the critical rebuttal.

These referred sensations are still localized within peripersonal space.

They are absolutely constrained by a bodily frame of reference.

It's like a spotlight mounted on your shoulder.

The light beam travels through empty space, but its direction and constraints are defined entirely by the body it's attached to.

So in the invisible hand illusion, the participants perceived that empty space is being occupied by a body part they just couldn't see.

That is the necessary interpretation.

The body representation isn't gone, it's displaced.

The body is perceived as being at a different, illusory location, a phantom in empty space.

Therefore, the sensation is still localized within a bodily frame of reference, even if the body itself is currently invisible or phantom.

So we can interpret these peripersonal sensations as a highly specific breakdown or disruption of the normal remapping process.

Precisely.

The system is designed to take information from PPS and remap it onto the body surface.

Peripersonal sensations are the result of an exceptional disruption of this process.

The sensation remains fixed in the PPS frame because the remapping fails or because the internal body map itself is momentarily displaced into empty space.

The sensation is still fundamentally tethered to the body's protective bubble.

In other words, even when the sensation is felt in empty space, it does not qualify as truly non -embodied exosomesthesia, that feeling on the moon, because it is absolutely constrained by the body's internal spatial map and its immediate protected frame of reference.

So to recap our extensive deep dive into the extended body hypothesis, we started with that provocative notion from the extended mind hypothesis.

There is nothing sacred about skull and skin for cognition, and we asked if that same radical freedom applies to our conscious sense of bodily awareness.

For the weak version, that bodily awareness extends beyond biological boundaries.

We found extensive confirmation.

Tools are successfully incorporated at the sensorimotor and perceptual levels.

The body map is highly plastic and it dynamically adjusts the tools we use.

However, this embodiment is profoundly limited by the absence of intransitive or effective sensations like pain.

And this isn't a design flaw, it's a functional safeguard.

Because we can't feel pain in tools, the effective body representation remains stable, acting as a protected core template.

And for the strong version, that bodily awareness is not constrained by the apparent boundaries of the body, allowing for true exosomesthesia that is far away, we had to reject the hypothesis.

Even the most puzzling cases, like sensations felt in empty space, are still localized relative to a fixed bodily frame of reference inside the body's protective peripersonal space.

So the chapter argues forcefully that for our conscious fundamental sense of self, there still seems to be something sacred about our apparent skull and skin.

The body maintains a stable, protected self centered on that effective map, even when it's functionally and perceptually extended into the world.

So the key takeaway isn't just that the body is flexible, but that it's fundamentally selfish.

It prioritizes the survival map of its biological tissue above all else.

It requires a constant, effectively charged reference point, which leaves us with a final provocative thought for you to consider based on those findings from the rubber hand illusion study.

We know that feeling pained in an object generates the experience of ownership.

If future technology, maybe a direct neural implant connected to a tool, genuinely allowed you to experience that effective barrier to feel pain and tickles localized in a mechanical limb, would you then experience full ownership over that tool?

And if you did achieve full effective embodiment, how would that fundamentally change the ethics of its use and what or who you were willing to sacrifice for the sake of survival?

That is the ultimate question of the extended body, something to ponder as you dynamically incorporate your mouse or your car into your sense of self today.

That was your deep dive.

We'll see you next time.