Chapter 5: The Parietal Lobes

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So imagine waking up in a hospital room, right?

And you look down at your side and there's an arm resting on the bed.

Sounds pretty normal so far.

Right.

But you can see it clearly.

It's attached to your shoulder.

But you feel this overwhelming, undeniable certainty that this arm belongs to a complete stranger.

Oh, wow.

Yeah.

A stranger who like somehow crawled into bed with you while you were sleeping.

That sounds like a bizarre hallucination, but it's not.

It is a very real glitch in your brain's spatial map.

It is deeply unsettling, isn't it?

But I mean, it's just one of the incredibly strange phenomena we encounter when we look at the parietal lobes.

Definitely.

And welcome to our deep dive into the fascinating world of neuropsychology.

Think of this as your own personalized one -on -one pittering session.

Today we're pulling directly from chapter five of Introduction to Neuropsychology, second edition.

Yep.

And our mission today is pretty clear.

We are going to completely demystify this chapter, going in exact chronological order, to really unpack how our brains build our reality.

You know, usually when we talk about a medical diagnosis, we expect it to be straightforward.

Like you break a leg, an x -ray shows the jagged white line, and the doctor says, well, there it is.

Exactly.

It's visible.

But with neuropsychology, and especially this chapter, it feels like trying to photograph a software glitch using a camera that was only built to look at hardware.

That captures the dilemma perfectly.

I mean, we are looking at the software of the mind, and the parietal lobes are where that software gets incredibly complex.

Because it's not just about muscles, right?

Right.

Damage to this area doesn't just paralyze a muscle.

It alters your fundamental perception of space, of objects, and as you mentioned, with the arm, even of yourself.

It's just wild.

And there are other things too, right?

Like people who only eat half the food on their plate, or find putting on a shirt to be this impossible puzzle.

Oh, absolutely.

The symptoms are baffling if you don't know the underlying architecture.

So to understand these wild behavioral symptoms, I feel like we really have to start with how the brain brings data in from the outside world.

The source material divides the parietal lobe into two main areas.

Yeah, the anterior and the posterior.

Right.

So let's look at the front part first, the anterior region.

What exactly is happening there?

So that front area, specifically a ridge called the post -central gyrus, is your brain's raw data intake center.

We call it the sensory strip,

or somatosensory perception.

Raw data, like touch and stuff.

Exactly.

It's where your brain processes touch, temperature, pain, and something called kinesthesia.

Kinesthesia.

That's the awareness of where your limbs are positioned and spaced.

You got it.

And the fascinating thing about this area is how highly organized it is.

Like every point on your body maps to a specific point -to -point location on this strip of cortex.

Okay, reading the text, I kept picturing this as a kind of sensory real estate map.

A real estate map is a great way to think about it.

But the land isn't divided up evenly, is it?

It's like highly sensitive areas.

Your lips, your fingers, they buy up these massive, sprawling mansions of brain tissue.

But the less sensitive areas, like the middle of your back, just get shoved into a tiny cramped studio apartment.

Yes, exactly.

The allocation is entirely based on necessity.

The amount of cortex devoted to a body part is directly proportional to its sensory acuity, not its actual physical size.

This makes sense.

Right, because your back is huge, but it doesn't need to read Braille.

Your fingertips do.

And to test how sharp that map is, neurologists use something called the two -point threshold test.

How does that actually work in practice?

Well, the clinician takes a tool that looks like a geometer's compass, you know, a pair of dividers with two small points.

While the patient is looking away, the clinician gently touches their skin, sometimes with one point, sometimes with two, and they keep moving the two points closer and closer together until they find the absolute minimum distance where the patient can still reliably say, oh, I feel two distinct points, not one.

Oh, I see.

So on your fingertip, that distance is tiny.

Tiny.

But on your back, the points can be inches apart and your brain still blends them into a single sensation.

That is so weird to think about.

But all this sensory data, the touch, the temperature, the pain, it doesn't all travel up to the brain on the same highway, does it?

Yeah.

The text mentions these two distinct pathways.

That does, yeah.

There is a brilliant evolutionary reason for it, too.

The pathways are functionally split.

The lemniscle pathway is primarily a high speed broadband connection for fine touch and vibration.

So that's the detailed stuff.

Right.

Highly detailed, nuanced information.

But temperature and pain travel on a completely separate route called the extra lemniscle pathway.

But why keep them separate?

Wouldn't it just be like more efficient to bundle them together?

Well, think about what happens when you touch a hot stove.

Okay, yeah.

You don't need a highly detailed, nuanced map of the exact texture of the burner, right?

You just need a blaring alarm that triggers an immediate reflex to pull your hand away.

Oh, that makes total sense.

Yeah.

The brain separates pain and temperature so they can trigger distinct, rapid defense mechanisms without waiting for the slower, higher resolution processing of fine touch.

Wow.

So what happens when this anterior intake center gets damaged?

Does the whole system just go dark?

It depends entirely on the injury.

Usually you see alterations in normal sensation.

You might experience anesthesia, which is total numbness.

Or on the flip side, you might experience dysesthesia.

Dysesthesia.

Break that down for me because that's not just a loss of feeling, right?

No, it's an altered sensation.

The touch signals are misfiring.

So a light brush against the skin might feel like burning or pins and needles or an intense uncomfortable pressure.

Like the brain is receiving corrupted data.

Exactly.

You also see a loss of that kinesthetic sense we mentioned.

And if you lose your kinesthetic sense, you're tracking of where your limbs are, that leads to clumsiness, right?

Yeah.

Because, I mean, if my brain doesn't know exactly where my hand is in space, it can't possibly orchestrate the smooth movement of reaching for a glass of water.

Exactly right.

It causes a secondary motor clumsiness.

But uncovering these deficits historically led to some massive disagreements in the medical community.

Oh, right.

The asymmetry debate.

Yes, from the 1960s and 70s.

The big question was, do the left and right hemispheres handle this sensory data differently?

Because you had one group of researchers,

Sims and her colleagues in 1960,

studying soldiers with war injuries.

And they found that if a soldier had damage on the left side of the brain, the sensory loss was highly localized.

But if the damage was on the right side, the sensory deficits were just spread all over the place.

But then a decade later, another researcher named Corkin studied a completely different group of people and found the exact opposite.

Wait, really?

Yeah.

In 1970, Corkin found no difference between the hemispheres at all.

The sensory loss was perfectly mirrored, depending on where the damage was.

So how did they get such radically different answers to the exact same anatomical question?

Well, it wasn't about the anatomy at all.

It was about the nature of the trauma.

You see, Sims studied soldiers with missile wounds.

Oh, like gunshots and shrapnel.

Exactly.

Which cause massive violent shockwaves.

The damage is messy, widespread, and causes secondary swelling and bleeding.

Corkin, however, studied patients who had undergone controlled surgical procedures to remove small bits of tissue.

Ah, I see.

So the researchers weren't really comparing apples to apples.

A bullet wound creates a very different kind of brain injury than a surgical scalpel.

Precisely.

It's a crucial lesson for anyone studying neuropsychology.

The how and why of the brain injury are just as important to the outcome as the where.

OK, so we've established how the raw data comes into the sensory strip.

But raw data isn't perception.

Like my computer's keyboard receives raw data when I type, but it doesn't understand the words.

That's a great analogy.

So how does the brain actually interpret this touch data?

This is where we cross the arbitrary border from the primary sensory strip into the posterior parietal lobe, specifically the secondary and tertiary cortex areas.

The superior and inferior lobules, right?

Exactly.

The brain has to take the raw feelings of cold, hard, and curved and stitch them together to conclude, oh, I am holding a key.

When the secondary area is damaged, we encounter the somatosensory agnosias.

Agnosia is literally meaning a loss of knowing.

Right.

Sensation without perception.

The primary cortex feels the object perfectly, but the secondary cortex can no longer assign meaning to it.

The first type the chapter describes is a stereognosis or a symbolia.

That's the inability to recognize objects just by touch, right?

Right.

So if I have this and you hand me a key while I'm blindfolded, what happens?

You would feel the cold metal.

You would trace the jagged edges with your thumb, but the concept of key would completely elude you.

Wow.

Yeah.

Your brain just cannot synthesize those individual tactile features into a unified 3D mental object.

And to assess for that, they use things like the Cygwin -Goudard form board, right?

Having patients feel geometric shapes while blindfolded and trying to fit them into matching holes.

Yes.

For a healthy brain, that's simple.

You feel a triangle.

You build a mental model of a triangle.

You find the triangular hole.

But for someone with a stereognosis, they're just feeling disconnected edges and points.

They can't synthesize the whole shape.

That's fascinating, but it's still just about objects outside ourselves.

Let's get back to that hook from the beginning, the person disowning their own arm.

How does this loss of knowing apply to our own bodies?

That is a condition known as asymptognosia relating to the patient's own body.

And it gets very strange when dealing with right -sided lesions.

Right -sided lesions cause anasognosia, right?

An unawareness or downright denial of their own illness or limb handicaps.

Exactly.

And the extreme version of that is somatoparafrenia.

The patient who calls the nurse complaining about the stranger's arm in their bed.

Now, I have to push back on this on behalf of anyone listening.

Go for it.

It's easy to understand not recognizing a key by touch.

But how can the brain look at its own arm, visibly attached to its own shoulder, and genuinely deduce that it belongs to someone else?

It's mind -boggling, but it forces us to rethink what ownership actually is.

We assume knowing our own body is just like magic, an innate property of being alive.

Right, it's just my arm.

But ownership is actually a complex, continuous calculation.

Your brain performs every second.

So it's actively computing this is mine.

Yes.

When raw sensory data arise from your arm, your secondary cortex is supposed to attach a cognitive tag to it that says self.

Oh, I see.

If that area of the brain is destroyed, the raw data still comes in.

You can see the arm, maybe even feel it slightly.

But the self tag is missing.

So the logic centers take over.

Exactly.

Your higher logical centers evaluate the situation.

I see an arm.

It doesn't have the self tag.

Therefore, logically, it cannot be mine.

That is wild.

The brain is so committed to its own internal logic that it would rather invent a story about a stranger in the bed than admit its own labeling system is broken.

It is deeply unsettling, and it can be dangerous.

The text details a related condition called a symbolia for pain.

Oh, right.

In this case, the pain pathways we talked about earlier are intact.

The patient feels the pain, but the secondary areas fail to incorporate it as a threat.

Meaning they might just leave their hand resting on a hot stove until someone else actually smells burning.

Right.

The alarm bell is ringing, but the fire department has been disconnected.

Exactly.

And the manifestations change depending on the hemisphere.

If the lesion is on the left side of the parietal lobe, we see auto -pagnosia.

Which is different from denial, right?

Right.

This isn't denial of the body.

It's an inability to localize or name body parts.

A common subset of this is finger agnosia.

Tested via the in -between test and the two -point finger test, so they wouldn't be able to tell you which finger you just touched.

Yep.

And what's crucial to understand about all of these tactile deficits is that they aren't just quirks of the skin.

They're fundamental spatial failures.

How do we know that?

Because patients who can't recognize objects by touch almost always have visual object agnosia as well.

Which seems entirely counterintuitive.

I mean, why would a touch deficit in your hand be linked to a visual deficit in your eyes?

Because recognizing an object, whether by feeling it or looking at it, requires the same underlying mechanism.

The brain's ability to form and manipulate spatial representations.

Like tested by Gollum figures or the Moony Closure Faces test.

Exactly.

Or Warrington and Taylor's unconventional views of objects test.

Think about looking at a bucket from directly above.

It just looks like two concentric circles.

But I know it's a bucket because my brain mentally rotates it in 3D space to match my memory of a bucket.

Precisely.

If your parietal lobe can't mentally rotate the visual image of the bucket, it also can't mentally assemble the tactile edges of a key in your pocket.

The spatial manipulation engine is broken in both cases.

Okay, so if the brain builds a spatial map of our personal body,

how does it handle the extra personal space, the world around our body?

That is the domain of the posterior parietal lobe.

This area constantly maps where objects are relative to you and where objects are relative to each other.

And when it's damaged, we see visuospatial agnosia, severe spatial disorientation.

Right.

Clinicians assess this with things like the pool reflections test for mental rotation,

the stick test for reproducing matchstick patterns, and spatial memory tests like the tactual performance test and the Benton visual retention test.

You know, I think we've all been in a car with a friend who gives terrible driving directions.

They confidently point left and yell, turn right.

What a parietal lobe patient experiences is just an extreme version of that.

That is actually a vital distinction to make.

When your friend points left but says right, they know perfectly well where the destination is in space.

Right.

They just pulled the wrong vocabulary word out of their mental dictionary.

It's a normal verbal error.

So what's happening with the parietal patient?

They are suffering a fundamental, profound spatial deficit.

They aren't confused by the words.

They are fundamentally confused by the physical relationship between left and right in the environment.

Like tested by the money roadmap test.

Exactly.

And everyday tasks and route following become incredibly difficult.

Researchers use the locomotor map following tasks, specifically Weinstein maps.

That's the one with the dots on the floor, right?

Yes.

They put nine dots on the floor and hand the patient a map with a route drawn on it.

The patient has to walk the route, but there's a catch.

They aren't allowed to rotate the map.

Right.

They are strictly forbidden from physically turning the paper as they turn their body.

Oh, that sounds incredibly difficult.

If I turn around to walk backward, but I can't turn the map, I have to constantly mentally reverse everything on the paper to match the real world.

Forcing them to hold the map steady requires continuous mental 3D rotation.

A patient with a right parietal injury will find this near impossible because they can no longer translate the 2D space of the map onto the 3D extra personal space around them.

So it's spatial orientation is how we map the world.

What happens when half of that map is just deleted?

Because that brings us to one of the most mind bending phenomena in the chapter,

spatial neglect.

Also known as hemi inattention, often caused by right sided lesions.

It results in the patient completely ignoring the left side of their universe.

And the real world examples of this are staggering.

Pages showing up to the clinic with massive bruises up and down their left arm because they keep walking into door frames on that side.

Men who shave only the right half of their face, people who eat only the food on the right side of their plate.

And if a nurse spins that plate 180 degrees, the patient says, oh, more food and eats the rest.

It's like the left side of the world doesn't exist.

It doesn't exist for their conscious mind.

But what's truly astonishing is a study from 1978 by Bizioch and Luzzati.

The Milan Square Study.

This blew my mind.

Tell everyone how they set this up.

The researchers asked patients who had lived in Milan their whole lives to close their eyes and imagine standing in the city's main square, Piazza del Duomo, facing the cathedral.

They asked the patients to describe the buildings around them.

The patients perfectly described all the cafes and buildings on the right side of the square, but they completely omitted the left side.

Because neglect applies to imagined scenes and memory, not just real time vision.

But then the researchers pulled a trick, right?

They said, OK, in your mind, walk to the cathedral steps, turn around and face the other direction.

Yes.

And suddenly, the patients began perfectly describing the buildings they had just ignored.

Because from this new mental vantage point, those buildings were now on their right.

Wow.

And they completely neglected the buildings they had just perfectly described a moment ago.

It's exactly like a computer rendering a video game.

If a glitch makes the computer stop rendering the left half of the screen, the player can't see it.

But the game's code still knows what's over there.

The data is in the machine.

It's just walled off from the player's awareness.

That is a brilliant analogy.

And it leads directly into the concept of implicit processing.

The brain still processes the neglected information, it just never reaches conscious awareness.

We see this vividly in the Marshall and Halligan study from 1988, the burning house study.

Exactly.

They showed a neglect patient two line drawings of a house, one stacked above the other.

The right sides of both houses looked completely identical, but on the left side of the bottom house,

massive jagged flames were billowing out of a window.

And because of the spatial neglect, the patient looks at the paper and says, they are exactly the same.

They genuinely cannot see the fire.

But then the researcher asks a seemingly silly question.

Which house would you rather live in?

Right.

Consistently, reliably, the patient points to the top house, the one not on fire.

They can't tell you why.

They'll make up an excuse like, oh, this one looks roomier.

Because their brain processed the profound danger of the fire outside of their conscious awareness and guided their decision.

The game code knew the house was on fire.

Exactly.

Oh, and we should also mention there's a difference between within object neglect and between object neglect.

Right.

So within object, we'd be drawing half of each individual flower on a page.

And between object, we'd be drawing only the complete flowers that are on the right side of the page, ignoring the left flowers entirely.

Yep.

It's a fascinating nuance.

So how do you treat a deficit this bizarre?

Do you just keep pointing to the left and saying, hey, look over here?

Unfortunately, no.

Verbal reminders are relatively ineffective.

The most successful treatment discussed in the text is contralateral limb activation.

Meaning you make them move the limb on the neglected side.

If they are neglecting the left side of space, you force them to perform physical movements with their left arm.

Moving the limb generates deep cerebral activation in the damaged hemisphere, which somehow helps reboot that attentional network and counteract the neglect.

And as they recover, they go through some very specific phases.

The text mentions allasthesia.

Allasthesia is a fascinating middle step.

The patient finally starts processing a touch on their left arm, but their spatial map is still distorted.

So they confidently point to their right arm and say, you touched me there.

And then they progress to simultaneous extinction.

Let me make sure I understand this one.

They can feel a touch on their left side.

Fine.

They can feel a touch on the right side.

Fine.

But if the doctor touches both arms at the exact same millisecond, what happens?

The slignal from the left side vanishes.

It's extinguished.

Why does that happen?

Think of attention as a limited bandwidth connection.

When stimulated individually, each side has enough bandwidth to get the message to the brain.

OK.

But when stimulated simultaneously, the two signals compete.

Because the damaged hemisphere is weaker, the healthy hemisphere completely outcompetes it for the brain's limited attentional resources, essentially muting the weaker signal.

It's a battle for processing power, which makes you realize just how much heavy lifting the parietal lobe does.

But it's not just physical space, right?

Section 5 of the chapter makes this massive pivot from space to symbols and sequences.

Yes, the symbolic synthesis.

And I have to admit, seeing the loss of calculating ability in a chapter about spatial processing really caught me off guard.

It surprises many people.

We think of math as pure logic.

But think about how you actually solve a complex math problem in your head.

Say 154 plus 87.

Well, I instantly visualize them stacked on a mental chalkboard.

I see the 4 and the 7.

I get 11.

I visualize writing the 1.

And then I mentally, literally move the other one over to the top of the next column.

Which is an entirely spatial task.

You are manipulating symbols on a geometric grid in your mind's eye.

Patients with parietal lesions who suffer from a calculia don't necessarily forget what a number is.

Their mental chalkboard shatters.

They lose track of where they place the digits, or they forget which direction to carry the one.

That reframes so much.

Yeah.

And it's not just abstract symbols.

It's the planning of complex physical actions, too, known as apraxia.

Apraxia is the loss of intentional, purposeful movement, despite having no muscle weakness or paralysis.

The muscles work fine.

The blueprint for how to use them is gone.

The text differentiates between a few types.

There's ideomotor apraxia, where you can't perform a simple gesture on command, like waving goodbye.

Right.

But then there's ideational apraxia, which seems much more complex.

Ideational apraxia is a failure of ordered sequences.

If you hand a patient a box of matches and an unlit candle, they know what the objects are.

But they can't organize the spatial and temporal steps required to strike the match, hold it to the wick, and blow it out.

The overarching plan is fragmented.

And the text brings up Geschwind's theory for this, right?

Yes.

Geschwind theorized it involves the arcuate fasciculus connecting the left posterior parietal lobe to the frontal lobe.

The connection between planning and execution is severed.

And then there's dressing apraxia, which, when you frame it as a spatial problem, makes perfect sense.

It really does.

Because putting on a shirt isn't just moving your arms.

Clothing is a highly plastic, floppy, shapeless object.

You have to mentally map that 2D flexible object onto the rigid 3D geography of your own personal body map.

It is a phenomenal feat of geometric calculation that we take for granted every morning.

And we see similar constructional failures when we ask these patients to draw.

That's constructional apraxia.

Tested by Benton's block designs, like Coe's blocks, and three -dimensional constructional praxis.

And you can actually look at the drawing errors they make and deduce which hemisphere is damaged.

Exactly.

If the right hemisphere is damaged, the patient's drawing will be fragmented and misplaced.

The pieces will be scattered, lines overlapping chaotically, completely losing the overall shape.

But what if it's the left hemisphere?

If the left hemisphere is damaged, the drawing will be structurally cohesive.

It will look like a house, but it will be incredibly oversimplified and lacking all internal detail.

Which perfectly illustrates how the different sides of the brain tackle different aspects of a problem.

Which brings us to the final portion of the chapter.

The deepest parts of the posterior parietal lobe acting as the ultimate integrator.

Yes, integrating the senses, memory, and language.

It handles intersensory association.

For example, cross -modal matching, like tactile visual integration.

So feeling an object and matching it to a picture.

Exactly.

And generally, the left hemisphere handles what the identity is, while the right handles where the location or form is.

And it even impacts short -term memory.

The text brings up Patient KF from a 1974 study by Shalise and Warrington.

Right, Patient KF had a parietal lesion and showed this abnormally rapid forgetting of auditory information, like trying to remember a spoken list of words.

Which highlights how spatial mapping and auditory loops are intertwined.

But historically, neurologists desperately tried to neatly categorize the fallout of this posterior parietal damage.

They really did.

They looked at four specific symptoms.

The inability to write, which is a graphia.

The inability to calculate, a calculia.

Right -left confusion.

And finger agnosia.

And they slapped a label on it.

Gerstmann syndrome.

Yep.

Historically linked to the left angular gyrus.

But the text is pretty definitive.

The current medical opinion completely rejects this as a unitary neatly packaged syndrome.

Why were doctors so obsessed with boxing these up in the first place?

I mean, brain lesions rarely respect arbitrary academic boundaries.

It's human nature, really.

When clinicians see a cluster of highly unusual symptoms appear together a few times, they want to name it and assign it a pinpoint location in the brain.

It provides an illusion of control.

But as the textbook concludes.

High -level functions like language reception or math don't live in one tiny isolated clump of cells.

They are distributed networks.

Which the McPhee and Zang will study demonstrated back in 1960.

They summarized laterality beautifully.

They found that right -sided lesions destroyed pure spatial tasks like cube counting and paper cutting.

Right.

But left -sided lesions destroyed tasks where the patient had to use verbal processes to solve a spatial problem, like the Weigel sorting test.

It all comes down to the cognitive strategy.

Your brain routes the task differently depending on whether you are using raw mental geometry or talking yourself through the steps.

So if we look back at the whole picture we've painted today from this textbook chapter,

the anterior parietal lobe, that sensory strip, is our raw data center, quietly building the physical map of our own skin and bones.

But the posterior parietal lobe is the master integrator.

It maps the extra personal space you are sitting in.

It choreographs your complex physical actions.

And it provides the hidden mental chalkboards for abstract symbols like math and language.

It is a staggering progression from the simple feeling of a sharp point on your finger to the highest levels of human reasoning.

It really makes you appreciate the sheer automatic computational power your parietal lobes are executing right this second.

Just allowing you to comprehend spatial boundaries or mentally map out your day without a single conscious thought.

And it leaves you with a provocative question to explore on your own.

Oh.

If our ability to do a simple math equation, or our ability to imagine standing in a city square relies entirely on hidden spatial maps in our parietal lobes, what other seemingly abstract thoughts, philosophical beliefs, or core memories are secretly governed by hidden geometry in our minds?

That is definitely something to chew on.

Thank you so much for joining us for this specialized tutoring session.

From all of us at the Last Minute Lecture team, keep questioning the map and we'll catch you next time.