Chapter 4: The Temporal Lobes

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Right now, as you listen to this, there are invisible sound waves vibrating against your eardrum.

Yeah, and light bouncing off all the objects around you right into your retinas.

Exactly.

And in a purely physical sense, that's, I mean, it's all just meaningless physics, just raw data.

Pure chaos, really.

Right.

But somehow you aren't experiencing a chaotic storm of, you know, frequencies and light beams.

You're experiencing a recognizable reality.

You hear words.

You recognize the room you're sitting in.

You have this, this continuous sense of who you are.

And the biological hardware responsible for that continuous sense of reality is, well, it's what we're decoding today.

Welcome to this Deep Dive.

Think of this as your personal one -on -one tutoring session.

Our mission today is to help you master chapter four of Introduction to Neuropsychology.

And we are focusing entirely on the temporal lobes.

Yeah, temporal lobes.

And we aren't just going to list off brain parts like a dry textbook.

We are going to trace the exact path of how a sound becomes a memory.

And how a memory becomes, you know, your actual personality.

The brain has to act as this ultimate meaning maker.

Meaning maker.

I like that.

And the clinical cases that illustrate this are genuinely mind -bending.

Oh, absolutely.

When this specific area of the brain misfires or gets damaged, the results just completely defy common sense.

Defy common sense is putting it mildly.

I mean, just to tease what you're going to learn today, we're going to talk about a patient who ate an entire lemon, peel and all, simply because his brain forgot what a lemon was.

Yeah.

And someone who lost the ability to form new memories after getting a miniature fencing foil shoved straight up his nose.

Which is just wild.

And oh, and we'll explore this terrifying sounding surgical technique that involves putting exactly half of a patient's brain to sleep while they're wide awake and talking.

It sounds like pure science fiction, I know, but it's actually standard neurological practice.

It's nuts.

So to understand what the temporal lobes do, you first got to visualize where they sit and how they're shaped.

So imagine looking at a human brain from the side.

Right.

You'll see this forward pointing protrusion on each side of the head, roughly sitting right behind your temples.

The Tanswik describes the shape as looking kind of like a folded wing of a bird sweeting forward.

That's a great visual.

And if we look at the outside surface, you know, the lateral surface is divided into three horizontal strips.

Right.

You've got the superior, the middle and the inferior temporal gyri.

Gyri just being the anatomical term for those fleshy ridges on the brain surface.

But that lateral view really only tells half the story, right?

Exactly.

If you were holding a brain and gently pulled the front tip, the temporal pole away from the rest of the cortex, you'd reveal this whole hidden world underneath.

You'd see the inner mesial surface, including a tucked away area called the insula.

Yeah.

And if you follow that tissue underneath the lobe and up the inside, you hit these much deeper structures.

Like the fusiform gyrus and the parahippocampal gyrus.

Right.

And right at the top border, there's this hook -like structure called the uncus.

And these hidden underneath areas are incredibly important to keep in mind.

Oh, absolutely vital.

But wait, hold on.

I want to stop you right there for a second.

When you look at diagrams in a standard psych class, you mostly just see that outside lateral surface.

Why is it so crucial for us to memorize these hidden underneath areas like the parahippocampal gyrus?

Because those deep hidden areas are the secret link to your emotions and your memories.

They connect intimately with the limbic system.

In fact, that's why it's called the parahippocampal gyrus.

It literally sits physically next to the hippocampus.

Ah, that makes sense.

Yeah.

This inner tissue actually belongs to a much older evolutionary system called the paleocortex.

So it's like a bridge to the older primal brains.

Exactly.

So when we get to the bizarre personality changes and profound memory losses later in this chapter,

that ancient paleocortex connection is the mechanism behind it all.

Okay, got it.

Oh, and we have to mention one more anatomical landmark before we move into function,

Heschel's gyrus.

Yes.

Located right inside that top horizontal strip folding inward.

Which leads us perfectly into the primary day job of the temporal lobe's outer surface, hearing.

Right.

Just as the frontal lobes have hierarchical levels for movement, the temporal lobes have three distinct levels for processing sound or audition.

Okay, so level one is the primary auditory cortex, right around that Heschel's gyrus.

Yep.

This is where the initial sensory reception of sound happens.

And the wiring here is highly specialized.

Specialized how?

Well, the auditory pathways coming up from your ears are both crossed and uncrossed.

Meaning a sound entering your right ear travels to the temporal lobes on both the left and right sides of your brain.

Oh, wow.

But those pathways aren't equal, right?

No, they aren't.

The cross pathway, so the contralateral one moving from the right ear to the left hemisphere, is anatomically thicker.

It's like the dominant cable.

Exactly.

And researchers prove this using what they call dichotic listening tests.

Where they play competing audio into both ears at the same time.

You got it.

The brain consistently favors the info coming through that thicker cross pathway.

So because of this dual wiring system, if you get a lesion in one pathway, you don't go completely deaf.

Right.

But your auditory detection thresholds on the opposite side will be noticeably impaired.

Interesting.

And we actually know an incredible amount about this first level of hearing thanks to Wilder Penfield.

His methods in the mid -20th century were just wild.

Yeah.

Operating on patients with severe epilepsy.

Right.

And to figure out exactly what damaged tissue to remove, he kept the patients awake under local anesthetic.

Because the brain tissue itself doesn't have pan receptors.

Exactly.

So he'd stimulate the primary auditory cortex with this tiny electrical probe.

And the awake patient would tell him what they experienced in real time.

Which is so cool.

And when he stimulated this primary level, they didn't hear it like complex symphonies, did they?

No.

They heard raw, unformed sounds.

A specific buzzing tone, a sudden pitch, or a sound that seemed to come from a very specific direction in the room.

Which brings us to level two.

The secondary auditory cortex.

This is in the superior temporal gyrus.

And when Penfield moved his probe and stimulated this area, the experience completely changed.

They started hearing meaningful things, right?

Yeah.

The specific voice of a family member, or the distinct sound of a running tap.

And this is also the level where we see the brain heavily lateralize.

Left versus right, taking on entirely different jobs.

Very much so.

Though left temporal lobe becomes highly specialized for language, it handles things like Wernicke's area.

So if you suffer damage there, you lose the ability to distinguish between phonems.

You might struggle to hear the difference between a bass sound and a paw sound.

Exactly.

While the right temporal lobe, on the other hand, becomes sort of the brain's music critic.

We assess this using the seashore tests, right?

Yeah.

A patient with a right temporal lesion will likely fail tests of tonal memory.

They lose the ability to identify timbre.

Timbre being that complex harmonic quality that makes a piano sound different from a violin, even when they play the exact same note.

Precisely.

And then we hit level three, the tertiary auditory cortex.

This moves further into the anterior and middle divisions.

And this is where total comprehension happens.

Right.

If you suffer a lesion here, you develop auditory agnosia.

Agnosia, literally meaning a loss of knowledge.

Yeah.

A researcher named Vignolo did some incredible work mapping this out.

He found a very clear split based on which hemisphere was damaged.

Like if a patient had a left temporal lesion, they failed the meaningful sounds identification test.

Yep.

They would hear a dog barking, but they couldn't tell you what the sound represented.

It was just noise to them.

But if they had a right temporal lesion, they failed the meaningless sounds discrimination test.

Right.

And you can even develop a condition called amusia, which is this profound tone deafness where you just completely lose the ability to recognize rhythms or melodies.

Man.

So to lock down these three levels of hearing, think of it like this.

Level one is just hearing a raw piercing frequency.

Level two is recognizing that the frequency is a familiar digital beep.

Yeah.

And level three is associating that beep with the critical knowledge that your alarm clock is going off and you need to get out of bed.

That's a perfect analogy.

It's the journey from raw frequency to complete semantic meaning.

Now, because of where the temporal lobe physically sits on the side of the head, it's not just about hearing.

It's intrinsically tied to vision.

Yeah.

The visual pathways, those massive bundles of cables, creating sight from your eyes to the processing centers have to physically run underneath the surface of the temporal lobes.

This is called the optic radiation.

Right.

Wait, you're losing me here.

Vision happens in the occipital lobe right at the very back of the skull.

Yeah, that's right.

So how on earth does an injury to the temporal lobe on the side of your head make you blind?

Well, it highlights a crucial rule of neuropsychology that's easy to forget when you're just looking at 2D textbook drawings.

Brain lesions are three -dimensional.

So a surface injury to the temporal cortex, like a tumor or a stroke, can penetrate deep enough into the subcortical tissue to sever those optic cables running underneath.

Wow.

And when those cables are cut, you get a very specific visual deficit called upper homonymous quadrant loss.

Exactly.

Meaning you experience blindness in the upper half of your vision.

On the side, opposite the injury, in both eyes.

But the temporal lobe's role in vision is more than just like acting as a conduit for cables.

But much more.

The tertiary visual cortex actually bleeds over into the middle and inferior temporal gerry.

And lesions in this specific area cause a pretty devastating condition called prosopagnosia, face blindness.

This usually results from a right temporal lesion.

And to be clear, it's not that their eyes stop working.

Right.

If they look at a face, they see the nose, the eyes, the mouth, perfectly clearly.

But the temporal lobe fails to attach the associative meaning to those features.

They can look right at their spouse or their kids and have zero recognition.

In severe cases, they look in a mirror and don't recognize their own reflection.

Which is terrifying.

And when you lose the ability to process the highest levels of sensory meaning, your selective attention completely falls apart.

Yeah, you can no longer filter out competing stimuli or make sense of a complex scene.

There's a brilliant visual assessment for this called the McGill picture anomalies test.

Oh, I love this one.

Yeah.

You show the patient a detailed sketch of a normal scene, say a carpenter building a workbench in a cluttered shop.

But the artist has hidden something incongruous in the drawing.

Like a boomerang sitting in the woodpile.

Exactly.

And patients with right temporal lesions really struggle to scan the scene and spot the unexpected element.

Because the temporal lobe sit at the anatomical crossroads of hearing and vision,

they're responsible for cross modal integration.

So when these areas misfire, like during temporal lobe epilepsy, patients will experience these terrifying errors where data from one sense gets scrambled and integrated into another.

Yeah.

And if attention is the cognitive act of noticing the present moment,

well, memory is how we store it.

Right.

So if the temporal lobe helps us recognize what we're looking at right now, what happens when it needs to store that recognition for tomorrow?

That brings us to the MDSIC syndrome.

Okay, let's get into it.

When you suffer bilateral damage, meaning damage to both the left and right sides of those deeper medial structures we talked about earlier,

the present moment instantly vanishes.

You can no longer lay down new memories.

And the most famous case of this in all of neuropsychology is case HM.

In 1953, HM underwent experimental surgery to cure his severe epilepsy.

The surgeons removed the mesial part of his temporal lobes on both sides, parts of the hippocampus, the amygdala, and the uncas.

And the surgery successfully cured his epilepsy.

And remarkably, his intellect and working memory stayed entirely intact.

Like if you asked him to repeat a seven digit phone number, he could do it perfectly.

Yeah.

But the removal of those deep structures left him with profound anterograde amnesia.

He couldn't form a single new conscious memory post 1953.

He'd read the same magazines over and over without ever getting bored.

He described his entire existence as feeling, quote, like waking from a dream.

Just incredibly sad.

But researchers testing HM discovered two massive exceptions that completely revolutionized our understanding of human memory.

Right.

So HM completely failed a complex, visually guided stylus maze.

But when researchers simplified the maze, he actually learned the solution over 155 trials.

And more importantly, he retained that learning for years.

But the even wilder discovery was the mirror drawing task.

Oh yeah.

They had HM sit down and trace a complex star shape on a piece of paper, but he could only see his hand and the paper through a mirror.

Everything is reversed.

It's an incredibly frustrating and difficult task.

But over several days, HM got faster, smoother, and made fewer errors.

He learned the physical skill flawlessly.

And the profound realization here is that every single time he sat down to trace that star, he was terrified and confused.

Swearing to the researchers he had never seen the task before in his life.

It proves that motor learning, you know, that knowing how to do something is handled by a completely different biological system than conscious declarative memory.

The knowing that you did something.

It's like the body and the subconscious brain are diligently taking notes, even when the conscious mind has entirely lost the pen.

That's a great way to put it.

And HM is the most famous, but he isn't the only case.

Consider case NA.

This is the one with the bizarre accident, right?

His roommate was making a mock lunge with a miniature fencing foil.

Yeah.

And by sheer terrible luck, the foil went straight up NA's nasal cavity and penetrated deep into his brain.

And NA also developed severe anterograde amnesia.

For years, the medical community debated what exactly the fencing foil damaged.

Many argued it must have hit the mammillary bodies.

But a 1989 MRI finally put the debate to rest.

The imaging proved the foil had actually damaged his left thalamus.

Right.

And the mammillary bodies had only atrophied later as a secondary result.

And this specific thalamic damage ties closely into Korsakoff's disease.

Yeah.

Korsakoff's is a severe memory disorder typically caused by chronic alcoholism or Wernicke's encephalopathy, which damages the anterior thallus and the amygdala.

These patients suffer similar anterograde amnesia.

Like a patient will greet their doctor.

The doctor leaves the room for five minutes.

And when they return, the patient greets them again as a total stranger.

But unlike HM, Korsakoff patients frequently engage in something called confabulation.

Because the brain abhors a vacuum.

Exactly.

They simply invent detailed imaginary stories to fill the gaping holes in their reality.

And interestingly, they also show a tiny bit of unconscious learning.

We see this with the Gallen incomplete figures test.

Right.

Visual puzzles where a patient is shown a few scattered meaningless lines.

Over several trials, more lines are added until it forms a picture of an elephant or an umbrella.

And Korsakoff patients show slight savings.

Meaning their brains recognize the image a tiny bit faster on repeated trials.

Even though their conscious minds have no memory of taking the test yesterday.

The clinical evidence of how memory degrades is just staggering.

The textbook compares two patients who both suffered from herpes simplex encephalitis, KCW and KSS.

KCW is truly heartbreaking.

The infection destroyed his hippocampus bilaterally.

He was left with essentially a 30 -second loop of memory.

He kept a journal where he would compulsively cross out his previous entry and write, Every few minutes, I am finally awake.

And he also lost profound amounts of semantic knowledge.

This is the patient we teased earlier.

His semantic memory degraded so much he was observed eating a whole lemon peel and all.

Because his brain no longer contained the concept of what a lemon was or how you were supposed to eat it.

Yet because he was a trained academic musician before the illness, he completely retained his ability to read and play complex 16th and 20th century sheet music.

It's remarkable now.

Contrast CW with KSS, who contracted the same infection.

Right.

SS developed a temporal gradient to his amnesia.

Meaning he remembered things from his deep past, like the 1930s, much better than events that happened right before his illness.

And unlike Korsakov patients, he didn't confabulate to fill the gaps.

And he retained his general sense of self -knowledge.

So what does looking at HM, NA, CW and SS actually tell us about how memory works?

It highlights this massive ongoing technical debate in neuropsychology.

Is amnesia a failure of encoding the memory into the brain in the first place?

Or a failure of retrieving the memory once it's safely stored?

But what is absolutely undisputed is the hardware.

The hippocampus, the medial temporal cortex, and the anterior thalamus are the essential biological structures required for laying down and retrieving our long -term reality.

So we know what happens when both sides of the hardware are destroyed, the present vanishes.

But what happens if only one temporal lobe is injured?

This brings us back to that concept of lateralization.

Left versus right.

Right -sided lesions systematically destroy spatial memory.

We can see this using the CORSI block tapping task, where the examiner taps a random sequence on a board of nine blocks, and the patient has to repeat it.

Normal subjects can easily learn a repeating physical sequence.

Patients with right temporal damage cannot.

They also fail visual spatial tests, like the Ray -Ostrich figure test.

That's copying a complex abstract geometric drawing.

Right.

If you ask a right lesion patient to copy the drawing while looking at it, they do it perfectly.

Which proves their basic vision and attention are fine.

Exactly.

But if you take the drawing away and ask them to draw it from memory 30 minutes later, the spatial memory is entirely gone.

They bomb the test.

Left -sided lesions, on the other hand, systematically destroy verbal memory.

Yeah.

They struggle with nonsense syllables.

Or the Wechsler memory scale logical memory subtest.

Where if you read a short news story to a left lesion patient and ask them to recall the semantic details 30 minutes later, their retention score drops to about 33 % of what they could remember immediately.

They also struggle immensely with paired associate learning.

That's trying to memorize two linked, unrelated words.

Although researchers found an interesting workaround here.

If you slow the test way down, left lesion patients actually improve.

Because giving them extra time allows them to invent verbal mediators.

They have time to make up a little sentence in their head to link the two words together, which aids the damaged encoding process.

Yeah.

And much of our clinical data on these unilateral one -sided memory deficits comes from patients undergoing an anterior temporal lobectomy.

A surgery to remove damaged tissue to cure focal epilepsy.

Right.

But before surgeons can remove that tissue, they employ the WADA technique.

Okay, wait.

Are you saying they literally pull the fuse on half the house's power to see which rooms go dark?

Because the WADA technique sounds absolutely terrifying.

It does.

They inject a barbiturate called intracarotid sodium, a metal, directly into the carotid.

...

brain to sleep while they are wide awake, just to see if they lose the ability to speak.

It is exactly like pulling a fuse.

It's a highly stressful, incredibly brief window of time, but it is in an absolute medical necessity.

To map exactly which hemisphere holds the verbal memory and language centers.

Yeah.

Before they make a single cut.

Because if they guess wrong and remove the dominant language the patient loses their ability to communicate permanently.

Wow.

Just wow.

And before we completely leave the mechanics of memory, we have to mention Eleanor McGuire's famous neuroimaging study.

The London taxi drivers.

Yes.

To get their cab license, drivers have to memorize the incredibly complex, twisting map of London, a grueling process known as the knowledge.

And brain scans of these drivers showed massive activation in their right hippocampus when they were recalling spatial routes.

Though it's worth noting the scientific debate here.

As the textbook points out, some researchers, like Rosenbaum, argue that it isn't just the hippocampus doing the heavy lifting.

Right.

They suggest the parahippocampal region surrounding it are just as critical for that intense spatial navigation.

Which brings us to the final and perhaps most unsettling piece of the puzzle, personality and altered states.

Yeah.

If the temporal lobes are responsible for integrating our senses and storing our personal history, then abnormal electrical firing in these areas doesn't just change what we perceive.

It fundamentally changes who we are.

We see this clearly in temporal lobe epilepsy or TLE.

These patients don't always experience the classic full body, physical convulsions we associate with seizures.

No, sometimes the seizure is just a short absence or a hidden electrical storm in the temporal tissue.

But chronic abnormal firing here often leads to a very specific personality syndrome.

And the behavioral shifts are stark.

Patients can become highly pedantic and overly detailed in their speech.

They might talk obsessively about their personal problems, clinging to people at social gatherings to the point where it's hard to escape them.

They can suffer from intense paranoia, sudden aggressive outbursts, and researchers note a surprisingly high rate of sudden overwhelming religious conversions.

Their moment to moment conscious experience of reality can also fracture.

They suffer from complex hallucinations.

Not just seeing a flash of light, but hearing fully formed conversing voices or suddenly smelling highly offensive phantom odors.

Right, and they report profound out of body experiences or depersonalization, feeling as though they are hovering above themselves.

And they experience wild distortions of time and familiarity,

like déjà vu, where a completely novel first time experience feels intensely, overwhelmingly familiar.

Or the exact opposite, jamais vu, where your own living room, a place you've sat a thousand times, suddenly feels like an alien environment you have never been in before in your entire life.

The mechanism behind déjà vu is fascinating when you view it through the lens of temporal lobe function.

It's basically a glitch in the brain's timeline coding.

Exactly.

It's as if the temporal lobe accidentally takes the psychological feeling of this has already happened and stamps it onto a sensory file that you are experiencing for the very first time.

That is precisely the mechanism at play.

The brain's meeting maker applies the wrong meaning to the present moment.

And the most extreme behavioral shifts happen when a patient suffers extensive bilateral lesions to the temporal lobes.

This results in what's known as the Kluverbuce syndrome.

This syndrome represents a radical fundamental shift in human behavior.

Patients develop profound hypersexuality.

An indiscriminate, insatiable sexual drive that can be directed at anyone or even toward inanimate objects.

And paradoxically, they display this behavior while having completely flat, blunted emotional responses.

They also develop a bizarre compulsion to explore entirely inappropriate objects by putting them in their mouths, much like an infant would.

It's a stark, unsettling reminder of what happens when the higher -level cortex loses its regulatory control over those underlying limbic, paleocortical animal drives we discussed at the very beginning of this session.

It really is.

So if we synthesize everything we've covered in Chapter 4, the temporal lobes are the grand biological synthesizers of the human experience.

They take raw acoustic frequencies and translate them into your favorite song.

They take visual geometry and turn it into the face of someone you love.

They take fleeting, temporary present moments and forge them into permanent personal histories.

They literally build the narrative of your reality.

And as we've seen through these clinical cases, when the hardware malfunctions, that narrative breaks down entirely.

The present disappears, faces lose their meaning, and behavior reverts to pure impulse.

Which leaves you with a pretty profound question to mull over as we wrap up this deep dive.

Yeah, if a microscopic electrical storm in your temporal lobe can induce a sudden religious conversion, an out -of -body experience, or transform you into an entirely different personality,

where exactly does the biological machinery of the brain end and the concept of the self actually begin?

That is the ultimate unanswered question of neuropsychology.

Thank you from the Last Minute Lecture Team.