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Hey everyone, it's Inés and welcome back to Draw Curiosity!
Now, I can already hear the knuckles cracking, the finger-flexing and the frantic keyboard tapping
“Geez, pop-science these days is subjected to so much oversimplification and misinformation!”
“Umm… you know that’s a myth… right?” “ … “
but - it’s OK, don’t worry,
I haven’t lost 90% of my brain - *cough* April 1st calendar flashes up *cough* -
this is and remains to be a myth, all of our brain is very much in use. If 90% of our brain were
dormant, you can be assured that evolution would have given us a healthy reduction in brain size
given that the brain, despite only occupying 2% of our body mass consumes 20% of our energy budget,
and what a waste that would be if 18% of our hard earned energy expenditure was not even being used!
That being said, whenever I hear this myth, it always makes me think - what is even meant
by we only USE 10% of our brains? What do people mean by the word use here? How much
and how little can we use, and how far can I go to twist the semantics of this word to find
an instance where this statement is true? So all of this, and a little bit more, coming right up!
< Introduction - Draw Curiosity > <TITLE CARD -
a wild theory>
For instance, let’s say by use we are referring to neurons firing and sending a signal onwards
to another neuron. Neurones have different and extremely variable firing rates, but an average
rate cited across the literature is between 1 and 30 times per second. When a neuron is fired,
the membrane goes from a resting to a depolarised potential, and is later subsequently repolarised
and returns to its baseline resting potential. On average, this process takes around 1 - 3
milliseconds. So now, taking a conservative measure let’s say that each neuron fires
30 times per second, and is depolarised for a total of 3 milliseconds each time - that means
that your average neuron feasibly spends 90 milliseconds per second depolarised,
which means that it is firing less than 10% of the time. Therefore, under the assumption that all
neurons share this firing rate and depolarisation rate and that the timing of firing is random,
then yes, for any instantaneous moment in time, less than 10% of the brain is in “USE”.
[PAUSE] (for the sunglasses) Buuuut as soon as you extend that snapshot
to any meaningful amount of time, chances are all of your neurons would have repeatedly fired
and been in use - so can we really imply that 90% of the brain is not being used? And conversely,
would it be possible for us to live using only strictly 10% of our brain?
< The man who only had 10% of his brain >
Hey Inés! Have you not seen this interesting news story of a French civil servant
who lives a perfectly normal life, went to the doctor complaining of leg weakness,
and came out discovering he was missing 90% of his brain? See!
It is possible to live with only 10% of your brain.
[PAUSE for a Rick Roll] [record scratch]
...Except that actually this isn’t quite the case! This Frenchman was suffering a very,
very extreme case of hydrocephalus, which literally means head full of water,
and is a condition where cerebrospinal fluid accumulates in the brain. He had actually been
diagnosed at the age of 6 months, he had a shunt installed to fix it, had a brief relapse at 14.
Somewhere along the way, he must have developed chronic hydrocephalus, whereby the cerebrospinal
fluid had gradually built up over the years, until it caused the symptoms of leg weakness
which brought him to the doctors. But that being said, hydrocephalus does not cause brain atrophy,
and nowhere in the official journal article does it say that 90% of the brain is missing. In fact,
if anything, the entirety of his brain mass is there, but is simply compressed by the large
volume of cerebrospinal fluid, such that it has formed a thin, dense layer around the inside of
the skull. Because this process would have been gradual, the damage caused was likely lesser as
the brain would have had time to adapt to it. They apparently performed an IQ test on him,
and whilst he was found to have a lower IQ than average, he was not mentally disabled,
he had a happy family, he had a good job. So, no, this case is not a case of a man living
with 10% of his brain, but rather living with his brain temporarily squished into 10% of its space.
< Brain atrophy > Whilst this previous
case was a little bit of a red herring,
I did wonder whether it was possible for the brain to gradually atrophy until only 10% remained,
assuming that the atrophy occurred in such a way that most of the brain function was preserved.
And the answer? Well, our brains actually do naturally atrophy over time. After our brains
develop during childhood, there’s actually an increase in brain volume during adolescence,
and then there's a mild increase peaking in our mid 20s and plateauing until mid 30s.
So in fact, according to this I should be at peak big-headedness given that
between my hair and my peak brain weight, my cephalus should be at its heaviest.
However, after age 35, the brain gradually decreases in volume, first slowly, and after
the age of 60 this rate increases, with up to 1% of brain volume decreasing per year.
So at the age of 75, the brain is actually 85% of its original peak volume. Certain factors
have been found to be correlated with higher rates of brain atrophy, including increased age,
certain genetic factors, a higher BMI, elevated alcohol intake and higher white matter
hyperintensities, which are lesions in the white matter of the brain that are visible on MRI scans.
That being said, neurodegenerative diseases such as Alzheimers, Parkinsons and Huntingtons amongst
others are generally quite devastating once they are onset, and generally, once 15-20% of brain
mass is lost, the dementia is often severe and irreversible. So no, according to this we can’t
just chip away at 90% of our brains and hope for it to function as it did once upon a time.
< Hemispherectomy >
But Inés, I hear you ask, what about people who undergo brain surgery?
Haven’t you heard of people who've had significant chunks of their brain extracted due to disease,
injury or tumours, with the extreme being a hemispherectomy - the complete removal of a
full hemisphere of the cerebrum, and who are thriving perfectly well afterwards?!
And this is true, there are people out there living with significant portions of their
brains missing. The cerebrum occupies around 88.2% of the brain’s total volume, so in theory
someone who has undergone a hemispherectomy in theory, is missing 44.1% of their brain.
That being said, hemispherectomies are usually carried out in young children with
either intractable epilepsy, or with serious disease such as Rasmussen’s encephalitis,
which causes the death of one of the hemispheres. As you know, the brain is much more plastic and
able to relocate key functional areas the earlier in development with appropriate rehabilitation,
so whilst some functions, generally motor functions and half of the visual field,
are lost during a hemispherectomy, the brain is able to rewire itself and grant the individual
with a far higher quality of life than otherwise. There are also cases of hemispherectomies carried
out in adults, which despite having more severe side effects such as hemiplegia,
which is the paralysis on one side of the body, they have still been able to
thrive and have demonstrated levels of brain neuroplasticity beyond surgeon’s expectations.
There is also an interesting case of a girl in Germany who was actually born without her
right hemisphere and she conserves the entirety of her field of vision, which is very unusual.
Generally people missing a brain hemisphere are also missing half of their visual field,
and that's because the optic nerve crosses to the opposite side of the brain. In the case of
this girl, the optic nerve from her left eye had actually rerouted back into the left hemisphere,
so the information from both eyes was being processed. And this is probably because the lack
of hemisphere development occurred when she was in the womb, and so it’s likely that the rewiring
happened whilst her brain was still developing and the neurons were able to migrate into the present
hemisphere that had then taken over all of the functions. Her condition was actually discovered
because she suffered from seizures as a young child, these were successfully treated, and she
apparently does present slight hemiparesis, which means a light weakness on one side of the body.
So, I wouldn’t necessarily take this as evidence that we *can* do without half of our brain, but it
does highlight the degree of brain plasticity that we have, especially in our early years.
< The woman with no cerebellum >
There are also several cases in the medical literature of people missing their cerebellum.
Whilst the cerebellum, which means small brain in Latin, only accounts for 10% of the brain’s
volume, it contains 50% of the brain’s neurons and is primarily responsible for fine motor controls.
If you’ve ever had a neurological test performed, you might have been asked to stretch out your
finger and touch your nose, and generally people who have suffered damage to the cerebellum
have trouble executing this movement smoothly. There aren’t many cases of people born without
a cerebellum, and generally people born without it experience severe motor and balance difficulties.
But, as always, there are exceptions. There is a case of a 58 year old, who presented
with moderate generalised headaches, and was found to be lacking the majority of the cerebellum,
but otherwise displayed no signs of cerebellum agenesis, suggesting either that the remnants
that were present or the cerebrum had taken on the cerebellum's function.
There was also a case of a 24 year old woman from China, who while she did have some motor problems,
she had unsteady gait and slurred speech, she was found to be completely lacking her cerebellum.
However, much like in the previous case, people born with congenital damage to the cerebellum
were much more likely to have better outcomes than people who developed damage through tumour
or injury later in life - but even then in most cases the consequences are generally severe,
which is what make these two cases, particularly remarkable.
< Anencephaly: The most extreme case >
The most extreme cases of missing brains are conditions such as anencephaly, which literally
means “nothing within the head”, but generally means lacking the cerebrum, and hydranencephaly,
which also presents with varying degrees of undeveloped brain and the cranial cavity is
filled with cerebrospinal fluid. In both cases, the baby usually passes away several days or weeks
after birth, although there are several remarkable cases where the child has lived several years.
Two weeks ago, the oldest person with anencephaly passed away at 8 years,
and the eldest person to live with hydranencephaly, who lacked cerebrum
and cerebellum, so therefore only had a functional brain stem - just slightly over 2% of the brain,
passed away at 12 years old. Whilst he did require intensive care, was blind and largely
unable to communicate, he did have an independent heartbeat and breathing, and reportedly displayed
some reaction to stimuli in his environment, which makes his case exceptionally remarkable.
Now, whilst so far we’ve explored the limits of how little physical brain one can have,
and we’ve established that roughly speaking functional deficits will begin to occur under 85%
of total brain volume, there are exceptional cases where lacking a cerebral hemisphere,
having extremely compressed white and gray matter, or lacking a cerebellum
have been possible without the expected severity of functional deficits surrounding it.
But what if we went in the opposite direction? We have, perhaps disingenuously,
assumed that possibly 10% of our brain is active at any one time, so what if
the neurons in our brain were active and firing synchronously all of the time? What would happen?
< Epilepsy, Excitation and Inhibition >
Well, what would happen is that you would have is a seizure.
Supposing you don’t have epilepsy, which is a lowered threshold for certain stimuli
to lead to seizures, chances are if all of your neurons decided to fire at once,
you wouldn’t notice anything happen at all. And this is because inhibitory processes in the brain
would likely kick in. Every neuron in the brain receives action potentials from other neurons. All
of these inputs are summated to determine whether the neuron will be likely to fire a signal onto
the downstream neurones or not, and if so, with what intensity. Excitatory potentials make it more
likely for a neuron to fire, whilst an inhibitory potential makes it less likely to fire. In this
episode I've perhaps made a flawed equivalence between “use” and a neuron firing, but the truth
is that inhibitory processes in the brain are just as important as the excitatory ones, and a
brain should have a healthy balance of excitatory and inhibitory processes, such that if a massive
synchronous activity is detected in the brain, it is quickly inhibited. Therefore, even if neurons
aren’t firing excitatory potentials all of the time, but rather receiving inhibitory potentials,
they are still in use and are still parsing electrical messages in a very significant way.
But, returning to epilepsy, there are actually many types of epilepsy and epileptic seizures.
I'm not going to go into detail of all of them here, but you might want to know
there’s a difference between the focal ones, which affect defined regions of the brain
and are inhibited before they extend to the entirety of the brain, and there are also
generalised seizures, of which there are two main types, the grand mal and the petit mal.
Most people when thinking about an epileptic seizure think about the grand mal seizure,
the one with the shaking. They are also called tonic-clonic seizures,
where a trigger causes all the cerebral cortex to depolarise synchronously,
leading to a loss of consciousness and a tonic contraction of all of the muscles of the body,
followed by intermittent contraction and relaxation, which is the clonic activity.
As long as the seizure doesn’t become status epilepticus, which is a continuous generalised
seizure, eventually inhibitory neurons are activated in the brain and the seizure stops.
However, there is another type of generalised seizure known as the petit mal,
or absence seizures. As the name suggests, someone experiencing an absence seizure displays a short
episode of blank staring, possibly with chewing or eyelid fluttering, but with no recollection
of the episode occurring at all. Now the petit mal seizure starts in a similar way,
with a huge synchronous depolarisation of cortical neurons, but displays a different EEG pattern
to grand mal seizures. After the depolarisation spike, there is a wave form of activity suggesting
hyperpolarisation, as post-excitatory inhibition kicks in extremely quickly.
This is what likely prevents any form of overt convulsions from developing.
Not only that, but petit mal seizures occur almost exclusively in childhood, and it's
actually thought that possibly the motor cortex of children are more resistant to excitatory stimuli,
as they rarely lose their posture and they rarely start convulsing when experiencing an
absence seizure. Most children with absence seizures will eventually grow out of them,
and that suggests that the neuronal maturation in adolescence allows the brain to get better at
inhibiting any excessive synchronous activity in the brain, but the patients who don’t grow
out of them often find that the seizures become upgraded to grand mal seizures, which suggests
that their motor cortex is actually no longer as protected from those excitatory signals.
So there you go, hopefully you weren’t inhibited from feeling excited about learning about the
brain, about epilepsy, and the limits to which we can use and not use our brain!
Now, if you’re disappointed by the fact there isn’t 90% of our brains waiting to be unlocked,
why not stimulate your physical brain and unlock the untapped potential of a DIGITAL BRAIN
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Anyway, that's it from me today, as always,
I want to thank my FANTASTIC patrons on patreon for supporting this channel,
and as always, thank you so much for watching me and I’ll see you in the next one! Bye!