
How researchers discovered we have “two brains” [video] - hanniabu
https://www.bbc.com/reel/video/p072zr9l/the-curious-case-of-the-man-with-two-brains-
======
nwatson
The video doesn't portray the left-vs-right correctly. As the person and
screens are set up in the video, the left and right eye both experience the
left and right images presented on either side of the point-of-fixation.
However, the video portrays all information from left eye going to right
brain, and all information from right eye going to the left brain ... that's
not how it happens ...

... rather, the left-hemi-visual-field from both left- and right-eyes goes to
the right brain, and the right-hemi-visual-field from both eyes goes to the
left brain, as depicted in [1].

[1]
[http://fourier.eng.hmc.edu/e180/lectures/eye/node4.html](http://fourier.eng.hmc.edu/e180/lectures/eye/node4.html)

EDIT: grammar

~~~
xtagon
So it works the same whether you have one or both eyes open? That's intriguing

~~~
pygy_
Yes, and you lose a hemi-field in both eyes if you have a stroke in the visual
cortex on one side of the brain.

Central vision is preserved though, because the fovea projects to both
hemispheres.

~~~
vanderZwan
> _Yes, and you lose a hemi-field in both eyes if you have a stroke in the
> visual cortex on one side of the brain._

In my case the aura of my migraines (that I thankfully only had during
puberty) were limited to the left hemi-field. Which I guess suggests that the
migraine was "centered" in the right brain hemisphere.

~~~
pygy_
Correct!

~~~
vanderZwan
Do we know anything about migraines based on their symptoms? As in, has there
been any research where people with different migraine symptoms have been put
in a scanner during the migraine to locate which parts of the brain display
weird activity?

~~~
pygy_
The migraine aura is rather well understood, it is the result of cortical
spreading depression, a phenomenon where a "wave" of brain hyperactivity
spreads in as a ring on the surface of the cortex, leaving a hypo-active zone
in the middle.

It is possible to trigger the phenomenon by hurting the brain (e.g. sprinkle
acid on a rat's brain, WP has a neat video:
[https://en.wikipedia.org/wiki/Cortical_spreading_depression](https://en.wikipedia.org/wiki/Cortical_spreading_depression))
so we suppose that it is a defense mechanism in humans too, but we're not sure
what causes them.

I've done research on migraine, 10 years ago, but I haven't kept up with the
field. There are hypotheses, but no known cause for the headaches and
associated symptoms.

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corysama
Common human behavior is a lot more schizophrenic than any of us are
comfortable with understanding. I recommend the book "Hare Brain Tortoise
Mind" to everyone who will listen. It explains one form of multi-mindedness
that leads to you getting your best insights while in the shower --what it's
good at and what it's not.

"The Man Who Mistook His Wife For a Hat" is another classic.

~~~
devoply
schizophrenia in japan is called integration disorder. meaning the many
different parts of the mind, or many minds, are not integrating into a single
mind.

~~~
dmitryminkovsky
Reminds me of something from a really great piece about schizophrenia that I
read:

> In 2007, they announced a startling discovery. Stevens was trying to
> identify the proteins that recognized and eliminated neuronal synapses
> during visual development. “The strangest finding was that a protein that
> usually tags and removes pieces of dead cells, bacterial remnants, or
> cellular debris was also being reworked to tag and remove the synapses,” she
> said. Mice designed to lack tagging proteins—called complement proteins—had
> problems both in clearing cellular debris and in tagging and pruning their
> synapses.

> The Stevens and Barres study, published in the journal Cell in 2007,
> documented one of the most arresting instances of repurposing in biology: a
> protein designed to ticket germs and junk for destruction had been co-opted
> by the nervous system to ticket synapses for destruction. “It reinforces an
> old intuition,” my psychiatrist friend Hans, in Boston, told me. “The secret
> of learning is the systematic elimination of excess. We grow, mostly, by
> dying.”

[https://www.newyorker.com/magazine/2016/03/28/the-
genetics-o...](https://www.newyorker.com/magazine/2016/03/28/the-genetics-of-
schizophrenia)

------
sago
It seems rather intuitive to me.

Our brain is composed of neurons. Different subsets seem to have more or less
specialised functions. Any subset can be seen as it own 'thinker'. We think of
them as larger units based mostly on the amount of communication / how
independently they can function.

Sometimes it is helpful to think of our mind as been made of many smaller
'thinkers'. Though there is no crisp boundary, every neuron is its own
individual, at one extreme.

And I think it is sensible to speak of groupthink as a mind, with thinkers
contributed by lots of people. So at the other extreme all is one super mind.

I think it can be useful to think of 'god' that way (as in the will of god,
the desires of god, the thoughts of god), as much as a corporation, a culture,
a nation. Though I admit you have to take care not to get carried away with
the implications and end up mired in sloppy reasoning.

It is also not surprising that, because axon connections are many orders of
magnitude more efficient than language, the most useful grouping is at the
level of an individual.

~~~
ZeroFries
This works to some extent, but does not account for consciousness. A mind, at
the very least a moment of experience, is a coherent whole. Your left and
right visual fields are bound together. Thinking of neurons as entirely
discrete units makes the binding problem insoluble.

~~~
sago
> This works to some extent, but does not account for consciousness.

True, although consciousness itself is rather poorly defined. There are also
other related issues like qualia. But I'm not sure it needs to 'account' for
those things. To observe that there are certain mental phenomena that are
evident at different scales.

> A mind, at the very least a moment of experience, is a coherent whole.

That rather assumes your conclusion. In the example of a divided brain, a
stimuli may be presented to only half the brain. How would one know whether
one's experience of 'coherent whole'ness was accurate?

> Thinking of neurons as entirely discrete units makes the binding problem
> insoluble.

No more than thinking of base pairs in your DNA as discrete units makes higher
order phylogenesis insoluble. To recognise that discrete units are simple does
not mean that large aggregates of them cannot display fundamentally different
dynamics.

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plainOldText
Iain McGilchrist discusses at length the two-sided brain in his book _The
Master and His Emissary: The Divided Brain and the Making of the Western
World_ [1]

Fun fact: He wrote the book over a period of 20 years.

[1]
[https://www.amazon.com/gp/aw/d/0300245920](https://www.amazon.com/gp/aw/d/0300245920)

~~~
pkghost
I read the first half of this book about eight years ago and haven't stopped
noticing the same story in different places. It shows up in meditation
frameworks (Chuladasa's attention/awareness) to Jungian psychology
(anima/animus), in Jill Bolte Taylor's story "My Stroke of Insight", and many
more. Here's a list of my favorites:
[https://www.notion.so/cameronboehmer/de3eb3e0a9524b6989962c4...](https://www.notion.so/cameronboehmer/de3eb3e0a9524b6989962c46ae18082a?v=02b57b892bda44d48863bb0abcadf878)

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newnewpdro
I suspect we have a variable number of supplemental threads of execution
serving the primary one we live on.

Recalling distant memories asynchronously after struggling to remember
anything about it sometimes feels very much like another person interrupting
and delivering the information once it's found.

~~~
scarcely
But there's always an immediate trigger

~~~
newnewpdro
Sure, they're cooperating, for now :)

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ilogik
Relevant CGP Grey:
[https://www.youtube.com/watch?v=wfYbgdo8e-8](https://www.youtube.com/watch?v=wfYbgdo8e-8)

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ianwalter
See, even the brain is split into microservices (ducks).

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callesgg
We don't have two brains but if you split the brain it will become two brains.

This feels like people playing with words to grab attention.

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dmpanch
Idea of “decentralized” brain perfectly explained in Michael’s Gazzaniga book
“Who's in Charge? Free Will and the Science of the Brain”. He’s the first guy
who makes experiments on split brain and found decisions making center which
works like telling and explaining machine. Recommend.

~~~
HNLurker2
Reminds me of Willpower instinct book that has similar but with the hypothesis
that the prefrontal cortex is the "executive in charge of" type of brain.

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myprasanna
Does anyone know why the left vs right hand behaves the way it does at the end
of the video? Left as an exercise for us?

~~~
pkghost
The left hemisphere encodes tool use. Its preference for this is so strong
that even left-handed folks encode tool-use in the left hemisphere, which
means that the round trip from hand to brain goes left hand -> right
hemisphere (b/c each hemisphere controls the opposite side of the body) ->
left hemisphere (tool circuits) -> right hemisphere -> left hand, which is
waaay less efficient than right hand -> left hem -> right hand.
([https://www.ted.com/talks/iain_mcgilchrist_the_divided_brain](https://www.ted.com/talks/iain_mcgilchrist_the_divided_brain))

So, the left hand's failure to pantomime screwdriver use probably reflects the
commissurotomy patient's right hemisphere's inability to access her left
hemisphere's knowledge of tools. (That this only happens when eyes are closed
probably means the right hemisphere just observed and copied the right hand's
behavior when eyes were open.)

Had a fun chat with someone interested in this at a bar a few weeks ago who
offered a theory that the best pro basketball players are right hand/right eye
dominant or left hand/left eye dominant, and that anybody with mixed dominance
just can't cut it. Apparently the military just wont take you as a pilot or
sniper if you have mixed hand/eye dominance?

Calling it now: laterality is central to who we are, and is currently
underappreciated—especially in AI research (though I'd be delighted to hear
from AI folks who disagree!).

EDIT: lots

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JBiserkov
Relevant XKCD: [https://xkcd.com/2120/](https://xkcd.com/2120/)

