First,
let us consider the human ability to remember memories involving long
sequences. An average person shows this ability by being able to
remember the words and notes of many different songs. Each song is a
particular sequence that must be remembered. You might think to
yourself, “I don't know the words of many songs,” but if I were
to have you browse though a list of the 500 most famous songs in
history, you would find that you remembered the words and notes of
lots of them. Of course, on the stage you might see something like a
concert in which a singer sings from memory for 80 minutes. There are
opera singers who have memorized various different roles in the
operatic repertory, which may add up to many hours of words and notes
they have memorized. In his prime years as an opera singer, Placido
Domingo had at least 20 hours of opera roles he could sing upon
demand (and would sometimes fill in for a sick singer, singing one of
the many roles he knew by heart). Similarly, there are a number of
Muslims who have memorized the entire Quran, a book of some 114
chapters or surahs.
We
take this kind of sequential recall for granted, because this is the
way we have always experienced memory working. Similarly, if we were
able to exactly remember every word in each book we had read, we
would take that for granted, and think it nothing special. But we
must ask: does the brain have any type of structure that might allow
such sequential recall to occur, if memories were all stored in the
brain?
There
is no such thing in the brain. The brain consists of billions of
neurons, which are connected together. But there is a reason why the
brain does not seem to have the right type of arrangement to allow
for sequential recall of long sequences of information. The brain
consists of neurons, and each neuron is connected to many other
neurons. There is no “next” for a particular neuron. Neurons are
not arranged in any type of chain-like structure that might support a
recall of sequential memories.
Consider
the physical way in which a book allows for a sequencing of
information. Words are arranged in a linear order on a particular
page. Also, the page order and binding of a book imposes a sequence
on its contents, a sequential ordering we would not see if the pages
were in a jumbled heap.
Or
consider the DNA molecule. The DNA molecule is structured in a way
that allows for a sequential storage of information. A DNA molecule
is rather like an incredibly long and thin rope, in which a sequence
of letters is written on the rope. Within the genetic code used by
DNA, there is a particular sequence that acts as a “stop” signal,
rather like a period in a sentence. With this physical structure, DNA
allows for both a sequential storage of information and a demarcation
of information.
But
consider the human brain. There seem to be no physical
characteristics that might allow for sequential storage of
information. It seems that if you try to store information in the
brain, it should be like tossing a set of alphabet blocks onto a
giant heap in a junkyard.
I
could schematically depict a set of neurons with a visual like the
one below. The little circles represent individual neurons. The
diagram greatly understates the number of connections between
individual neurons.
Consider the recall
of sequential information. Imagine you are trying to recall a series
of words. We might imagine that individual parts of the sequence are
stored in individual neurons – perhaps something a little like the
schematic visual below.
But how could you
recall the sequence in its correct order? Nerve cells are scattered
throughout three dimensional space, with each neuron having many
connections to other neurons. If information is stored in nerve
cells, there would seem to be no way for a sequence to be stored in a
way that would allow a sequential recall involving a long series,
such as happens when an actor playing Hamlet recalls all of his many
lines in the correct order. We can't imagine the brain simply going
from one neuron to the “next” neuron to retrieve a sequence of
information. This is because neurons don't exist in chains in which a
particular neuron has a “next” neuron. Each neuron is connected
to many other neurons.
Below we see a map
of Dupont Circle in Washington D.C.
Once your car gets
on Dupont Circle, there is no “next” place to go. You've reached
an interchange in which there are 10 roads feeding out of the
circular interchange. Similarly, in the photo below we see neurons.
Each one of the parts coming out of the nerve cell is a path that can
be traversed from this nerve cell. Such an arrangement should not
offer any support for storing a sequence of information such as the
lines in a play or the notes in a song. There's no “next” route
leading from one neuron to the next neuron. Every neuron is like
Dupont Circle, except that there are even more paths leading out of
the neuron.
Now
let's consider another aspect of human sequential memory: the fact
that it is insertable. What this means is when we memorize a
sequence,
it is very easy to insert anywhere a new item in the sequence.
You
can demonstrate this by trying a test such as this. Most Americans
know the beginning of the song “America the Beautiful”
O
beautiful, for spacious skies
For amber
waves of grain
But
what if we try to recall this sequential data with an insertion? Try
memorizing the following variation, and see how long it takes to
recall it while looking away from this page:
O
beautiful, for spacious skies
For amber
waves of tasty grain
This
task is extremely easy. We have no problem inserting an item in the
middle of something we have memorized. Similarly, if you (like most
Americans) have memorized the famous line “we hold these truths to
be self-evident: that all men are created equal,” then it is very
easy to change that memory into something a little different, such
as: “we hold these truths to be self-evident: that all men are
created pretty much equal.”
So
evidently humans can remember sequences, and it is quite easy for us
to make an insertion anywhere in the sequence. But from a
neurological standpoint, such a thing should be impossible. For let
us imagine that there is some sequence of neurons that stores a
sequential memory, ignoring the difficulty that a neuron has no
“next” and so neurons do not seem to be suitable for storing a
sequence. Then let us imagine we are inserting an item somewhere in
the middle of this sequence. This should be as physically difficult
as inserting a new steel link in the middle of a steel chain (or in
the middle of a chain link fence). We cannot imagine that a new
neuron gets created (in the middle of the sequence) to store the new
word or words that are being inserted, because new neurons are not
created for each new item inserted into our memories.
There
is still one other problem in postulating that sequential memory is
stored in the brain. This is the “end of sequence” problem. Try
recalling the song “America the beautiful.” Typically your recall
will end nice and neatly with the line “From sea to shining sea.”
When we remember a sequence of items such as the words in a song, we
remember only up until the end point of the sequence. But how could
we possibly do that, if memories are stored in neurons? Given the
organization of neurons in the brain, storing a memory in neurons
would seem to be like tossing a bucket of letter blocks on to a
mountainous heap of blocks containing letters, words and image
fragments – not at all an arrangement that allows you to tell where
the end point of a particular sequential memory is to be found. Or if
we imagine instead an analogy of writing on a few vines in a thick
Amazon-type jungle of very densely packed vines, the “end of
sequence” problem still is there.
In
short, if long-term memory were stored in the brain, it seems we
shouldn't be able to remember long sequential memories, we should not
be able to remember where such sequences end, and we shouldn't be
able to easily insert new items anywhere in such a sequence – all
things we are actually very capable of doing.
The
points discussed here constitute another argument for believing that
our long-term memories are not actually stored in the brain, but are
stored in some greater mental reality that is merely related to the
brain.
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