This is a good question, and actually is not
something that is very well understood. The "why"
that you ask has a couple possible meanings here,
and so I'll try to address both. By your question,
you might mean "why is it useful to store
visual memory more than audio?". This question
is not easily answered by science because it can't
easily be tested. We can't really design an
experiment (yet, at least) that changes how
someone or an animal stores memory and then look
for an advantage or disadvantage compared to a
person who stores it "normally." So in that sense
of "why," the answer can't be approached yet. If
by "why" you mean "what physical process causes
this to happen," that is a question we can answer.
The short version is that there are two
different pathways to your brain that record
things differently. Nerves in your ears that
sense sound send one signal, and nerves in your
eyes that receive light send a separate signal.
These signals don't merge before being wired
into your brain. You can think of it like
wiring - it is actually very much similar.
I am not sure what you mean by stored twice,
but there are two possibilities. First, you might
mean that we have two eyes, and so we look at
things with both eyes and they send nearly
identical signals to the brain, because both eyes
are focused on it. On the other hand, each ear is
focused primarily on one side of hearing. This is
one aspect of this concept. Another might be that
when you see something, your brain receives a
pattern. When it is learned, that sometimes
involves a part of your brain (called the
hippocampus, it looks like a seahorse)
replaying the memory of it. But, this is also true
for things you have heard, though such a signal
seems to be less strong. These pathways are
currently under study in a lot of labs in the
world (including some I have worked in/with) and
the answers are not yet crystal clear. I
think they will be much more so in the next 20-30
years, which means that if this interests you,
it's definitely something that you can study in
the future and maybe discover some new things of
your own. There's a lot of opportunity for it.
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