How do our brains process speech? - Gareth Gaskell

The average 20-year-old knows between
27,000 and 52,000 different words.

By age 60, that number averages between
35,000 and 56,000.

Spoken out loud, most of these words last
less than a second.

So with every word, the brain has a quick
decision to make:

which of those thousands of options
matches the signal?

About 98% of the time, the brain chooses
the correct word.

But how? Speech comprehension is different
from reading comprehension,

but it’s similar to sign language
comprehension—

though spoken word recognition has
been studied more than sign language.

The key to our ability to understand
speech

is the brain’s role as a
parallel processor,

meaning that it can do multiple different
things at the same time.

Most theories assume that each word
we know is represented

by a separate processing unit that has
just one job:

to assess the likelihood of incoming
speech matching that particular word.

In the context of the brain, the
processing unit that represents a word

is likely a pattern of firing activity
across a group of neurons

in the brain’s cortex.

When we hear the beginning of a word,

several thousand such units
may become active,

because with just the beginning of a
word, there are many possible matches.

Then, as the word goes on, more and
more units register

that some vital piece of information
is missing and lose activity.

Possibly well before the end of the word,

just one firing pattern remains active,
corresponding to one word.

This is called the ‘recognition point.’

In the process of honing in on one word,

the active units suppress
the activity of others,

saving vital milliseconds.

Most people can comprehend up to
about 8 syllables per second.

Yet, the goal is not only
to recognize the word,

but also to access its stored meaning.

The brain accesses many possible meanings
at the same time,

before the word has been fully identified.

We know this from studies which show
that even upon hearing a word fragment––

like ‘cap’ ––

listeners will start to register multiple
possible meanings,

like captain or capital,
before the full word emerges.

This suggests that every time we hear a
word

there’s a brief explosion of meanings in
our minds,

and by the recognition point the brain
has settled on one interpretation.

The recognition process moves more
rapidly with a sentence

that gives us context than in a random
string of words.

Context also helps guide us towards the
intended meaning of words

with multiple interpretations, like ‘bat,’
or ‘crane,’

or in cases of homophones
like ‘no’ or ‘know.’

For multilingual people, the language
they are listening to is another cue,

used to eliminate potential words
that don’t match the language context.

So, what about adding completely new
words to this system?

Even as adults, we may come across a
new word every few days.

But if every word is represented as a
fine-tuned pattern of activity

distributed over many neurons,

how do we prevent new words from
overwriting old ones?

We think that to avoid this problem,

new words are initially stored in a part
of the brain called the hippocampus,

well away from the main store of words
in the cortex,

so they don’t share neurons
with others words.

Then, over multiple nights of sleep,

the new words gradually transfer over
and interweave with old ones.

Researchers think this gradual acquisition
process

helps avoid disrupting existing words.