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.

So in the daytime,

unconscious activity generates explosions
of meaning as we chat away.

At night, we rest, but our brains 
are busy integrating new knowledge

into the word network.

When we wake up, this process ensures 
that we’re ready

for the ever-changing world of language.