How do our brains process speech? - Gareth Gaskell

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The average 20-year-old knows between 27,000 and 52,000 different words. Spoken out loud, most of these words last less than a second. With every word, the brain has a quick decision to make: which of those thousands of options matches the signal? And about 98% of the time, the brain chooses the correct word. How is this possible? Gareth Gaskell digs into the complexities of speech comprehension.

Lesson by Gareth Gaskell, directed by Art Shot.

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.