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Title:
How do our brains process speech? - Gareth Gaskell
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Description:
View full lesson: https://ed.ted.com/lessons/how-do-our-brains-process-speech-gareth-gaskell
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.
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Speaker:
Gareth Gaskell
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The average 20 year old knows
between 27,000 and 52,000 different words.
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By age 60, that number averages
between 35,000 and 56,000.
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Spoken out loud, most of these words
last less than a second.
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So with every word, the brain
has a quick decision to make:
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which of those thousands of options
matches the signal?
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About 98% of the time, the brain chooses
the correct word.
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Speech comprehension is different
from reading comprehension,
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but it’s similar to sign language
comprehension—
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though spoken word recognition
has been studied more than sign language.
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The key to our ability
to understand speech
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is the brain’s role
as a parallel processor,
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meaning that it can do multiple
different things at the same time.
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Most theories assume
that each word we know
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is represented by a separate processing
unit that has just one job:
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to assess the likelihood of incoming
speech matching that particular word.
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In the context of the brain,
the processing unit that represents a word
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is likely a pattern of firing activity
across a group of neurons
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in the brain’s cortex.
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When we hear the beginning of a word,
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several thousand such units
may become active,
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because with just the beginning
of a word,
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there are many possible matches.
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Then, as the word goes on,
more and more units register
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that some vital piece of information
is missing and lose activity.
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Possibly well before the end of the word,
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just one firing pattern remains active,
corresponding to one word.
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This is called the "recognition point."
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In the process of honing in on one word,
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the active units suppress
the activity of others,
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saving vital milliseconds.
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Most people can comprehend
up to about 8 syllables per second.
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Yet, the goal is not only
to recognize the word,
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but also to access its stored meaning.
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The brain accesses many possible meanings
at the same time,
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before the word has been fully identified.
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We know this from studies which show
that even upon hearing a word fragment—
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like "cap"—
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listeners will start to register
multiple possible meanings,
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like captain or capital,
before the full word emerges.
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This suggests that every time
we hear a word
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there’s a brief explosion of meanings
in our minds,
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and by the recognition point the brain
has settled on one interpretation.
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The recognition process moves
more rapidly
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with a sentence that gives us context
than in a random string of words.
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Context also helps guide us towards
the intended meaning of words
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with multiple interpretations,
like "bat," or "crane,"
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or in cases of homophones
like "no" or "know."
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For multilingual people, the language
they are listening to is another cue,
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used to eliminate potential words
that don’t match the language context.
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So, what about adding completely
new words to this system?
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Even as adults, we may come across
a new word every few days.
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But if every word is represented
as a fine-tuned pattern of activity
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distributed over many neurons,
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how do we prevent new words
from overwriting old ones?
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We think that to avoid this problem,
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new words are initially stored in a part
of the brain called the hippocampus,
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well away from the main store
of words in the cortex,
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so they don’t share neurons
with others words.
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Then, over multiple nights of sleep,
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the new words gradually transfer
over and interweave with old ones.
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Researchers think this gradual
acquisition process
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helps avoid disrupting existing words.
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unconscious activity generates explosions
of meaning as we chat away.
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At night, we rest, but our brains
are busy integrating new knowledge
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into the word network.
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When we wake up, this process ensures
that we’re ready
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for the ever-changing world of language.