How do our brains process speech? - Gareth Gaskell
-
0:07 - 0:14The average 20 year old knows
between 27,000 and 52,000 different words. -
0:14 - 0:20By age 60, that number averages
between 35,000 and 56,000. -
0:20 - 0:24Spoken out loud, most of these words
last less than a second. -
0:24 - 0:29So with every word, the brain
has a quick decision to make: -
0:29 - 0:32which of those thousands of options
matches the signal? -
0:32 - 0:36About 98% of the time, the brain chooses
the correct word. -
0:36 - 0:37But how?
-
0:37 - 0:41Speech comprehension is different
from reading comprehension, -
0:41 - 0:44but it’s similar to sign language
comprehension— -
0:44 - 0:49though spoken word recognition
has been studied more than sign language. -
0:49 - 0:51The key to our ability
to understand speech -
0:51 - 0:55is the brain’s role
as a parallel processor, -
0:55 - 0:59meaning that it can do multiple
different things at the same time. -
0:59 - 1:01Most theories assume
that each word we know -
1:01 - 1:06is represented by a separate processing
unit that has just one job: -
1:06 - 1:11to assess the likelihood of incoming
speech matching that particular word. -
1:11 - 1:15In the context of the brain,
the processing unit that represents a word -
1:15 - 1:20is likely a pattern of firing activity
across a group of neurons -
1:20 - 1:22in the brain’s cortex.
-
1:22 - 1:24When we hear the beginning of a word,
-
1:24 - 1:27several thousand such units
may become active, -
1:27 - 1:29because with just the beginning
of a word, -
1:29 - 1:32there are many possible matches.
-
1:32 - 1:36Then, as the word goes on,
more and more units register -
1:36 - 1:41that some vital piece of information
is missing and lose activity. -
1:41 - 1:43Possibly well before the end of the word,
-
1:43 - 1:48just one firing pattern remains active,
corresponding to one word. -
1:48 - 1:51This is called the "recognition point."
-
1:51 - 1:54In the process of honing in on one word,
-
1:54 - 1:57the active units suppress
the activity of others, -
1:57 - 1:59saving vital milliseconds.
-
1:59 - 2:04Most people can comprehend
up to about 8 syllables per second. -
2:04 - 2:07Yet, the goal is not only
to recognize the word, -
2:07 - 2:10but also to access its stored meaning.
-
2:10 - 2:14The brain accesses many possible meanings
at the same time, -
2:14 - 2:17before the word has been fully identified.
-
2:17 - 2:22We know this from studies which show
that even upon hearing a word fragment— -
2:22 - 2:23like "cap"—
-
2:23 - 2:27listeners will start to register
multiple possible meanings, -
2:27 - 2:32like captain or capital,
before the full word emerges. -
2:32 - 2:35This suggests that every time
we hear a word -
2:35 - 2:38there’s a brief explosion of meanings
in our minds, -
2:38 - 2:43and by the recognition point the brain
has settled on one interpretation. -
2:43 - 2:46The recognition process moves
more rapidly -
2:46 - 2:51with a sentence that gives us context
than in a random string of words. -
2:51 - 2:55Context also helps guide us towards
the intended meaning of words -
2:55 - 2:59with multiple interpretations,
like "bat," or "crane," -
2:59 - 3:03or in cases of homophones
like "no" or "know." -
3:03 - 3:07For multilingual people, the language
they are listening to is another cue, -
3:07 - 3:13used to eliminate potential words
that don’t match the language context. -
3:13 - 3:17So, what about adding completely
new words to this system? -
3:17 - 3:21Even as adults, we may come across
a new word every few days. -
3:21 - 3:25But if every word is represented
as a fine-tuned pattern of activity -
3:25 - 3:27distributed over many neurons,
-
3:27 - 3:32how do we prevent new words
from overwriting old ones? -
3:32 - 3:34We think that to avoid this problem,
-
3:34 - 3:39new words are initially stored in a part
of the brain called the hippocampus, -
3:39 - 3:43well away from the main store
of words in the cortex, -
3:43 - 3:46so they don’t share neurons
with others words. -
3:46 - 3:49Then, over multiple nights of sleep,
-
3:49 - 3:54the new words gradually transfer
over and interweave with old ones. -
3:54 - 3:58Researchers think this gradual
acquisition process -
3:58 - 4:01helps avoid disrupting existing words.
-
4:01 - 4:03So in the daytime,
-
4:03 - 4:07unconscious activity generates explosions
of meaning as we chat away. -
4:07 - 4:12At night, we rest, but our brains
are busy integrating new knowledge -
4:12 - 4:14into the word network.
-
4:14 - 4:18When we wake up, this process ensures
that we’re ready -
4:18 - 4:21for the ever-changing world of language.
- Title:
- How do our brains process speech? - Gareth Gaskell
- Speaker:
- Gareth Gaskell
- 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.
- Video Language:
- English
- Team:
- closed TED
- Project:
- TED-Ed
- Duration:
- 04:21
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