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← How do our brains process speech? - Gareth Gaskell

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Showing Revision 3 created 07/13/2020 by lauren mcalpine .

  1. The average 20 year old knows
    between 27,000 and 52,000 different words.
  2. By age 60, that number averages
    between 35,000 and 56,000.
  3. Spoken out loud, most of these words
    last less than a second.
  4. So with every word, the brain
    has a quick decision to make:
  5. which of those thousands of options
    matches the signal?
  6. About 98% of the time, the brain chooses
    the correct word.
  7. But how?

  8. Speech comprehension is different
    from reading comprehension,
  9. but it’s similar to sign language
  10. though spoken word recognition
    has been studied more than sign language.
  11. The key to our ability
    to understand speech
  12. is the brain’s role
    as a parallel processor,
  13. meaning that it can do multiple
    different things at the same time.
  14. Most theories assume
    that each word we know
  15. is represented by a separate processing
    unit that has just one job:
  16. to assess the likelihood of incoming
    speech matching that particular word.
  17. In the context of the brain,
    the processing unit that represents a word

  18. is likely a pattern of firing activity
    across a group of neurons
  19. in the brain’s cortex.
  20. When we hear the beginning of a word,
  21. several thousand such units
    may become active,
  22. because with just the beginning
    of a word,
  23. there are many possible matches.
  24. Then, as the word goes on,
    more and more units register
  25. that some vital piece of information
    is missing and lose activity.
  26. Possibly well before the end of the word,
  27. just one firing pattern remains active,
    corresponding to one word.
  28. This is called the "recognition point."
  29. In the process of honing in on one word,
  30. the active units suppress
    the activity of others,
  31. saving vital milliseconds.
  32. Most people can comprehend
    up to about 8 syllables per second.
  33. Yet, the goal is not only
    to recognize the word,

  34. but also to access its stored meaning.
  35. The brain accesses many possible meanings
    at the same time,
  36. before the word has been fully identified.
  37. We know this from studies which show
    that even upon hearing a word fragment—
  38. like "cap"—
  39. listeners will start to register
    multiple possible meanings,
  40. like captain or capital,
    before the full word emerges.
  41. This suggests that every time
    we hear a word

  42. there’s a brief explosion of meanings
    in our minds,
  43. and by the recognition point the brain
    has settled on one interpretation.
  44. The recognition process moves
    more rapidly
  45. with a sentence that gives us context
    than in a random string of words.
  46. Context also helps guide us towards
    the intended meaning of words
  47. with multiple interpretations,
    like "bat," or "crane,"
  48. or in cases of homophones
    like "no" or "know."
  49. For multilingual people, the language
    they are listening to is another cue,
  50. used to eliminate potential words
    that don’t match the language context.
  51. So, what about adding completely
    new words to this system?

  52. Even as adults, we may come across
    a new word every few days.
  53. But if every word is represented
    as a fine-tuned pattern of activity
  54. distributed over many neurons,
  55. how do we prevent new words
    from overwriting old ones?
  56. We think that to avoid this problem,
  57. new words are initially stored in a part
    of the brain called the hippocampus,
  58. well away from the main store
    of words in the cortex,
  59. so they don’t share neurons
    with others words.
  60. Then, over multiple nights of sleep,

  61. the new words gradually transfer
    over and interweave with old ones.
  62. Researchers think this gradual
    acquisition process
  63. helps avoid disrupting existing words.
  64. So in the daytime,

  65. unconscious activity generates explosions
    of meaning as we chat away.
  66. At night, we rest, but our brains
    are busy integrating new knowledge
  67. into the word network.
  68. When we wake up, this process ensures
    that we’re ready
  69. for the ever-changing world of language.