Key language areas, and lesion deficits


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Key language areas, and lesion deficits

  • Key language areas, and lesion deficits

  • Lots of interactive brain areas

    • It’s not just a couple of areas on the left
  • Interpreting brain activation:

    • Who cares which bit of the brain lights up?
    • We want brain imaging to tell us about linguistic information processing, or linguistic representations


Some brain areas are specialised for language

  • Some brain areas are specialised for language

    • Broca's area: speech production
    • Wernicke's area: speech perception
    • On the left side of the brain (in 95% of people)
    • This is pretty much the only left-brain / right-brain saying that is actually true
  • What does "specialised for language" actually mean?

    • If you lose these areas, you lose language
    • When you use language, you use those areas
    • BUT: That does not mean that they only do language
    • E.g. Broca's area may be involved in music perception


Tan's brain: lesion (injury) in left frontal cortex

  • Tan's brain: lesion (injury) in left frontal cortex



Auditory cortex: all sounds pass into here

  • Auditory cortex: all sounds pass into here

    • Mostly specialised for low-level features, e.g. raw frequency
    • Bilateral (on both left and right sides of the brain)
  • Wernicke's area (Carl Wernicke, 1874)

    • Patient with very poor speech comprehension
    • Good speech production
    • Lesion on left side, just behind auditory cortex
    • Specialised for processing "higher level" sounds: speech
















And what does it not tell us?

  • And what does it not tell us?





Takes a 3D picture of the inside of body, completely non-invasively

    • Takes a 3D picture of the inside of body, completely non-invasively
    • One picture, just shows the structure


Shows brain activity (indirectly)

    • Shows brain activity (indirectly)
    • Takes a series of pictures over time, e.g. one every three seconds
    • The “f” in fMRI means functional, i.e. you get a movie of brain function, not a still image of brain structure


An MRI machine is just a big magnet (30,000 times stronger than Earth's magnetic field)

    • An MRI machine is just a big magnet (30,000 times stronger than Earth's magnetic field)
    • The only things it can measure are changes in the magnetic properties of things inside the magnet: in this case, your head
    • When neurons are active, they make electrical activity, which in turns creates tiny magnetic fields
    • BUT far too small for MRI to measure (100 million times smaller than Earth's magnetic field)
    • So, how can we measure neural activity with MRI?


Two lucky facts make fMRI possible

  • Two lucky facts make fMRI possible

    • When neurons in a brain area become active, extra oxygen-containing blood gets pumped to that area. Active cells need oxygen.
    • Oxygenated blood has different magnetic properties than de-oxygenated blood. Oxygenated blood gives a bigger MRI signal
  • End result: neurons fire => MRI signal goes up

  • This fMRI method is known as BOLD imaging: Blood-Oxygenation Level Dependent imaging. Invented in 1992.



Individual nerve cells (neurons) represent information

  • Individual nerve cells (neurons) represent information

    • Sensitive to “preferred stimuli”, e.g. /ba/
    • These stimuli make them active
    • Firing activity: send electrical spikes to other neurons


Information is distributed across large populations of neurons, and across brain areas

  • Information is distributed across large populations of neurons, and across brain areas

  • There's no “grandmother cell”: the one single cell that recognizes your grandmother

  • To really understand the brain, we'd need somehow to read the information from millions of individual neurons at once!



Aim:

  • Aim:

  • Typical design:

    • Present blocks, e.g. 30s of task, 30s of rest
    • Measure fMRI activity regularly every few seconds
    • Look for brain areas which are more active during the task periods, compared to rest periods




Colour represents statistical significance of how well the voxel's activation correlates with the task.

  • Colour represents statistical significance of how well the voxel's activation correlates with the task.

  • The hi-res grayscale anatomical picture underneath the coloured blobs is a completely different type of image, from a different type of scan. Shows the anatomy at the spot where the significant voxel's time-course was recorded.



Interpreting what brain activation means

  • Interpreting what brain activation means



Attention

  • Attention

  • Intention

  • Spatial reasoning

  • Numerical magnitude



“You love your iPhone, literally”

  • “You love your iPhone, literally”

  • http://www.nytimes.com/2011/10/01/opinion/you-love-your-iphone-literally.html

  • “But most striking of all was the flurry of activation in the insular cortex of the brain, which is associated with feelings of love and compassion. The subjects' brains responded to the sound of their phones as they would respond to the presence or proximity of a girlfriend, boyfriend or family member.

  • In short, the subjects didn't demonstrate the classic brain-based signs of addiction. Instead, they loved their iPhones.”



If you have Ebola, you will start off by having flu like symptoms

  • If you have Ebola, you will start off by having flu like symptoms

  • “I am having flu like symptoms”

  • “Oh no! I must have Ebola!”



Who cares which bit of the brain lights up?

    • Who cares which bit of the brain lights up?
    • We want brain imaging to tell us about linguistic information processing, or linguistic representations
    • Example study:
    • Does a person’s brain have well-structured representations for performing a given language task?










Predicted pattern-separability, if it matches perception:

  • Predicted pattern-separability, if it matches perception:

  • English: F3-difference > F2-difference

  • Japanese: F3-difference = F2-difference

  • fMRI pattern separability contrast:

    • F3-separability minus F2-separability
  • Is this neural difference greater for English than Japanese?

  • Key point: the classifier doesn't get told anything about people's behaviour, or about who is English or Japanese





The brain is astonishingly good at processing language

  • The brain is astonishingly good at processing language

    • Nobody understands how it achieves this
    • But we do have some exciting leads
  • Lots of brain areas, all representing multiple types of information, all communicating with each other

    • Not just Broca’s and Wernicke’s areas
    • Not just in the left hemisphere
  • Challenges for neuroscience

    • What information processing tricks does the brain use?
    • What representations does it use, how does it use them?



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