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Language and the brain Rajeev Raizada Dept. of Brain & Cognitive Sciences [email protected] raizadalab.org Language and the brain: why bother with brain stuff in the first place? 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 Language areas in the brain 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 Broca's area: crucial for speech production Paul Broca (1861): patient "Tan" • Severe deficit in speech production: could only say "tan" • Good language comprehension Tan's brain: lesion (injury) in left frontal cortex Auditory cortex and Wernicke's area 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 Auditory cortex and Wernicke's area From http://www.physiology.wisc.edu/neuro524/ Language areas in the brain From University of Washington's Digital Anatomist project An example of Broca's aphasia in the news: Gabby Giffords http://www.youtube.com/watch?v=rx3nfUKvrZ8 Start at 2mins Wernicke's aphasia Deficit of comprehension, not production. Fluent speech, but lacking coherent meaning https://www.youtube.com/watch?v=aVhYN7NTIKU There's more to language in the brain than just Broca's and Wernicke's Friederici, A. D. (2012). The cortical language circuit: from auditory perception to sentence comprehension. Trends in cognitive sciences, 16(5), 262-268. The claim "language is on the left" is a total over-simplification Specht, K. (2013). Mapping a lateralization gradient within the ventral stream for auditory speech perception. Frontiers in human neuroscience, 7. The claim "language is on the left" is a total over-simplification Peelle, J. E. (2012). The hemispheric lateralization of speech processing depends on what "speech" is: a hierarchical perspective. Frontiers in human neuroscience, 6. What does brain imaging do, and what can it tell us? And what does it not tell us? Problem: This picture does not show any neural mechanisms MRI Magnetic Resonance Imaging • • Takes a 3D picture of the inside of body, completely non-invasively One picture, just shows the structure http://www.coppit.org/brain/ fMRI functional Magnetic Resonance Imaging • • • 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 http://www.fmrib.ox.ac.uk/image_gallery/av/ What are we actually measuring with fMRI? • • • • • 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? What makes fMRI possible: Don't measure neurons, measure blood 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. But neurons do the real work, not blood. Neurons represent and process 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 /ba/ /ba/-sensitive neuron Populations of neurons process information together 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! The basic design of an fMRI experiment Aim: • • Find which brain areas are active during a given task E.g. discriminating speech sounds, producing speech 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 Example time-courses Time-course of task versus rest periods Task Rest Task Rest Rest MRI signal from voxel that correlates well with task: Active Signal from voxel that does NOT correlate with task: Inactive TIME What are those little coloured blobs, actually? 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 timecourse was recorded. The key problem Interpreting what brain activation means Reverse inference Why it's hard to infer processing from activation: Brain areas are multi-functional ??? Attention Intention Spatial reasoning Numerical magnitude Parietal cortex A famously horrible example “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.” An example of reverse inference from everyday life 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!” 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 Example study: Does a person’s brain have well-structured representations for performing a given language task? Well-structured representations for doing a task, versus poorly-structured Sumo wrestlers Basketball Students Weight Weight Faculty players Height Height Wanted: • One single task, one set of stimuli • BUT: Some people can do task, other people cannot Different languages carve up acoustic space differently: /ra/ and /la/ in English and Japanese speakers Raizada et al., Cerebral Cortex (2010) Formants and perceptual discriminability Raizada et al., Cerebral Cortex (2010) Prediction: fMRI patterns for /ra/ and /la/ are more separable in the brains of English speakers than in Japanese speakers English speakers: Can perceive /r/-l/ distinction fMRI patterns are separable /ra/ /ra/ /ra/ /ra/ /ra/ /la/ /ra/ Japanese speakers: Cannot perceive /r/-l/ distinction fMRI patterns are not separable /ra/ /la/ /la/ /ra/ /ra/ /la/ /la/ /la/ /la/ /ra/ /la/ /ra/ /la/ /la/ /la/ /ra/ /ra/ /ra/ /la/ /la/ /la/ /ra/ /la/ /ra/ /la/ Will neural pattern separability match perceptual discriminability? 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 Individual differences: Neural pattern separability predicts behavioural performance Raizada et al., Cerebral Cortex (2010) Correlation after partialling out effects of group membership: r = 0.389, p < 0.05 Summary 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?