The neurolinguistic toolbox Jonathan R. Brennan Introduction to Neurolinguistics, LSA2017 1
Psycholinguistics / Neurolinguistics Happy Hour!!! Tuesdays 7/11, 7/18, 7/25 5:30-6:30 PM @ the Boone Center Introduction to Neurolinguistics, LSA2017 2
! Agenda»! Imaging brain structure» Four ways to study brain function in humans 1. Correlating structural damage with language deficits 2. Mapping function in space via blood flow 3. Tracking function in time with electromagnetic dynamics 4. Brain stimulation» Pros and cons Introduction to Neurolinguistics, LSA2017 3
Magnetic Resonance Imaging (MRI) records brain structure in three dimensions Introduction to Neurolinguistics, LSA2017 4
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Dronkers et al. 2007 Brain 8
MRI Summary» Noninvasive 3D measurement of anatomical structure» Relies on different concentrations of water in different tissues» Some key terms used in papers» Resolution of the images (1x1x1mm voxels)» Strength of magnet ( 1.5 Tesla )» Sequence of magnetic pulses ( a fast spin echo series )» Kinds of relaxation signals measured by a sequence ( T1 weighted ) Introduction to Neurolinguistics, LSA2017 9
Diffusion tensor imaging (DTI) uses MRI to map the diffusion of water and estimate how brain regions are connected Introduction to Neurolinguistics, LSA2017 10
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! Agenda» Imaging brain structure» Four ways to study brain function in humans 1.! Correlating structural damage with language deficits 2. Mapping function in space via blood flow 3. Tracking function in time with electromagnetic dynamics 4. Brain stimulation» Pros and cons Introduction to Neurolinguistics, LSA2017 12
! Correlating Lesions with Deficits is proven but severely limited Imaging brain Structure and Connections can be done with various kinds of MRI Imaging brain function can be done via blood flow (PET, fmri) which is slow but focused or via electrical activity (EEG, MEG) which is fast but blurred Introduction to Neurolinguistics, LSA2017 13
The deficit/lesion method relies on clinical evaluation to determine language impairment, and connects syndromes with location of brain damage. Introduction to Neurolinguistics, LSA2017 14
Examples Broca s Aphasia: Speech production impaired. Associated with damage to inferior frontal gyrus Wernicke's Aphasia: Speech comprehension and semantic deficits. Associated with damage to posterior temporal lobe Anomia: Word-finding difficulty, association with various patterns of damage Conduction aphasia: Intact comprehension and fluent speech peppered by errors; difficulty repeating phrases. Associated with damage to the connecting tissue between posterior temporal and inferior frontal lobes ("arcuate fasciculus") Alexia: Impaired letter/word recognition with normal vision and spoken language. Associated with damage to the left inferior occipital lobe Introduction to Neurolinguistics, LSA2017 15
Challenges for deficit/lesion mapping! Can be used to reason about causality!! Research driven by available patient populations» Lesion identification in single subjects» Extent of damage» Not all damage equally visible in MRI» Generalizing across groups?» Symptom-based grouping... no two sets of symptoms alike» Lesion-based grouping... no two lesions alike» Plasticity and compensatory mechanisms illunderstood Introduction to Neurolinguistics, LSA2017 16
Lesion overlap identifies damage common across individuals with similar syndromes Introduction to Neurolinguistics, LSA2017 17
Voxel-based lesion symptom mapping (VLSM) Bates et al., 2003 Nat Neurosci 18
! Agenda» Imaging brain structure» Four ways to study brain function in humans 1. Correlating structural damage with language deficits 2.! Mapping function in space via blood flow 3. Tracking function in time with electromagnetic dynamics 4. Brain stimulation» Pros and cons Introduction to Neurolinguistics, LSA2017 19
The brain is bloody and electric Introduction to Neurolinguistics, LSA2017 20
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In the 1890s, Angelo Mosso reasons that thinking increases blood flow to the brain Click for more details 23
Functional MRI (fmri) tracks changes in blood oxygenation to indirectly measure the location of brain activity Introduction to Neurolinguistics, LSA2017 24
Blood-oxygenation level dependent (BOLD) signal Introduction to Neurolinguistics, LSA2017 25
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The hemodynamic response function (HRF) describes the BOLD signal's dynamics Introduction to Neurolinguistics, LSA2017 27
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Measure functional connectivity by correlating time-courses across different regions Introduction to Neurolinguistics, LSA2017 30
Check out MRI and fmri data yourself! Lots of free tools out there, like Mango! Introduction to Neurolinguistics, LSA2017 31
fmri Summary» Absolutely dominates cognitive neuroscience» Measures changes in blood oxygenation as a proxy for neuronal function» High spatial resolution! 1-2 mm» Low temporal resolution» Hemodynamic response is sluggish, 6-8 sec» Also, whole-brain images take ~2 sec to collect (the "repetition time", or TR)» Another con: fmri is loud!!! Introduction to Neurolinguistics, LSA2017 32
! Agenda» Imaging brain structure» Four ways to study brain function in humans 1. Correlating structural damage with language deficits 2. Mapping function in space via blood flow 3.! Tracking function in time with electromagnetic dynamics 4. Brain stimulation» Pros and cons Introduction to Neurolinguistics, LSA2017 33
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Electroencephalography (EEG) measures electrical potentials in real time that are generated by thousands of synchronized neurons with electrodes placed on the scalp Introduction to Neurolinguistics, LSA2017 36
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Average EEG signals around some stimulus event to create the Event-related potential (ERP) Introduction to Neurolinguistics, LSA2017 38
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Electrocortiography (ECoG) involves placing electrodes directly on (or into) the brain»! Invasive -- typically part of of neurosurgery preparation, e.g. to treat severe epilepsy»! Spatial coverage limited and constrained by medical demands» " Record field potentials from dendrites of neural within immediate vicinity (< 1 mm)» " High spatial and temporal resolution» " Sensitive to fainter high-frequency (>80 Hz) signals Introduction to Neurolinguistics, LSA2017 42
Magnetoencephalography (MEG) measures magnetic fields generated by electrical currents from thousands of neurons and can locate these neural sources with good spatial resolution Introduction to Neurolinguistics, LSA2017 43
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MEG sensor results (left) can be converted to source estimates (right) Introduction to Neurolinguistics, LSA2017 48
Electromagnetic brain dynamics summary» EEG measures voltages at the scalp with! high temporal resolution but " low spatial resolution» ECoG measures voltages directly on the cortex with! high spatial and temporal resolution but is very " invasive and only approprite with certain populations» MEG meausures magnetic fields at the scalp with! high temporal resolution and can be used to reconstruct source locations with good spatial resolution» Look out for these sorts of terms in your readings:» What were the epochs of data? ( -.1 to 1 s from stimulus onset )» How were signals filtered? ( 1-40 Hz band-pass filter )» How were artifacts rejected? ( Removed blinks artifacts with visual inspection )» What algorith was used to estimate the current source ( an equivalent current dipole or a minimum norm distributed source model ) Introduction to Neurolinguistics, LSA2017 49
Electromagnetic stimulation can be used to test causal links between regional brain activation and cognition» Direct Cortical Stimulation (DCS) invasive ; indicated before certain medical procedures» Transcranial Magnetic Stimulation (TMS): Non-invasive cortical stimulation.» Strong magnetic field induces current which stimulates action potential along axons» Single pulse evokes inhibitory response: Virtual Lesion» < 5 ms repetition is excitatory» Some deficit/lesion limitations apply, e.g. behavioral consequences may be due to cascade from targeted region Introduction to Neurolinguistics, LSA2017 50
! Agenda» Imaging brain structure» Four ways to study brain function in humans 1. Correlating structural damage with language deficits 2. Mapping function in space via blood flow 3. Tracking function in time with electromagnetic dynamics 4. Brain stimulation»! Pros and cons Introduction to Neurolinguistics, LSA2017 51
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Cheat-sheet» Structural MRI images different tissue types in 3D with high resolution. The DTI variant measures white-matter connectivity.» fmri images changes in metabolic activity, measuring activation with! high precision in space, but " low precision in time» EEG records changes in electrical activity at the scalp, measuring activation with " low precision in space, but! high precision in time» ECoG invasively records electrical activity at the cortex with! high spatial and temporal resolution in " special populations» MEG records changes in magnetic activity at the scalp, measuring activation with! high precision in time, and " moderate precision in space» TMS Inhibits or excites action potentials with high spatial and temporal resolution to test! causal hypotheses Introduction to Neurolinguistics, LSA2017 54