The neurolinguistic toolboxJonathan R. Brennan

Introduction to Neurolinguistics, LSA2017 1

Psycholinguistics / Neurolinguistics

Happy Hour! ! !

Tuesdays 7/11, 7/18, 7/255:30 - 6:30 PM @ the Boone CenterIntroduction 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 consIntroduction to Neurolinguistics, LSA2017 3

Magnetic Resonance Imaging (MRI) records brain structure in three dimensions

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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”)

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Diffusion tensor imaging (DTI) uses MRI to map the

diffusion of water and estimate how brain

regions are connected

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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 consIntroduction 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

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The deficit/lesion method relies on clinical evaluation to determine language impairment, and connects

syndromes with location of brain damage.

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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

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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 ill-understood

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Lesion overlap identifies damage common across individuals with similar

syndromes

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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 consIntroduction to Neurolinguistics, LSA2017 19

The brain is bloodyand electricIntroduction 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

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Blood-oxygenation level dependent (BOLD) signal

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The hemodynamic response function (HRF) describes the BOLD signal's dynamics

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Measure functional connectivity by correlating time-courses across different regions

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Check out MRI and fMRI data yourself! Lots of free tools out there, like Mango!

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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 ! ! !

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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 consIntroduction 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

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Average EEG signals around some stimulus event to create the Event-related potential (ERP)

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Electrocortiography (ECoG) involves placing electrodes directly on (or into) the brain

» ! Invasive -- typically part of of neuro-surgery 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

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Magnetoencephalography (MEG) measures magnetic fields generated by electrical currents from thousands of

neurons and can locate these neural sources with good spatial resolution

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MEG sensor results (left) can be converted to source estimates (right)

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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”)

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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

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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 consIntroduction 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

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