neuroanatomy of language 3 sept 20, 2013 – day 11

NEUROANATOMY OF LANGUAGE 3SEPT 20, 2013 – DAY 11

Brain & LanguageLING 4110-4890-5110-7960NSCI 4110-4891-6110Harry HowardTulane University

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Course organization• The syllabus, these slides and my recordings are

available at http://www.tulane.edu/~howard/LING4110/.• If you want to learn more about EEG and neurolinguistics,

you are welcome to participate in my lab. This is also a good way to get started on an honor's thesis.

• The grades are posted to Blackboard.

9/20/13 Brain & Language - Harry Howard - Tulane University

http://www.tulane.edu/~howard/LING4110/

REVIEWMacrostructure

9/20/13 Brain & Language - Harry Howard - Tulane University 3


Macrostructure review• Three axes of the brain

• Vertical = dorsal - ventral• Horizontal

• Longitudinal = anterior - posterior• Lateral = lateral - medial

• Connections• corpus callosum, arcuate fasciculus

• Naming conventions• Gyrii ~ sulcii• Brodmann's areas• Stereotaxic (“Talairach”) coordinates

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EEG, ERP & MEG


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Basic ideas• The electrical activity of neurons produces currents

spreading through the head.• These currents reach the surface of the scalp in the form

of voltage changes and magnetic fields, both of which can be measured non-invasively.

• Voltage changes measured at the scalp are called an electroencephologram (EEG).

• Magnetic fields measured at the scalp are called an magnetoencephologram (MEG).


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Electrical activity of neurons• Each neuron consists of a cell

body, short threadlike projections called dendrites, and one longer thread called an axon.

• Impulses travel within a neuron in the form of electrical signals, but between them as chemical signals.

• The electrical signal is changed to a chemical one at the synapse, and then changed back to an electrical one on the other side of the synapse.


pre-synaptic, axon

post-synaptic,dendrites


Electrical-chemical-electrical communication at the synapse

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Pre- and post-synaptic currents


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Layout of dendrites of pyramidal cells


apical dendrites

basaldendrites

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Axons vs. apical dendrites• Axons are oriented

randomly along the cortical sheet, which results in their potentials cancelling one other out.

• Apical dendrites are oriented in parallel along the cortical sheet, which results in their potentials to reinforce one another and sum together, creating a large “dipole”, which is measurable with EEG/MEG.


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Currents and fields• Primary or intracellular current (what we want to know about) does not summate across axons.

• Summation of parallel dendrites in cortical sheet creates:• Secondary or

extracellular or volume currents

• Magnetic field perpendicular to primary current


The paired positive and negative ‘ends’ of the volume current are known

as a dipole.

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Another take on currents and fields


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Currents, fields and a dipole


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Another dipole


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The basic fact about dipoles

A dipole has a direction … … which in cortex is perpendicular to its surface


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But, what do we know about the shape of the cortex?


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Summary• The electrical activity of neurons produces currents that

spread through the head.• These currents reach the surface of the scalp in the form

of voltage changes and magnetic fields, both of which can be measured non-invasively.

• Voltage changes measured at the scalp are called an electroencephologram (EEG).

• Magnetic fields measured at the scalp are called an magnetoencephologram (MEG).


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Scalp EEG• Scalp EEG is collected from tens to hundreds of electrodes positioned on different locations at the surface of the head.

• EEG signals (in the range of millivolts) are amplified and digitalized for later processing.

9/20/13

Magnetoencephalography (MEG) • … records magnetic fields produced by using arrays of SQUIDs (superconducting quantum interference devices).


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Comparison of EEG & MEGEEG MEG

Signal measured from electrical fields of secondary (volume) currents

magnetic fields generated by primary currents

Signal magnitude large (10 mV), easy to detect tiny (10 fT), difficult to detect

Dipole orientation sensitive to tangential and radial dipoles

sensitive only to tangential dipoles

Signal purity affected by skull, scalp, etc. unaffected by skull, scalp, etc.

Temporal resolution ~ 1 ms ~ 1 ms

Spatial resolution ~ 1 cm ~ 1 mm

Experimental flexibility allows some movement requires complete stillness

Cost cheap expensive


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An EEG

9/21/11

Brain & Language - Harry Howard - Tulane University 239/21/11

ERP• Event-related brain potentials (ERPs) are positive and

negative voltage fluctuations (or components) in the EEG that are time-locked to the onset of a sensory, motor, or cognitive event.

• ERPs reflect brain activity that is specifically related to some event, usually the onset of a stimulus.

• This activity cannot be directly observed in the EEG:• the EEG is a composite of simultaneously occurring brain

activity,• so it doesn't reflect just the activity associated with the event of

interest.• In other words, the "signal" (the brain response to some

event) is swamped by the "noise" (the brain activity that is unrelated to that event).


Signal averaging• The solution to this problem is to present not just one

instance of the event, but many instances. • Epochs of brain activity, each one time-locked to the onset

of the event, are then averaged together.• The "random" activity washes out during averaging,

whereas the brain activity of interest - namely, what is constant over presentations of the event - stays in the signal.

• Through this signal-averaging procedure, it is possible to isolate the brain response that is specifically elicited in response to some event of interest.


ERP procedure and componentsnamed by their polarity and peak latency (in ms)

NEXT TIMEP3Any leftovers from Ingram §3; go on to §4.

☞ Go over questions at end of chapter.


neuroanatomy of language 3 sept 20, 2013 – day 11

Documents

brain languageling

electrical communication

form of electrical signals

cortical sheet

chemical signals

postsynaptic currents

form of voltage changes

magnetic fields