1. Cortex Sept 22, 2017 – DAY 11 Brain & Language
LING
NSCI
Harry Howard
Tulane University

2. Brain & Language - Harry Howard - Tulane University
22-sep-2017
Brain & Language - Harry Howard - Tulane University
Course organization
Fun with
I am still working on grading.

3. Brain & Language - Harry Howard - Tulane University
22-sep-2017
Brain & Language - Harry Howard - Tulane University
Review

4. Axons vs. apical dendrites (in cortex)
22-sep-2017
Brain & Language - Harry Howard - Tulane University
Axons vs. apical dendrites (in cortex)
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.

5. Tonotopy in the ascending auditory pathway
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Brain & Language - Harry Howard - Tulane University
Tonotopy in the ascending auditory pathway
Hayhoe07-CentralAuditoryPath.pdf

6. Brain & Language - Harry Howard - Tulane University
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Brain & Language - Harry Howard - Tulane University
Winer05-CentralAudPathway.pdf

7. Brain & Language - Harry Howard - Tulane University
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Brain & Language - Harry Howard - Tulane University
Experimental animals

8. Brain stem frequency-following response
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Brain & Language - Harry Howard - Tulane University
Brain stem frequency-following response
The human brain stem frequency-following response (FFR) registers phase-locked neural activity to cyclical auditory stimuli.
Galbraith et al., (1995)
We show that the FFR can be elicited by word stimuli, and when speech-evoked FFTs are reproduced as auditory stimuli they are heard as intelligible speech
Galbraith, Gary C., Paul W. Arbagey et al. “Intelligible Speech Encoded in the Human Brain Stem Frequency-Following Response.” NeuroReport 6.17 (1995)

9. Brain & Language - Harry Howard - Tulane University
22-sep-2017
Brain & Language - Harry Howard - Tulane University
Timing, pitch, and timbre in the stimulus (black) and brain stem response (gray) waveforms
Kraus, Nina, Erika Skoe et al. “Experience-Induced Malleability in Neural Encoding of Pitch, Timbre, and Timing.” Annals of the New York Academy of Sciences (2009):

10. Brain & Language - Harry Howard - Tulane University
22-sep-2017
Brain & Language - Harry Howard - Tulane University
subcortical Audition

11. Can experience have any effect?
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Brain & Language - Harry Howard - Tulane University
Can experience have any effect?
Grand average brain stem responses to the speech syllable “da” for both musician (red) and non-musician (black) groups in the audiovisual condition.
Top: Amplitude differences between the groups are evident over the entire response waveform.
Middle: Musicians exhibit faster (i.e., earlier) onset responses.
Bottom. Fourier analysis shows musicians to have more robust amplitudes of the F0 peak (100 Hz) and the peaks corresponding to the harmonics (200, 300, 400, 500 Hz) (left).
Kraus, Nina, Erika Skoe et al. “Experience-Induced Malleability in Neural Encoding of Pitch, Timbre, and Timing.” Annals of the New York Academy of Sciences (2009):

12. Brain & Language - Harry Howard - Tulane University
22-sep-2017
Brain & Language - Harry Howard - Tulane University
Pitch tracking
The thin black line represents the pitch contour of the stimulus (Mandarin tone 3), and the thick gray line represents the extracted pitch trajectory of the brain stem response. The musician's brain response follows the pitch of the stimulus more precisely, a phenomenon known as pitch tracking.
Kraus, Nina, Erika Skoe et al. “Experience-Induced Malleability in Neural Encoding of Pitch, Timbre, and Timing.” Annals of the New York Academy of Sciences (2009):

13. One hour of phonetic discrimination training vs passive listening
22-sep-2017
Brain & Language - Harry Howard - Tulane University
One hour of phonetic discrimination training vs passive listening
Grand-average FFR power spectra (i.e., subtracted polarities) at T0 (pre, red line) and T1 (post, blue line) of the trained group (upper part) and passive-listening (lower part).
f0 = fundamental frequency,
f1–f6 = harmonics.
Elmer, Stefan, Marcela Hausheer et al. “Human Brainstem Exhibits Higher Sensitivity and Specificity Than Auditory-Related Cortex to Short-Term Phonetic Discrimination Learning.” Scientific Reports 7 (2017): 7455.

14. The descending pathway
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Brain & Language - Harry Howard - Tulane University
The descending pathway
thalamus
McLachlan10
midbrain

15. Brain & Language - Harry Howard - Tulane University
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Brain & Language - Harry Howard - Tulane University
Some conclusions
Cortex is generally held to be the seat of learning ~ neural plasticity.
The fact that subcortical nuclei can alter their response -- presumably under the direction of auditory cortex -- shows that Realism stops there and Constructivism takes over.
We still don't know where auditory objects are constructed, though.

16. Brain & Language - Harry Howard - Tulane University
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Brain & Language - Harry Howard - Tulane University
cortex

17. Vertical organization: lamination
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Brain & Language - Harry Howard - Tulane University
Vertical organization: lamination

18. Functional Histology of the Cerebral Cortex
Neocortex has 6 layers designated I, II, III, IV, V, VI
Pyramidal cells predominate in layers III and V
Granule cells in layers II and IV
neuro_chap15.ppt
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Brain & Language - Harry Howard - Tulane University

19. Brain & Language - Harry Howard - Tulane University
Types of Cortex
Cytoarchitecture varies in different areas
Number and size of cells
Thickness of layers
neuro_chap15.ppt
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Brain & Language - Harry Howard - Tulane University

20. Brain & Language - Harry Howard - Tulane University
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Brain & Language - Harry Howard - Tulane University
Brodmann's areas

21. Brodmann's areas, functions
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Brain & Language - Harry Howard - Tulane University
Brodmann's areas, functions

22. Horizontal organization: minicolumn
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Brain & Language - Harry Howard - Tulane University
Horizontal organization: minicolumn

23. Brain & Language - Harry Howard - Tulane University
22-sep-2017
Brain & Language - Harry Howard - Tulane University
EEG & MEG

24. Brain & Language - Harry Howard - Tulane University
22-sep-2017
Brain & Language - Harry Howard - Tulane University
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.

25. Magnetoencephalography (MEG)
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Brain & Language - Harry Howard - Tulane University
Magnetoencephalography (MEG)
… records magnetic fields produced by using arrays of SQUIDs (superconducting quantum interference devices).

26. Brain & Language - Harry Howard - Tulane University
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Brain & Language - Harry Howard - Tulane University
An EEG

27. Electrical-chemical-electrical communication at the synapse
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Brain & Language - Harry Howard - Tulane University
Electrical-chemical-electrical communication at the synapse

28. Pre- and post-synaptic currents
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Brain & Language - Harry Howard - Tulane University
Pre- and post-synaptic currents

29. 22-sep-2017
Brain & Language - Harry Howard - Tulane University
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.
Magnetic fields are also created by volume currents, but approximately cancel and sum to zero. Assuming a spherical head, they would precisely sum to zero.

30. Currents, fields and a dipole
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Brain & Language - Harry Howard - Tulane University
Currents, fields and a dipole
Kahkonen-Fig_1a.gif

31. Another take on currents and fields
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Brain & Language - Harry Howard - Tulane University
Another take on currents and fields

32. Axons vs. apical dendrites
22-sep-2017
Brain & Language - Harry Howard - Tulane University
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.

33. The basic fact about dipoles
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Brain & Language - Harry Howard - Tulane University
The basic fact about dipoles
A dipole has a direction …
… which in cortex is perpendicular to its surface

34. But, what do we know about the shape of the cortex?
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Brain & Language - Harry Howard - Tulane University
But, what do we know about the shape of the cortex?

35. Brain & Language - Harry Howard - Tulane University
22-sep-2017
Brain & Language - Harry Howard - Tulane University
Another dipole

36. Brain & Language - Harry Howard - Tulane University
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Brain & Language - Harry Howard - Tulane University
Inverse problems
Ideally, one would like to localize the precise neural sources that generate ERPs.
This is an example of an “inverse problem”, because it tries to deduce the cause of an observation from the observation itself:
“An inverse problem is a general framework that is used to convert observed measurements into information about a physical object or system that we are interested in.”

37. How to calculate the source of a dipole
22-sep-2017
Brain & Language - Harry Howard - Tulane University
How to calculate the source of a dipole + - + -
How do we know which one is correct?
We can’t. There is no correct answer.
Dipole source localization is an ill-defined problem.
That is to say, the inverse solution to the dipole source-localization problem is impossible to compute with certainty, because any given scalp distribution could, in principle, be generated by any number of source configurations within the brain.

38. Brain & Language - Harry Howard - Tulane University
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Brain & Language - Harry Howard - Tulane University
But …
… researchers have developed powerful tools that provide good estimates of dipole localization, given some reasonable assumptions.

39. Brain & Language - Harry Howard - Tulane University
22-sep-2017
Brain & Language - Harry Howard - Tulane University
LORETA
One such method is known as LORETA (low resolution brain electromagnetic tomography), which provides an estimate of the current distribution throughout the entire 3-dimensional space within the brain.
It does so by taking into account what is known about the structure of the brain and skull.
An example of a LORETA solution, mapped onto a normalized brain space, is provided above.

40. Brain & Language - Harry Howard - Tulane University
22-sep-2017
Brain & Language - Harry Howard - Tulane University
Comparison of EEG & MEG
EEG
MEG
Signal measured from
electrical fields generated by 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
Spatial resolution
~ 1 cm
~ 1 mm
Experimental flexibility
allows some movement
requires complete stillness
Cost
cheap
expensive

41. Brain & Language - Harry Howard - Tulane University
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Brain & Language - Harry Howard - Tulane University
NEXT TIME P3
Auditory cortex