Transcript The Anatomy of Language Sydney Lamb Rice University, Houston

Ling 411 – 09
Brain Mapping
and
Functional Brain Imaging
Methods of localization
 Lesion studies
• The traditional method
• For a long time, the only method
 Intra-operative mapping
• Started by Penfield and Roberts , 1960’s
 Transcranial magnetic stimulation (TMS)
• Recently developed
• Very promising
 Functional brain imaging
• Currently very popular
• Many techniques
Intra-operative Mapping
Intra-operative brain mapping
 Performed on exposed neural tissue
• After craniotomy
• Used only in pathological conditions
 E.g., epilepsy
 Methods in use
• Electrical stimulation mapping
• Electrocorticograms
• Microelectrode recordings
Electrical stimulation mapping
(A type of intra-operative mapping)
 Early work by Penfield and Roberts
• Montreal
 More recently, George Ojemann –
neurosurgeon, U. of Washington
• Book: Conversations with Neil’s brain
•
Calvin, 1994)
Neil, a patient, suffers from epilepsy
 Currently, in Texas Medical Center
• Nitin Tandon, UT
(with W.
George Ojemann and Neil’s Brain
(electrical stimulation mapping)
 George Ojemann – neurosurgeon, U. of
Washington
• Book: Conversations with Neil’s brain
•
Calvin, 1994)
Neil, a patient, suffers from epilepsy
(with W.
 Intraoperative probing of part of Neil’s
brain
• In the area suspected of causing seizures
• Probing to spare vital linguistic functions
• Additional probing for research
Probing Neil’s brain (Ojemann)
 Aim: to localize functions
 Area activated – “size of pencil eraser”
• I.e., about 1 sq cm
• Number of neurons under 1 sq cm of cortical
surface: 14,000,000
 Test for “naming sites”
• Problem
“Naming sites” found in Neil’s brain
Probing Neil’s brain – “Naming sites”

Problems with “naming sites”
•
•

Naming is a complex function
 Therefore, not localizable
Ojemann doesn’t distinguish different kinds of
objects
Additional problem in interpreting results:
•
•
Input for testing is only pictures – visual
stimuli
Same problem comes up with results of many
imaging studies
“Naming sites” identified in the experiment
In Broca’s area
2. In Wernicke’s area
3. In supramarginal gyrus
1.
•
•
•
N.B.: Angular gyrus not considered
Was not under the section of skull
removed
Might also be involved (?)
“Naming sites” – English and Spanish
Transcranial Magnetic Stimulation - TMS
 Magnetic stimulation disrupts electrical
activity
 TMS disrupts activity only while it is being
applied
• Recovery is immediate
 Can induce temporary dysfunction of
specific areas – e.g. Broca’s area
 Usefulness depends greatly on areal
precision, a function of expense
Brain imaging and functional brain imaging
 Brain imaging
• Gets static image
• Used for example in locating lesion areas
• E.g. MRI
 Functional brain imaging
• Images of brain performing more or less
•
specific function
 E.g., linguistic, motor, sensory, attention
 That is the ideal, never actually realized
E.g. fMRI
Functional Brain Imaging Techniques
 Electroencephalography (EEG)
 Positron Emission Tomography (PET)
 Functional Magnetic Resonance Imaging
(fMRI)
 Magnetoencephalography (MEG)
• Magnetic source imaging (MSI)
 Combines MEG with MRI
Electroencephalography (EEG)
 An old technique, from the days before
mapping techniques were developed
• Was used for recording brain wave
activity, rather than for imaging
 Any neuronal activity in the brain
generates electric current flow
 Current flows through the cranium and
scalp
 The changes in electric potential are
detected by electrodes placed on the
scalp
EEG Mapping
 Nowadays multiple electrodes can be
placed all over the scalp, allowing the
recording of the electric activity from
many different sites simultaneously
 Allows the construction of topographic
maps of the momentary electric activity on
the scalp
 Also permits study of the time series of
these maps with millisecond resolution
• But very poor spatial resolution
Multiple electrodes for mapping
http://brainmapping.unige.ch/researchtopics.php
ERP Mapping
 ERP – event related potentials
 Traditional analysis: ERP waveforms at certain
electrode positions
 ERP mapping attempts to determine points in time
when map configurations change and/or when they
differ between experimental conditions
 Relies on the fact that, whenever the spatial
configuration of the electric field on the scalp
differs, different neuronal populations are active
in the brain, reflecting an alteration of the
functional state of the brain
Christoph M. Michel, Margitta Seeck and Theodor Landis,
Spatiotemporal Dynamics of Human Cognition
News Physiol. Sci 1999 Oct, 14:206-214
EEG-MRI Coregistration
 Separate MRI images are taken
 Reference points are used to get same
positioning
• Impossible to get them accurate
• But can get within a few mm
EEG-MRI Co-registration
•Spinelli L, Gonzalez Andino S, Lantz G, Seeck M, Michel CM.
Electromagnetic inverse solutions in anatomically constrained
spherical head models. Brain Topography 2000; 13: 115-126.
Some Properties of EEG-ERP Mapping
 Spatial resolution: Very approximate
• The volume currents picked up by the EEG
electrodes are distorted as they pass through
cranium and scalp [see next slides]
• Hence, imperfect correspondence between surface
distribution and primary activation
• 2nd problem: inverse dipole modeling
 With multiple dipoles, impossible to get a
unique solution
 Temporal resolution: Excellent
Detecting electrical activity
 Activation of neural fibers is electrical
activity
 Most fibers are too short to produce
detectable signal even when active
• Relatively longer fibers:
 Apical dendrites of pyramidal neurons
 Cortico-cortical axons
Dipoles
 The activity of a single fiber is too weak to
be detected
• Therefore we need multiple parallel fibers
acting in concert
 Sets of neighboring apical dendrites firing
synchrounously
 Such a set, when active, constitutes a dipole
Source and volume currents
Dipole
Papanicolaou 1999: 32
Volume Currents
 Volume currents (read by an EEG) become
distorted as they follow lines of least electrical
resistance
 Flow through layers of tissue offering different
degrees of resistance (e.g., white matter, gray
matter, meninges, cerebrospinal fluid)
 Become further distorted by the skull, which
provides the most resistance where it is thicker
Positron Emission Tomography (PET)
(1) tomography: pictures of slices
tomo- ‘slice’
graph “picture’
(2) produced by a technique based on
emission of positrons
Axial sections:
commonly used in brain imaging
• “From the
top/bottom”
• Accomplished
by use of
computerized
tomography
http://www.indiana.edu/~m555/axial/axial.html
PET Machine
http://www.radiologyinfo.org/content/petomography.htm
In a PET Machine
http://en.wikipedia.org/wiki/Positron_Emission_Tomography
Positron Emission Tomography (PET)
 Measures the distribution of particular organic
molecules and compounds (e.g., water, glucose,
neurotransmitters) in the brain
 The organic molecules and compounds are not
detectable because they do not emit
electromagnetic signals
 Positron-emitting isotopes of these organic
molecules and compounds are introduced into the
blood intravenously
 After a short time period, the isotopes are
dispersed throughout the brain
Positron Emission Tomography (PET)
 These isotopes, along with the blood, flow to the
areas of the brain with the highest metabolic
needs
 These areas are assumed to be the most active at
the given point in time
 The positrons in the isotopes collide with
electrons
 These collisions produce photons, which can be
detected at the surface of the head
 The greater the activation of an area, the more
positrons originate from that area
Positron Emission Tomography (PET)
 Tomography is accomplished by computer using
sophisticated algorithms
 The final PET images show areas of different
hues, each hue representing a different degree
of activation of the underlying brain structures
 The final PET images are superimposed on a
structural image of the brain (MRI or CT scan)
Some PET Images
PET images courtesy of UCLA Department of Molecular and Medical Pharmacology
© 1995-2005, Healthwise, Incorporated, P.O. Box 1989, Boise, ID 83701. All Rights Reserved.
More PET
Images
http://encarta.msn.com/media_461519549_761555359_-1_1/Positron_Emission_Tomography.html
Some properties of PET
 Spatial resolution: 5-10 mm
 How good is that?
• Under one sq mm of cortical surface
 130,000 neurons
 1400 minicolumns (at est. avg. 93 neurons/col)
 Temporal resolution: “…on the order of
minutes…” (A. Papanicolaou, Fundamentals of
Functional Brain Imaging (1998), p. 14)
PET study of object categories
Hanna Damasio, Thomas Grabowski, Daniel
Tranel, Richard Hichwa, Antonio Damasio, A
neural basis for lexical retrieval. Nature
380, 11 April 1996, 499-505
Different categories of concrete objects
found to be represented in different
extrasylvian areas of left hemisphere. Both
normal subjects and those with brain
damage were tested.
Categories tested
 Animals
 Tools
 Unique persons
• E.g., J.F.K.
Subjects, method, and findings
 127 subjects with focal brain lesions
• Category-related defects correlate with
different neural sites
 9 normal subjects, tested with PET
• Differential activation of left temporal sites
comparable to those of the lesion study
 Method: visual naming experiment
• Three categories: tools, animals, unique persons
Patients with defects in
more than one catetory
 If two categories had defects, they were
• Animals and tools
or
• Animals and unique persons
 If both tools and persons affected, then
animals were also
 Q: What do these findings suggest?
Deficits vis-à-vis areas of damage
 Abnormal access for names of unique
persons correlated with damage in left
temporal pole
 Abnormal access for names of animals
correlated with damage in left inferotemporal area
 Abnormal access for names of tools
correlated with damage in posterolateral
inferotemporal and temporo-occipitoparietal junction area
Similar results from PET
experiment on normal subjects
 Increased rCBF (regional cerebral blood
flow) in left temporal pole for naming
unique persons
 Some increase of rCBF also in right TP for
naming unique persons
 Animals and tools activated left posterior
inferotemporal areas, more posterior for
tools
Functional Magnetic Resonance Imaging
(fMRI)
 Measures the amount of oxygenated blood
supplied to different areas of the brain
 When a group of neurons increases its
signaling rate, its metabolic rate increases
 When the metabolic rate increases, the
amount of hemoglobin in the blood
decreases
Functional Magnetic Resonance Imaging
(fMRI)
 The decrease in hemoglobin becomes
apparent approximately 2 seconds after
the increase in the neurons’ signaling rate
 Then, oxygenated blood flows into the
depleted area, resulting in excessive
amounts of hemoglobin in the area
• This flood of oxygenated blood to the
depleted area occurs 5 to 8 seconds after
the low level of hemoglobin is detected
Functional Magnetic Resonance Imaging
(fMRI)
 The fMRI results are superimposed
on a structural MRI
MRI Machine
http://www.radiologyinfo.org/photocat/photos.cfm?Image=philips5.jpg&&subcategory=Brain
Another MRI Machine
http://www.radiologyinfo.org/photocat/photos.cfm?Image=hitachi.jpg&&subcategory=Brain
Functional Magnetic Resonance Imaging
(fMRI)
 Temporal resolution: not very specific
 Image reflects the increase in oxygenated
blood 5 to 8 seconds after the neurons fire
 Records all activation that occurs within
the recording interval; does not separate
early versus late activation
 For example, there is no way to separate
activation of, for example, primary
auditory cortex and higher-level
association cortices
fMRI: Example
http://www.fmrib.ox.ac.uk/fmri_intro/fusion.gif
Another example
Areas of the
brain used in
working memory
www.firstscience.com/ SITE/ARTICLES/love.asp
Functional Magnetic Resonance Imaging
(fMRI)
 Spatial resolution: good
 However, it is unclear whether
the imaged area is precisely the
area involved in the activity
• The flow of oxygenated blood into
the depleted area may also flow into
neighboring vessels in areas where
neural firing did not occur
Active area
Area that “lights up”
(hypothetical example)
REVIEW
Functional Brain Imaging Techniques
 Electroencephalography (EEG)
 Positron Emission Tomography (PET)
 Functional Magnetic Resonance Imaging
(fMRI)
 Magnetoencephalography (MEG)
• Magnetic source imaging (MSI)
 Combines MEG with MRI
end