Transcript Brain Imaging & the Mirror Neuron System Lisa Aziz-Zadeh 1

Brain Imaging &
the Mirror Neuron System
Lisa Aziz-Zadeh
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Brain Imaging Methodologies
• Fuctional Magnetic Resonance Imaging
(fMRI)
• Transcranial Magnetic Stimulation (TMS)
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MRI and fMRI
• MRI: Images of brain structure.
• fMRI: Images of brain function.
• Tissues differ in magnetic
susceptibility (grey matter, white
matter, cerebrospinal fluid)
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Physiological basis for Blood
Oxygen Level Dependent
(BOLD) fMRI
• Neural activity leads to increased blood flow.
• Increased flow exceeds increased oxygen
extraction, resulting in decreased deoxyhemoglobin content.
• Deoxyhemoglobin is paramagnetic, so reducing
the deoxy-Hb/oxy-Hb ratio increases the signal.
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FMRI signal and neural activity
• Recently it has
been shown that
the BOLD signal
reliably reflects
neural activity
(Logothetis et al., 2001).
• BOLD signal was correlated
with both local field potentials
(reflecting input) and multiunit activity (reflecting output)
• Note the lag of the BOLD
response relative to the
neural activity.
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Acquiring functional images
• Low resolution
• Rapid sequence
• Cognitive
manipulation
Signal
• Statistics
Time
Condition
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Statistical parameter maps
• Every voxel has associated statistics.
• SPMs are superimposed on anatomical
images, thresholded and clustered.
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http://www.simplyphysics.com/
MRIntro.html
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Transcranial Magnetic
Stimulation (TMS) as a Brain
Mapping Tool
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How TMS works
• Pass a current through a hand held coil, whose
shape determines the properties and the size
of the field
• The coil is driven by a machine which switches
the large current necessary in a very precise
and controlled way
• The coil is held on the scalp and the magnetic
field passes through the skull and into the
brain
• Small induced currents can then make brain
areas below the coil more or less active,
depending on the parameters used.
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Safety Issues
• Generally thought to be free from
harmful effects
• Examination of brain tissue submitted
to thousands of TMS pulses has shown
no detectable structural changes
• It is possible in unusual circumstances
to trigger a seizure in normal patients,
but using the proper guidelines
eliminate this risk
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Different Types of TMS
• Single Pulse
– In the motor cortex, this usually causes a
excitation in the brain.
• rTMS
– Multiple pulses in a short interval
– Usually causes a “temporary lesion”
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TMS as a Treatment
Technique
• Epilepsy
– May be able to lower the number of
seizures a patient
• Depression
– TMS treatments have been shown to cause
improvement in severe cases of depression
– An alternative to ECT
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TMS as a tool in Research:
Understanding the Motor
System
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TMS to understand the motor
system
• Understanding the motor system
– Participants watch different things that we think may
activate the motor system
– If these stimuli do activate the motor system, the
participant’s muscles are just beneath the threshold
of movement
– We record the muscle activity when we give TMS
over the motor cortex
– If the stimulus had an effect, then we see bigger
muscle twitches than if the stimulus had no effect
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TMS
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Using TMS with fMRI
• fMRI
-uses amount of blood flow used by the brain
to determine which areas are the most
active (more active areas use more oxygen)
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fMRI
• Problem:
– are the areas shown to be used in an fMRI
image ESSENTIAL to the brain function, or
are do they activate peripherally?
• ROLE OF TMS:
– Using the “inhibitory lesion technique” we
can turn off the specific brain area and see
if it is ESSENTIAL for the task. If the
person can not perform the task during
rTMS, it is essential.
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The Mirror Neuron System
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Mirror Neurons: Area F5
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Cortical Mechanism
for
Action Recognition
provides an early
Observed
Action
description of the
action
STS
adds additional somatosensory
information to
the movement to be imitated
Parietal mirror neurons (PF)
(inferior parietal lobule)
copies of the motor
plans necessary to
imitate actions for
monitoring
purposes
Frontal mirror neurons (F5) (BA 44)
codes the goal of the
action to be imitated
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Frontoparietal networks for
action recognition
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Expected Activity for Mirror
Areas
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25
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15
10
5
0
Imitation
Execution
Action
Observation
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Imitation of Hand Actions
A
B
C
Iacoboni et al. 1999
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Iacoboni et al. 1999
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Posterior Parietal Cortex
Execution
155 00
+
Observation
+
154 00
153 00
152 00
151 00
150 00
Iacoboni et al. 1999
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Superior Temporal Sulcus
and Imitation
Execution
Observation
122 00
120 50
119 00
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Summary
• Anatomical similarity between human and
nonhuman primate frontoparietal mirror
systems
• Broca’s area codes the goal of the action (lift
the finger)
• PPC codes the precise kinesthetic aspects of
the movement (how much the finger should
be lifted)
• STS codes the visual information (input)
• Both left and right hemispheres are active
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Acousitic Mirror Neurons
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Auditory Mirror Neurons
peanut breaking squeekingduck
100
spk/s
vision & sound
0
1s
vision
sound
motor
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Christian Keysers
Acoustic Mirror Neurons in the
Monkey: Kohler et al (2002)
• Discriminated significantly between two different
sounds of actions (ripping paper, breaking a
peanut)
• Representation of actions in these neurons are
independent both of who performs the actions
and how they are perceived
• Multimodality may provide a first step towards
abstract, semantic representations, perhaps tying
to origin of language
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Can we get a similar result in
humans?
A study using TMS
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Transcranial Magnetic Stimulation
(TMS) Study
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Q:
– Do acoustic mirror neurons exist in the human brain?
– Is there hemispheric specialization for the auditory
modality?
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Single pulse TMS to left or right primary motor
(M1) hand area
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Motor Evoked Potentials (MEPs) recorded from the
left or right hand muscle (FDI)
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Subject listens to 3 auditory stimuli:
– Bimanual Hand Action Sound: Typing orTearing Paper
– Bipedal Leg Action Sound: Walking
– Control Sound: Thunder
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Predicted Results
• Prediction:
– MEPs will be largest when the action sound
matches the muscles of the stimulation site
– Left hemisphere specialization
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Percent Change in MEP from Control (%)
Results: Significant Facilitation to
Hand Stimuli in the Left Hemisphere
10
*
8
leg action sound
hand action sound
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4
2
0
-2
-4
-6
*
Left Hemisphere
Right Hemisphere
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Left Hemisphere MEP Means for a
Sample Subject
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Discussion
• Motor facilitation to action sounds
• Left hemisphere specialization
• All the components of an action seem to
be available to left hemisphere
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Mirror Neurons and
understanding another person’s
INTENTIONS
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• Mirror neurons respond to GOAL
ORIENTED actions, even when only the
intent is apparent but the action itself is
occluded
• May have implications for INTENTION
UNDERSTANDING
• fMRI studies in Humans show similar
findings (Iacoboni et al, 2004)
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Mirror Neurons and
Language
1) Language Evolution
2) Embodied Semantics
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Language Evolution
• Evolutionary progression from:
– goal oriented actions
– pantomiming without the goal present
– abstracting the pantomime
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Embodied Semantics
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“She had the marvelous sensation of being a part
of a vaster world and moving with it because of
moving in rhythm with another being. The joy
of this was so intense that when she saw him
approaching she ran towards him wildly,
joyously. Coming near him like a ballet dancer
she took a leap towards him, and he,
frightened by her vehemence and fearing that
she would crash against him, instinctively
became absolutely rigid, and she felt herself
embracing a statue. Without hurt to her body,
but with immeasurable hurt to her feelings.”
-Anais Nin, Stella
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Embodied semantics
• Hypothesis: The same brain area that
processes sensory-motor experiences also
processes the semantics related to that
experience
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“grasp the cup” = hand motor area
“kick the ball” = foot motor area
“I see what you mean” = visual area
“flew past me” = visual motion areas
“hear the music” = auditory areas
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Action Observation
Premotor Cortex
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Buccino et al, 2001
Plan
• Find regions of interest (ROIs) in the
premotor cortex based on action
observation of a given effector and
compare that ROI with a participant
reading a phrase with that same
concept
– Watch “grasping a cup” & Read “grasping a
cup” = same brain area
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Individual Subject ROI analysis:
Observation
Left Hemisphere
Right Hemisphere
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Individual Subject ROI analysis:
Literal Language
Left Hemisphere
Right Hemisphere
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Discussion
• Premotor areas that are activated most
for observation of hand/mouth actions
are also most activated for literal
phrases pertaining to the hand/mouth
• Left hemisphere specialization
• Support for embodied semantics
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“In all communication, sender and
receiver must be bound by a common
understanding about what counts; what
counts for the receiver, else
communication does not occur.
Moreover the process of production and
perception must somehow be linked;
their representation must, at some
point, be the same.”
-Alvin Liberman
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Creation of Man
Michaelangelo
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