IMAGING THE MIND
• Direct methods
– Electrical activity (EEG, MEG)
– Metabolic activity (EROS)
• Indirect methods
– Changes in regional Cerebral Blood
Flow (rCBF) (PET, MRI, fMRI)
Broca’s Brain
EEG and EVENT-RELATED
POTENTIALS (ERPs)
• Postsynaptic extracellular potentials
vary with neuronal activity
• Masses of pyramidal cells generate
a varying electrical signal, the EEG
• Changes in the EEG that are related
to psychological events (ERPs) can
be seen by averaging
• Various ERP “components” are
sensitive to memory processes
MAGNETOENCEPHALOGRAPHY
• methodology
– Incredibly weak magnetic signal
(femtoTeslas)
– Detected by SQUID ($3M, 16,000 lbs,
minus 269 deg C
– Works for neural fields tangental to
surface
EVENT-RELATED OPTICAL
SIGNALING (EROS)
• Infrared light source placed on
scalp
• Scattered light picked up by optic
detector
• Signal varies with metabolic state of
neurons
PET IMAGING
(Positron Emission Tomography)
• Subject ingests radioactive tracer
• Does cognitive task for several
minutes
• Metabolic activity increases regional
cerebral blood flow and glucose
metabolism in specific areas
• Tracer is deposited more in these
areas than others
• Isotopes decay and emit positrons
• These are detected and an image of
activity reconstructed
MAGENTIC RESONANCE
IMAGING (MRI)
• Align the spins of Water-based
hydrogen atoms by powerful
magnetic field
• Create a “gradient” in the field
• “pulse” the field with
a strong radiofrequency signal that
perturbs the alignment
• Using an RF detector, track the
return to alignment
• With really complex computing,
reconstruct the 3D density of tissue
in the brain
MRI vs. fMRI
MRI studies
brain anatomy
fMRI studies
brain function
Thanks to Jody Culham, fMRI for Dummies
http://defiant.ssc.uwo.ca/Jody_web/fmri4dummies.htm
FUNCTIONAL MAGNETIC
RESONANCE IMAGING
(fMRI)
• Oxygenated blood has different
magnetic properties than deoxy
• So comparing MRI between target
task and “control” task (a challenge)
reveals areas of task-related
activation
• D’Espesito & Ranganath (2000)
•
•
•
In scan 1, a subject is asked to remember a face. Areas at
the rear of the brain that process visual information are
active during this task, as is an area in the frontal lobe. In
scan 2, the subject is asked to "think about this face."
Surprisingly, the hippocampus is activated - the first time
this has been documented. The hippocampus was already
known to be important for memory, but these results show
that this part of the brain is specifically active during the
time when we are remembering new information.
In scans 3 and 4, the subject was asked to compare another
face to the remembered face. Some of the same visual areas
are activated as during the initial memory task, but other
areas, such as part of the frontal lobe, are involved in making
a decision about the memory.
Credit: Mark D'Esposito and Charan Ranganath Department
of Psychology & Helen Wills Neuroscience Institute
University of California, Berkeley (2000)
fMRI (cont’d)
• Event-related fMRI allows tracking
of the “hemodynamic response” to
individual events:
Source: Kwong et al., 1992
CHOOSING AN IMAGING
METHOD
• There is no single ideal method
• Advantages and disadvantages to
each
Method
Temp
Resol
Spatial
Resol
Depth
Tech
Diffic
Cost
EEG
Poor
Mod
Mod
Mod
Mod
ERP
Excel
Mod
Mod
Mod
Mod
PET
Poor
Mod
Good
High
High
Blocked
fMRI
Event-Rel
fMRI
MEG
Poor
Excel
Excel
High
High
Good
Excel
Excel
High
High
Excel
Good if
Tang
Mod
High
High
EROS
Excel
Excel
Mod
Mod
Low