IMAGING THE MIND

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IMAGING THE MIND

• Direct methods
• Electrical activity (EEG, MEG)
• Metabolic activity (EROS)
• Indirect methods
• Changes in regional Cerebral Blood Flow (rCBF)
  (PET, MRI, fMRI)

Brocas Brain
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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

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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

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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

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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

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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 witha strong radio-frequency
  signal thatperturbs the alignment
• Using an RF detector, track the return to
  alignment
• With really complex computing, reconstruct the 3D
  density of tissue in the brain

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MRI vs. fMRI
fMRI studies brain function
MRI studies brain anatomy
Thanks to Jody Culham, fMRI for
Dummies http//defiant.ssc.uwo.ca/Jody_web/fmri4du
mmies.htm
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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

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• DEspesito 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)

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fMRI (contd)

• Event-related fMRI allows tracking of the
  hemodynamic response to individual events

Source Kwong et al., 1992
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CHOOSING AN IMAGING METHOD

• There is no single ideal method
• Advantages and disadvantages to each