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EEG, Event-related potential (ERP),
Magnetoencephalography (MEG)
Brain Imaging Techniques
• fMRI or functional magnetic
resonance imaging
– Measures brain activation by
detecting the increase in oxygen
levels in active structures.
– Advantages: activity
measurement, good spatial
resolution and speed, no
radioactive substances
– Disadvantage: unable to resolve
details at the all-important circuit
level. Brain imaging tracks the
power consumption of a million
neurons, irrespective of whether
they are excitatory or inhibitory,
project locally or globally, and so
on.
SOURCE: Reprinted with permission from “Building Memories: Remembering and Forgetting of Verbal Experiences as Predicted by
Brain Activity” by A.D. Wagner et al., Science, 281, p. 1188-1191. © 1998 AAAS
• CT or computed tomography:
– A dye is injected into the blood.
– A series of X-rays is made from different
angles; the images reflect the density of blood
vessels in each area.
– A computer combines the X-rays into a series
of horizontal sections of the brain.
• PET or positron emission tomography
– Injecting radioactive substance into the bloodstream, which is
taken up by active parts of the brain.
– Advantages: ability to track changing activity in the brain, fast
– Disadvantages: expensive, requires sophisticated staff, must
be near a cyclotron, relatively slow
Research Techniques: Autoradiography
reveals which neurons are active
Adapted from Tootell, 1982
Macaque monkeys were trained to stare at a pattern (left panel) while injected with radioactive glucose. The radioactive
glucose was absorbed and metabolized by active neurons to a much greater extent than by other neurons. After the
experiment, the animals were sacrificed and the cortical radioactivity pattern was analyzed.
This method provides high resolution radioactive labeling of active neurons. The physical pattern of active neurons (right
panel, darker pixels correspond to greater neuronal activity) is clearly a geometrical representation of the pattern
physically laid-out on the cortex.
This experiment clearly demonstrates that the visual cortex relies on topographical representation of spatial information.
EEG, Event-related potential (ERP),
Magnetoencephalography (MEG)
An Electroencephalograph
• EEG /
electroencephalogram
– records the combined
activity from many neurons
(irrespective of whether
they are excitatory or
inhibitory, project locally or
globally, and so on) by using
multiple electrodes;
– has excellent temporal
resolution but poor spatial
resolution;
– is best used to detect
changes in arousal. Garrett: Brain & Behavior 4e
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EEG: Epilepsy
• Epilepsy (from the Ancient Greek
meaning "to seize") is a group of
neurological disorders
characterized by epileptic
seizures. Prevalence: 1 in 100
• Epileptic seizures are episodes
that can vary from brief and
nearly undetectable to long
periods of vigorous shaking.
• Often brought on by factors
such as lack of sleep, stress or
flickering light among others.
• In epileptic seizures a group of
neurons begin firing in an
abnormal, excessive, and
synchronized manner.
• Patient HM.
EEG and Evoked Potentials
Figure 4.9: Evoked Potential Produced by a Novel Tone
• Evoked potential
measurement:
– uses a computer to
average the EEG over
several stimulus
presentations;
– cancels out the “noise”
of the brain’s other
activity, leaving only the
unique response to the
stimulus.
Garrett: Brain & Behavior 4e
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Magnetoencephalography
Origin of the brain's magnetic
field.
• Transcranial magnetic stimulation is used to induce a transient
interruption of normal activity in a relatively restricted area of the brain.
It is based on the generation of a strong magnetic field near the area of
interest, which, if changed rapidly enough, will induce an electric field
sufficient to stimulate neurons.
• Permits excitation or inhibition.
Research Techniques
Figure 4.7: Scanning Electron Microscope
• Light microscopes:
– Cell bodies, dendrites, axons,
and large organelles in
neurons;
– Limited capability due to the
nature of light.
• Electron microscopes:
– Pass beams of electrons
through a thin slice of tissue
onto detector;
– High resolution, magnifying
objects up to 250,000 times;
– can reveal objects in 3-D
(scanning electron
microscope).
Garrett: Brain & Behavior 4e
13
Single unit recording
Whole-cell patch-clamp
Intracellular electrodes