Opportunity to Participate
• EEG studies of
vision/hearing/decision
making – takes about 2
hours
• Sign up at www.tatalab.ca
– Keep checking back there
for more time slots
• Two extra points added to
your final grade!
Functional Imaging
• blood flow overshoots baseline
after a brain region is activated
• More oxygenated blood in that
region increases MR signal
from that region
Experimental Design in fMRI
Signal
• A voxel in tissue that responds to the task shows
signal change that matches the timecourse of the
stimulus
Active
60 sec
Rest
60 sec
Active
60 sec
Rest
60 sec
Experimental Design in fMRI
• A real example of fMRI block design done
well:
– alternate moving, blank and stationary visual
input
Moving
40 sec
Blank
40 sec
Stationary
40 sec
Blank
40 sec
Experimental Design in fMRI
• Voxels in Primary
cortex tracked all
stimuli
Experimental Design in fMRI
• Voxels in area MT tracked only the onset of
motion
Experimental Design in fMRI
• Voxels in area MT tracked only the onset of
motion
• How did they know to look in area MT?
PET: another way to measure blood
Oxygenation
• Positron Emission Tomography (PET)
• Injects a radioisotope of oxygen
• PET scanner detects the concentration of this isotope as
it decays
Advantages of fMRI
• Advantages of MRI:
1. Most hospitals have MRI scanners that can be
used for fMRI (PET is rare)
2. Better spatial resolution in fMRI than PET
3. Structural MRI is usually needed anyway
4. No radioactivity in MRI
5. Better temporal resolution in MRI
Advantages of PET
• Advantages of PET:
1. Quiet
2. A number of different molecules can be labeled
and imaged in the body
Limitations of fMRI
• All techniques have constraints and limitations
• A good scientist is careful to interpret data
within those constraints
Limitations of fMRI
• Limitations of MRI and PET:
1. BOLD signal change does not necessarily mean a
region was specifically engaged in a cognitive
operation
2. Poor temporal resolution - depends on slow
changes in blood flow
3. expensive
Electrophysiology
Neurons are Electrical
• Remember that Neurons have
electrically charged
membranes
• they also rapidly discharge and
recharge those membranes
(graded potentials and action
potentials)
• Review relevant textbook
sections if this isn’t familiar to
you
Neurons are Electrical
• Importantly, we think the electrical signals are
fundamental to brain function, so it makes
sense that we should try to directly measure
these signals
– but how?
Intracranial and “single” Unit
• Single or multiple electrodes
are inserted into the brain
• “chronic” implant may be left in
place for long periods
Intracranial and “single” Unit
• Single electrodes may pick up
action potentials from a single
cell
• An electrode may pick up the
combined activity from several
nearby cells
– spike-sorting attempts to
isolate individual cells
Intracranial and “single” Unit
• Simultaneous recording from
many electrodes allows
recording of multiple cells
Intracranial and “single” Unit
• Output of unit recordings is
often depicted as a “spike
train” and measured in
spikes/second
Stimulus on
Spikes
Intracranial and “single” Unit
• Output of unit recordings is
often depicted as a “spike
train” and measured in
spikes/second
• Spike rate is almost never zero,
even without sensory input
– in visual cortex this gives rise
to “cortical grey”
Stimulus on
Spikes
Intracranial and “single” Unit
• By carefully associating changes in spike rate
with sensory stimuli or cognitive task, one can
map the functional circuitry of one or more
brain regions
Subdural Grid
• Intracranial electrodes typically cannot be
used in human studies
Subdural Grid
• Intracranial electrodes typically cannot be used in
human studies
• It is possible to record from the cortical surface
Subdural grid on surface of Human cortex
Electroencephalography and the
Event-Related Potential
• Could you measure these electric fields
without inserting electrodes through the
skull?
Electroencephalography and the
Event-Related Potential
• 1929 – first measurement of brain electrical
activity from scalp electrodes (Berger, 1929)
Voltage
Electroencephalography and the
Event-Related Potential
Time
-Place an electrode on the scalp and another one somewhere else on the
body
-Amplify the signal to record the voltage difference across these
electrodes
-Keep a running measurement of how that voltage changes over time
-This is the human EEG
Electroencephalography and the
Event-Related Potential
• 1929 – first measurement of brain electrical
activity from scalp electrodes (Berger, 1929)
– Believed to be artifactual and/or of no significance
– Currently Google Scholar search for “human EEG”
returns “about 383,000” hits
• That’s about 13 papers per day
Electroencephalography
• pyramidal cells span layers of cortex and have
parallel cell bodies
• their combined extracellular field is small but
measurable at the scalp!
Electroencephalography
• The field generated by a patch of cortex can be
modeled as a single equivalent dipolar current source
with some orientation (assumed to be perpendicular
to cortical surface)
Electroencephalography
• Electrical potential is
usually measured at
many sites on the head
surface
• More is sometimes
better
Electroencephalography
• EEG changes with various
states and in response to
stimuli