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Lecture 6

PSYC04H3 Lecture 6: EEG – Time-Frequency Analysis

Course Code
Lorna Garcia-Penton

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PSYC04: Neuroimaging Lab Clara Rebello
PSYC04 Lecture 5: EEG Time-Frequency Analysis
Physiological basis of electroencephalography and magnetoencephalography (EEG/MEG)
o Synchronicity of cell excitation (due to recurrent cortico-cortical and cortico-thalamo-
cortical projections) determines amplitude and rhythm of the EEG signal
Ohm’s Law: Deals with the relationship between voltage
and current in an ideal conductor; States that the current
through a conductor between two points is directly
proportional to the voltage across the two points
o V = IR
V is the voltage difference
I is the current in amperes
R is the resistance in ohms
o Right image is a visual depiction of this law
Artifacts: Noise sources that have overlaid onto the EEG
and corrupted the purity of the brainwave
signal when they are present
o Other non-neural activity
o EMG: Electrical “noise” generated by
facial muscle activity near the
o Blink: Eye blink

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PSYC04: Neuroimaging Lab Clara Rebello
ERPs (tinny) are obtained after segmentation and averaging
o Panel A shows single-trial EEG traces from 12 randomly selected trials (number inside
plot indicates trial number)
Data are from electrode FCz
o Panel B shows 99 single trials in gray and their average the ERP in black
o Panel C shows the same ERP with focused y -axis scaling.
Markers indicate what parts of the signals will be sent to the recording computer
Advantages of ERPs
o Simple and fast to compute and require few analysis assumptions or parameters
o High temporal precision and accuracy

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PSYC04: Neuroimaging Lab Clara Rebello
o Extensive literature in which to contextualize and interpret your results
o Provide a quick and useful data quality check of single-subject data
Limitations of ERPs
o Reveal relatively little of the information present in EEG data
Many kinds of dynamics in EEG data do not have a representation in the ERP
o Provide limited opportunities for linking results to physiological mechanisms
The neurophysiological mechanisms that produce ERPs are less well understood
Task-related information can be lost
during ERP averaging
o Diagram shows how
simulated data showing how
time-locked but not phase-
locked activity (left column)
is lost in ERP averaging
(middle column) but is visible
in band-specific power (right
o Each row in the left column
shows a different trial, and
each row in the middle and
right columns shows
averages from the first until
the current trial
Tallon-Baudry & Bertrand (1999)
o A) Successive EEG trials (simulated data) with a small amplitude gamma response phase-
locked to stimulus onset (blue bexes) and a gamma burst jittering in latency (green
o B) Averaging across single trials leads to the conventional evoked potential
o C) Time-frequency power representation of the evoked gamma response
Colour scale codes the variations of power (positive or negative) with respect to
a pre-stimulus baseline
Non-phase-locked activity is cancelled out
o D) Time-frequency power is computed for each single trial
o E)Time-frequency power is averaged across trials
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