Lecture 5 - imaging the brain part 2 Flashcards
EEG
Electrical potential
event-related potentials
components
-electrical potential = scalp recorded summation of PSP’s
quanitfied in 2 domains: time & frequency
-ERP = time locked activity. ( know whekn event happens, can detect what happens in EEG) NT binding.
- exogenous: related to things happening outside the body = sensory ERP. physical characteristics of stimulator
- endogenous = related to mental event.
Electrode Placement in EEG
10-20 system - standarized placement. 10/20% of midline distance from nasion (nose dip) to inion( back of head dip)
z= 0
odd = left
even = right
beta wave alpha wave delta wave deep sleep coma
- excited, alert, aroused
- relaxed, slow oscillation
- sleep, but there are theta bursts involved too.
- complex pattern in slow roll
1 Hz, brainstem activity
EEG - seizure.
onset
clipping
origin?
- can see where onset is, vs where it travelled based on time until dramatic EEG recording.
- clipping = flat tops on recording. means strength of voltage is stronger than range in eeg.
- info can tell you where seizure may originate
ERP
-amplified?
- averaged
Nd ?
- wave of brain activity during stimulus. signals amplified to be visualized. do task multiple times and average divided by stimuli to get solid wave.
Nd = difference wave
contingent negative variation (CNV)
- walter, 1964.
warning followed by interval followed by imperative stimulus
- EEG change in response to paradigm. - warning that you might get task, brain prepared. CNV = difference btw prepare and do.
P300
- sutton 1965
oddball paradigm - low probability stimuli embedded in train of high probability stimuli.
- every time you hit the lower probability stimuli, p300 shows up. ~300 mV.
N1 attention affect - dichotic listening paradigm (DLP)
- hillyard 1973
- covert attention = change attention without moving eyes. sound on same side as attention, N1 -ERP is larger.
endogenous process of shifting attention withou overt sign.
evoked potential
brainstem evoked potentials
visual evoked potentials
ERP defined by:?
-EP:ERP
-non-cortical PSP
VEP = ERP in response to visual stimulus
- peak size, time to peak post-stimulus, , topography, paradigm (stimulus unique to paradigm), slope of onset.
signal averaging
superimpose all signals. divide by number of superimposed signals. common is kept, deflections cancelled out.
filtering response
more repeats of trials = stabilize the response (average), filter out the endogenous actors and keep whats common to the task/stimuli.
within subject
btw subject
signal averagin
within = after many sessions = more similarities
btw = each person has different ERP.
- average ERP usually doesnt look like anyone’s, but is average of all.
caveats to ERP reliability
- grand averages
- large btw subject variability: diggerent brain., good within-subject reliability across sections
- latency jitter:: amplitudes misleading: average is related to everyones but not same as anyone’s
- less between session variability- sleep, drugs, hangover, diet.
ERP comparison to behaviour.
ERP elucidate specific cognitive processes underlying overt behaviour
- Stroop + P300. change in p300 = perceptual thing happens. if not, then no motor prep.
online measure in absence of overt behaviour
cover attention change, look at what brain is going. note correct vs error in responses.
small amplitude = ERP =? # of trials compared to RT experiments
greater number of trials than RT experiments
advantages of ERP
non-invasive
temporal resolution is decently good
cost - low.
major ERP components
-visual sensory responses
-VSR: see difference when attending to same side, different when attending to opposite side.
VSR
C1 - retinotopy & polarity
P1
C1: (80-130 ms)basic retinotopy = lower visual fiel maps to upper bank of calcarine fissure (V1 folded into this fissue. vice versa for upper visual field.
- polarity can vary. stimuli in LVF give +C1,, -C1 in UVF
- P1 100-130 mslateralized. early P1 - v2, later P1 = v4.
sensitive to direction of spatial attention
Auditory sensory responses
- BER
N1
Mismatch negativity (MMN)
brainstem evoked responses.
N1 = 70-150 msauditory cortex arousal - related to A1 processing. , vertex potential attention (at point Cz. related to attention), lateral potential -
MMN: 100-200msautomatic - novel stimuli elicit this. mapping enviro with auridoty info. orienting response always working. partt of endogenous activity.
schizophrenics - MMN and Gray Matter Reduction.
look at schizophrenics. - increase (less negative) MMN over time = should be stable.
shrinkage in brain area in A1 - correlated with increase in MMN. = decreased symptoms.
P300
- solicited by oddball paradigm.
Parietal P3b - sshows task is relevant. frontal P3a = novel processing
“context updating” updating awareness - is it here? no. now? no.
target probablity - lower prob = higher p300 peak
resource allocation - split attention = less resource to task = smaller p300
time latency - mask stimulus = longer for p300 to peak
dipole modeling and inverse problem
caveat?
try to find part of brain that is sending outthe signal.
- map image of brain, pick area and dipole within that. then create the wave that would come from that. = see how it matches to real EEG. If different fin a new area.
combine with fMRI for best results.
Image= spherical model of brain. not exactly what patient loos like.
conductivity thru different tissue is different too.
MEG
right hand rule
electric current = magnetic field.
use electrodes to detect magnetic field.
drawback: expensive
sensitive to other fields - interruption
dipole orientation - too deep in sulci, cant reach electrode
