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Method of Cognitive Neuroscience > Task 3 > Flashcards

Task 3 Flashcards

1
Q

Dipole

A
  • Region of positive charge is separated from a region of negative charge
  • EEG detects sum of dipole charges
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2
Q

Radial dipole

A

Oriented perpendicular to the scalp surface

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3
Q

Tangential dipole

A

Oriented parallel to the scalp surface

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4
Q

How electrodes measure dipoles

A
  • dipoles produce positive and negative deflection at different regions of the scalp
  • electrodes measure sum of dipoles
  • in order to avoid non-zero signal: neurons need to be arranged in parallel fashion and be synchronously active
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5
Q

Parallel arrangement

A

-if neurons all arrayed in same orientation -> signals can sum to form a larger signal

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6
Q

Synchronization of activity

A

necessary to yield a net charge on scalp facing side of dipole sheez rather tgan charges cancelling each other out
-needed for a signal large enough to be measured

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7
Q

EEG: Spatial and temporal resolution

A
  • high temporal accuracy, low spatial accuracy

- signal are transferred in real time

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8
Q

EEG and action potentials

A
  • NOT sensitive to action potentials

- too fast, too local, abolished by tiny time differences between nearby neurons

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9
Q

EEG and post-synaptic potentials

A
  • sensitive to (slower) post-synaptic potentials
  • negativity at apical dendrites/positivity at cell body
  • > neuron becomes a dipole
  • sum of dipoles measurable at the scalp if neurons have same input (excitatory or inhibitory) and same orientation
  • EPSP and IPSPs produce positive or negative deflection in EEG signal depending on whether positivity or negativity is closer to scalp
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10
Q

Negative deflection

A

-produced by EPSP close to cell body and IPSP at apical dendrites

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11
Q

Positive deflection

A

-produced by EPSP close to apical dendrites and IPSP close to cell body

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12
Q

Volume conduction

A
  • responsible for propagation of EEG signal within the brain
  • process by which a pool of ions repels nearby ions of the same charge
  • > opposite electrical charge attracts to each other and same charges repel each other
  • repelled ions repel other ions of the same charge
  • > results in wave of charge that travels through the extracellular space (=Signal propagation)
  • does not reflect electrical current within neuron itself
  • ions cannot leave the brain, therefore we need capacitive conduction
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13
Q

Capacitive conduction

A

Capacitor: 2 pools of charges separated by an insulating layer (=dielectric)

  • > Prevents ions from mingling (and resulting in neutrally charged pool)
  • > Charge differences build up across insulating layer as negative ions push up against one side and positive ions accumulate on the other side
  • > Sequence of layers from the brain to dura layers, skull layers, electrode gel, and electrode forms a series of conductive volumes separated by insulating layers
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14
Q

Electrodes and gel

A

highly conductive electrode gel saturates spaces beneath an electrode. Filling the air pockets between hairs

  • > provides conductive path from scalp to electrode
  • before beginning to record, one should allow the electrochemical interaction between electrode and gel to reach a steady stage
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15
Q

10 HZ = ? oscillations per second?

A

10 Hz = 10 oscillations per second

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16
Q

Reference electrode placement

A
  • ear love/nose
  • mastoid bone
  • ideal: electrode should be close to head electrode, yet not pick up brain activity
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17
Q

Voltage

A

= potential for an electrical charge to move between 2 locations

  • recorded from each electrode, resulting in a separate waveform for each electrode
  • any voltage is a voltage-difference between 2 locations (active electrode and reference electrode)
  • filters are used to remove very slow and very fast voltage changes: likely present noise emerging from non-neural sources
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18
Q

Frequency

A
  • number of full waves (up and down) per second

- measured in Hz

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19
Q

Amplitude

A
  • amount of volt form zero line to top peak

- units: microvolt

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20
Q

EEG Neurofeedback

A
  • give feedback on amplitude/frequency

- > participants learn what specific states of cortical arousal feel like and how to activate such states voluntarily

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21
Q

Peak-picking

A
  • latency of components: time between stimulus onset and peak
  • amplitude of components: voltage at the time of the peak
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22
Q

Signal and noise

A

Signal: proportion of measured voltage that reflects the brain
Noise: voltage that reflects other sources

23
Q

Signal to noise ratio:

A

measure of how much signal the system measures compared to noise
-high SNS: better quality signal

24
Q

Sources of noise

A
  • external noise: electrical power supply in building, computer and equipment -> room can be shielded
  • internal noise: breathing, muscle tension, blinking-> need of filtering methods and artifact detection
25
Q

Artifact rejection and correction

A

> 

Eyes:  blink -> large voltage deflection over the front of the head -> much larger than the ERP signals  eye movements: large potentials produced by eye movements -> confound experiments that use lateralized stimuli 
Trials containing blinks, eye movements or other artifacts -> typically excluded from averaged ERP waveforms  Shortcomings: large number of trials may need to be rejected, mental effort involved in suppressing eyeblinks (if you tell them not to blink) may impair task performance  	Reject trials on which blink/eye movements occurred at a time when they might change the sensory input  	Correct for artifactual voltage when timing of blink/movement should not change task performance
26
Q

Amplifier

A
  • maximizes SNR of measured voltage

- increases size of signal above the size of noise

27
Q

ERPs

A

-event related potentials
-electrical potentials related to internal or external event (stimulus, responses, decisions)
> can provide info about broad range of cognitive and affective processes
> reflect ongoing brain activity with no delay
>high temporal, but low spatial resolution

28
Q

Extracting averaged ERPs

A

-ERPs are small in comparison with rest of EEG activity
-event codes mark events that happened at specific time (e.g. stimulus onset)
> used as time-locking point to extract segments of the EEG
> large components: averaging 10-30 trials to obtain clear results
>small components: 100-500 trials

  • Works only well if timing of neural response is same across trials
  • Address problem: measure amplitude of an ERP components as mean voltage over a broad time range rather than as peak voltage
29
Q

ERP component

A
  • a voltage deflection that is produced when a specific neural process occurs
  • Many components will be elicited by a stimulus in a given task
  • sum together to produce the observed waveform, which consists of a set of positive and negative peaks
30
Q

How to isolate ERP components?

A
  • by means of difference waves in which the ERP waveform elicited by one trial type is subtracted from the ERP waveform elicited by another trial type
  • isolate neural processes that are differentially active for two trial types, separating these processes from many concurrently active brain processes that do not differentiate between trial types.
31
Q

Naming of ERP components

A

Most common:
-start if P or N: indicate that the component is positive going or negative going
-followed by a number: indicating peak latency of the waveform
-e.g.: N400
Or
-ordinal position of the peak within the waveform
-e.g.: P2: second major positive peak

Sometimes more functional names are given:

  • syntactic positive shift
  • error-related negativity
32
Q

Exogenous components -ERPs

A

P1, N1

  • <150ms after stimulus
  • obligatory
  • do not depend on instructions
  • sensitive to physical features of stimulus
  • depend on modality of stimulus (auditory text on headphones / text on computer screen)
33
Q

Endogenous components – ERPs

A
  • P2,P3,N4000
  • > 150ms after stimulus
  • depend on the task
  • less sensitive to physical features of stimulus
  • less dependent on modality
34
Q

ERP in visual oddball paradigm

A

Paradigm: 2 classes of stimuli: frequently occurring standard stimulus and infrequently occurring oddball stimulus

Example: 80% of the stimuli letter X and 20% letter O

35
Q

P3 wave - oddball paradigm

A

->much larger for infrequently occurring stimulus categories than for frequent stimuli

P3a/novelty P3:
elicited by highly distinctive improbable stimuli, even when the task does not require discrimination of these stimuli
> Frontal scalp distribution

P3b: sensitive to task-defined probability -> larger for improbable stimuli only when the task requires it
> Difference in amplitude between oddball and standard stimuli cannot occur until the brain has begun to determine the category of a given stimulus

-> P3 latency can be used to distinguish between pre- and post categorization processes

36
Q

ERP Study – covertly attending

A

Task: participants covertly attend to stimuli present at one location and ignore those presented at another while event-related potentials are recorded
-> P1 and N1

37
Q

P1

A

-begins at 60-70ms and peaks at 100ms (= sensory wave generated by activity in contralateral occipital visual cortex)
･ Sensitive to changes in physical stimulus parameters such as location in visual field and stimulus luminance
･ Modulations due to attention begin as early as 70-90ms, affecting this wave
･ Stimulus appears at location a subject is attending – P1 larger in amplitude
･ Supports early selection models of attention

38
Q

Inhibitory aftereffect / inhibition of return

A

-task-irrelevant flash of light affects the speed of responses to subsequent task-relevant target stimuli (= reflexive or exogenous cueing)
-responses after faster to targets that appear in the vicinity of the light flash
> BUT only for a short time after the flash (50-200ms)
> more than 300ms between task irrelevant flash and target: pattern reverses, participants respond more slowly

-> Occurs because we have a built in mechanism to prevent reflexively directed attention from becoming stuck for more than a couple of milliseconds

39
Q

Difficulties ERPs

A

> 

Low spatial resolution: difficult to localize ERPs purely on basis of scalp distributions 
Averaging cannot be performed if mental processes being studied is not reasonably well time-locked to a discrete, observable event 
Tens/hundreds of trials should be averaged, which cannot be done with every paradigm 
Slower processes difficult to see in ERPs
40
Q

ERP components of special populations

A

> 

ERPs in infants and young children: useful in revealing mental processes that are difficult to assess behaviorally 
ERPs in Aging: determine whether overall slowing of responses reflect slowing in specific processes 
ERPs in disorders: determine which exact processes are impaired 
ERPs as biomarkers: define specific treatment targets and assess effectiveness, especially pharmacological treatments
41
Q

Voluntary vs. reflexive attention

A

Voluntary: ability to intentionally attend to something (goal-driven process)
Reflexive: bottom-up stimulus-driven process in which a sensory event captures our attention

42
Q

Overt vs covert attention

A

Overt: when you turn your head to orient toward a stimulus
Covert: paying attention to something without orienting your head or eyes towards it

43
Q

Early selection

A

-stimulus can be selected for further processing OR it can be deemed irrelevant before perceptual analysis of the stimulus is complete

44
Q

Late selection

A
  • all inputs are processed equally by the perceptual system and attentional selection determines what will undergo additional processing and be represented in awareness
  • implies attentional processes cannot affect perceptual analysis of stimuli
45
Q

Modification of selection models

A

> 

room for possibility that info in the unattended channel could reach higher stages of analysis but with greatly reduced signal strength
46
Q

Cueing task

A

> 

used to manipulate the focus of voluntary spatial attention
  • Target stimulus is flashed onto screen at either cued location or another location
  • Valid trial – when a cue correctly predicts the location of the subsequent target
  • Invalid trial – target presented at location not indicated by cue
  • Neutral trial – cues give no information about most likely location of the impending target
47
Q

Endogenous cue

A

-orienting of attention to the cue is driven by the participant’s voluntary compliance with instruction and the meaning of the cue rather than its physical features

48
Q

Benefit of attention

A

participants respond faster when the cue correctly predicts the target’s location than when neutral cues are given

49
Q

Cost of attention

A

reaction times are slower when stimulus appears at unexpected location

50
Q

N170 - face processing

A
  • negative-going wave over visual cortex that typically peaks around 170ms after stimulus onset
  • N170 generator probably lies in visual cortex
51
Q

Research N170

A

> 

face processing partially automatic, but can be modulated by attention 
key role of expertise: bird experts exhibit enhanced N170 in response to birds, dog experts to dogs etc. 
tracking development of face processing: face specific processing present early in infancy -> faster and more sophisticated over time 
N170 anormal in autistic children
52
Q

Example: impaired cognition in schizophrenia

A

Why are RTs typically slowed in schizophrenia patients when they perform simple sensorimotor tasks?
-Impairment in perceptual processes, decision processes or in response processes?

53
Q

Design: modified oddball task

Schizophrenia low in sensorimotor tasks:
-Impairment in perceptual processes, decision processes or in response processes?

A

> 

Isolation of specific ERP components by means of difference waves: ERP waveform from one trial type is subtracted from ERP waveform from other trial type 
P3 wave: subtracting frequent trials from rare trials -> reflect time course of stimulus categorization
LRP (lateralized readiness potential): subtracting ipsilateral electrode sites from contralateral, relative to responding hand) -> response selection
54
Q

Findings:
Schizophrenia low in sensorimotor tasks:
-Impairment in perceptual processes, decision processes or in response processes?

A

> 

60ms slowing in patients 
Grand average waveforms: average waveforms were computed across trials for each participant -> averaged together
P3 waveform: indistinguishable for patients and controls 
LRP (lateralized readiness potential): delayed by 75ms in onset time and diminished by 50% in amplitude for patients vs. controls 
Slowed reaction time = result of slowing in response selection

Method of Cognitive Neuroscience (9 decks)

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