Task 3 Flashcards
Dipole
- Region of positive charge is separated from a region of negative charge
- EEG detects sum of dipole charges
Radial dipole
Oriented perpendicular to the scalp surface
Tangential dipole
Oriented parallel to the scalp surface
How electrodes measure dipoles
- 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
Parallel arrangement
-if neurons all arrayed in same orientation -> signals can sum to form a larger signal
Synchronization of activity
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
EEG: Spatial and temporal resolution
- high temporal accuracy, low spatial accuracy
- signal are transferred in real time
EEG and action potentials
- NOT sensitive to action potentials
- too fast, too local, abolished by tiny time differences between nearby neurons
EEG and post-synaptic potentials
- 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
Negative deflection
-produced by EPSP close to cell body and IPSP at apical dendrites
Positive deflection
-produced by EPSP close to apical dendrites and IPSP close to cell body
Volume conduction
- 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
Capacitive conduction
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
Electrodes and gel
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
10 HZ = ? oscillations per second?
10 Hz = 10 oscillations per second
Reference electrode placement
- ear love/nose
- mastoid bone
- ideal: electrode should be close to head electrode, yet not pick up brain activity
Voltage
= 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
Frequency
- number of full waves (up and down) per second
- measured in Hz
Amplitude
- amount of volt form zero line to top peak
- units: microvolt
EEG Neurofeedback
- give feedback on amplitude/frequency
- > participants learn what specific states of cortical arousal feel like and how to activate such states voluntarily
Peak-picking
- latency of components: time between stimulus onset and peak
- amplitude of components: voltage at the time of the peak
Signal and noise
Signal: proportion of measured voltage that reflects the brain
Noise: voltage that reflects other sources
Signal to noise ratio:
measure of how much signal the system measures compared to noise
-high SNS: better quality signal
Sources of noise
- 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
Artifact rejection and correction
>
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
Amplifier
- maximizes SNR of measured voltage
- increases size of signal above the size of noise
ERPs
-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
Extracting averaged ERPs
-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
ERP component
- 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
How to isolate ERP components?
- 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.
Naming of ERP components
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
Exogenous components -ERPs
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)
Endogenous components – ERPs
- P2,P3,N4000
- > 150ms after stimulus
- depend on the task
- less sensitive to physical features of stimulus
- less dependent on modality
ERP in visual oddball paradigm
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
P3 wave - oddball paradigm
->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
ERP Study – covertly attending
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
P1
-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
Inhibitory aftereffect / inhibition of return
-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
Difficulties ERPs
>
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
ERP components of special populations
>
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
Voluntary vs. reflexive attention
Voluntary: ability to intentionally attend to something (goal-driven process)
Reflexive: bottom-up stimulus-driven process in which a sensory event captures our attention
Overt vs covert attention
Overt: when you turn your head to orient toward a stimulus
Covert: paying attention to something without orienting your head or eyes towards it
Early selection
-stimulus can be selected for further processing OR it can be deemed irrelevant before perceptual analysis of the stimulus is complete
Late selection
- 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
Modification of selection models
>
room for possibility that info in the unattended channel could reach higher stages of analysis but with greatly reduced signal strength
Cueing task
>
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
Endogenous cue
-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
Benefit of attention
participants respond faster when the cue correctly predicts the target’s location than when neutral cues are given
Cost of attention
reaction times are slower when stimulus appears at unexpected location
N170 - face processing
- negative-going wave over visual cortex that typically peaks around 170ms after stimulus onset
- N170 generator probably lies in visual cortex
Research N170
>
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
Example: impaired cognition in schizophrenia
Why are RTs typically slowed in schizophrenia patients when they perform simple sensorimotor tasks?
-Impairment in perceptual processes, decision processes or in response processes?
Design: modified oddball task
Schizophrenia low in sensorimotor tasks:
-Impairment in perceptual processes, decision processes or in response processes?
>
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
Findings:
Schizophrenia low in sensorimotor tasks:
-Impairment in perceptual processes, decision processes or in response processes?
>
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
