Electrophysiology

Electrical activity from thousands/10s of thousands of neurons
Behind layers of tissue

Electrical activity associated with post-synaptic dendritic currents generated in cortical pyramidal cells

Summed current from local activity (the elcetrical activity generated by post-synaptic dendritic current)

Volume conducted
Brain tissue and skull act as an electrically conductive medium where electric waves propogate (send/spread out)
This propogating electical activity = secondary current

Microvolts

+ Technology relatively cheap and easy to maintain
+ Electrode placement is standardised (international 10-20 electrode system)
+ High temporal resolution
- Poor spatial resolution, impossible to know where exactly electrical data recorded occurred in the brain
- 'Inverse problem' = don't know which areas are responible for a particular EEG topography
- 'Forward problem' = computing scalp recorded EEG when we know the locations of the within brain current generators (+ potentially solvable)

Cz (middle and top of scalp)

Noise

Any measured activity unrelated to the task at hand/signal of interest

EEG modulation affected by the stimulus

EEG data recorded to a single stimulus presentation

Physiological artefacts (eye blinking)
Movement artefacts
Mental activity unrelated to task (unrelated thoughts)
External noise (electric noise from nearby lift, fan etc.)
Trial by trial fluctuations in brain activity
Sweating

Experimental condition is presented many times

Average EEG signal in response to many presentations of stimulus

Averaged EEG data
Grand-average ERP = all pps' data averaged together

Polarity (-ive or +ive)
Magnitude of the amplitude (range)
Timing (latency in ms)
Scalp topography (distribution of voltage across scalp)

Positive ERP component that occurs approximately 100 ms after stimulus presentation

Maximal negative deflection in the ERP occurring between 180 and 391 ms post-stimulus

EEG = primary recordings on all channels
ERP = recording time locked to an event

Ocular (eye-related) artefacts
Eye movement
Blinking

Electro-oculagram

No foolproof way to recover underlying components from observed waveforms

Difficult to tell what neural activity in the brain produced a particular EEG topography
Unsolvable with known accuracy
We can't know which brain areas are responsible for the recorded electrical activity

Technique for interpreting source localisation of EEG signals
Generators of signals often represented as 'equivalent current dipole'
Describes summed electrical activity of many neurons

Intracranial EEG
Magnetoencephalography (MEG)
fMRI

OxyHb repulses from magnetic field (diamagnetic)
DeoxyHb attracted to magnetic field (paramagnetic)

Automatic acoustic change detection
Elicited in a passive oddball paradigm
Reflects amplitude difference between deviants (eg. 10% ga) and standards (eg. 90% ba)

Pps not consciously attending stimuli

+ Study acoustic processing without contamination of attention
+ Can be used in non-verbal pps (babies/coma)

Hylliard and Anllo-Vento 1998
Attention modulates amplitude of ERPs
Contralateral to stimulation
Attending stimuli = more positive amplitude compared to unattended stimuli

Positive wave
Latency of 200-700ms (appears ~200-700ms after stimulus presentation) - increases with task difficulty
Also called P300 complex - probably sum of overlapping waves with similar amplitudes and latencies

P300
P3b
P3a

Occurs in conscious oddball tasks such as active categorisation paradigms (attending to stimuli)
Amplitude modulated by subjectively perceived frequency of task-relevant stimuli
Smaller wave = subjectively percieved probability of events
Larger wave = P300 amplitude
Usually assumed to reflect categorisation and working memory processes related to task-relevant stimuli

Occurs in conscious oddball tasks
Appears when improbable, unexpected, task-irrelevant distractor stimuli are presented
More frontal topography than P3b

Negative polarity
Latency ~400ms
Appears when pps encounter semantically unexpected stimuli
Magnitude of amplitude depends on degree of semantic anomaly

Both associated with high cognitive function

Worse resolution of perceptual conflict, stimulus perception disturbed
More confused when responding (motor conflict)

Type of motor potential
Marker of movement preparation in motor cortex
Can be detected even before a movement happens
Used to observe unconscious/latently planned movement

Choose which animal is larger in real life (animals shown as different sizes on the screen)
Size decision is fast, semantic decision is slow
Measured LRP