Electrophysiological Recordings of Brain Activity Flashcards
MICRO-ELECTRODE (SINGLE-CELL) RECORDINGS
- electrical activity can be measured from brain tissue using v thin electrodes (micro-electrodes) inserted into tissue
- also called single-cell recordings; data gen acquired from single brain cells (neurons)
- recordings = invasive (require surgery) so performed in humans only when under brain surgery
- firing/spike rate (frequency of action potentials cell generates) gen measured
HUMAN MICRO-ELECTRODE RECORDINGS
QUIROGA et al (2005)
- recorded cell spike rates in medial temporal lobe (in/around hippocampus) in patients undergoing surgery monitoring
- many showed selective responses to specific item (ie. face) irrespective of view/size/etc.
- some cells responding to face also responded to name of person; not simply coding for visual appearance (ie. Halle Berry cell)
- pattern of selective responses not only seen for faces but other pictures (ie. famous buildings)
MICRO-ELECTRODE RECORDINGS EVALUATION
POSITIVES
- most direct/precise brain activity measure
- electric activity in brain measured non-invasively from scalp surface via EEG (electroencephalography)
LIMITS
- invasive so v restricted in pp availability/brain areas than can be investigated
EEG
- change in voltage (electricity) recorded from scalp sensors
EPSP (EXCITATORY POSTSYNAPTIC POTENTIAL) AT DENDRITE END
pre-synaptic neuron -> neurotransmitter -> Na+ chemically-grated channel -> dendrite end -> K+ passive channel
EPSP x FP (RESULTING FIELD POTENTIAL)
- temporary deficit of positive charge develops in area of extracellular space where sodium enters neuron
- temporary surplus of positive charge develops in area near soma where K+ exits cell
- field potential = potential measured outside neuron
PHYSIOLOGICAL BASIS OF EEG
- unlike micro-electrode recordings which can detect spiking activity in neurons, EEG = not sensitive to spikes (aka action potentials)
- due to spatial extent of action potentials = too small/time too short for EEG reflection
- most sensitive to cortical tissue activity nearest scalp surface as electrical fields diminish w/distance so EEG less sensitive to brain structures further from scalp
- BUT can still detect some further structure activity (ie. hippocampus)
EEG POSSIBLE INFO EXTRACTION
- frequency/spectral analysis = examining how rapidly EEG signal oscillates
- EEG frequency/shape can be used to inform us on sleep beh/detect pathologies (ie. epilepsy)
- ERPs (event-related potentials) = EEG segments associated w/particular stimuli; separately analysed
FREQUENCY ANALYSIS
- frequency refers to oscillation number p/time unit (ie. x4 p/s = 4Hz)
- EEG has complex frequency patterns (ie. several frequencies can be noticed in idealised waveform aka. high/lower/low frequencies)
EEG x SLEEP
- Awake.
- Sleep.
- Sleep.
- gradual slowing of EEG (lower frequencies) as sleep deepens - Sleep.
- Sleep.
- REM
- fast (awake-like) EEG seen in REM (rapid eye movement sleep) during which most vividly recalled dreams believed to occur
EEG FREQUENCY TO DETECT CONSCIOUS AWARENESS
- gen higher frequency activity in EEG associated w/greater cortical activity
- recently researchers recorded high-frequency EEG over motor cortex to assess conscious awareness in patients w/vegetative state
- fMRI for this BUT EEG = much cheaper/portable/easily deployable to bedside
CRITERIA FOR VEGETATIVE STATE
- no overt motor responses
- no elaborate voluntary/willed behs from upper/lower limbs
- no evidence of visual orientation
- no eye fixation >5s
- no visual/auditory stimuli tracking
THE LANCET
CRUSE et al (2011)
- vegetative patients compared w/healthy pps
- high frequency examination in EEG over primary motor cortex showed:
1. 3/16 patients activated motor cortex areas according to instructions
2. blue-greater activity associated w/”squeeze hand” condition; red-greater activity associated w/”wiggle toes” condition
3. strongly suggests presence of conscious awareness in patients
EEG x EPILEPSY
- abnormal/excessive post-synaptic potentials synchronisation in epilepsy = large amplitude discharges; observable during seizures (ictal activity)/between seizures (inter-ictal activity)
ERPs (EVENT-RELATED POTENTIALS)
- ERP = methodology of analysing EEG recordings via extracting from EEG segments time-locked to specific events (stimuli/responses)
- dif stimuli separately averaged/compared
- components = waveform features (ie. peaks)
- labelled by order (P1 -> 1st positive peak)/latency (P100 -> positive 100ms post stimulus)
- peak size (amplitude) related to stimuli/tasks (ie. P1/N1 reflect perceptual analysis/attention; P3 associated w/decision about stimulus)
N400 COMPONENT AS SEMANTIC PROCESSING SIGNATURE
- N400 not simply elicited via any anomaly in sentence (ie. not elicited by physically anomalous (font size/colour) word/syntactic anomaly)
- specific to processing semantic info (meaning)
- not elicited only by semantic anomalies BUT also via semantic congruity degree
- amplitude inversely proportional to predictability of word in sentence; N400 reflects retrieving from word lexicon/meaning
- larger in amplitude for least expected words
AUTOMATIC WORD MEANING ACCESS
LIEN et al (2008)
- PRP (Psychological Refractory Period) paradigm; present 2 tasks in rapid succession
- on some trials interval between stimuli from 2 tasks (SOA) = v short
TASK 1) Tone (discriminate between 2 tones)
TASK 2) Word (decide if target word related to context word)
- if meaning accessed automatically (doesn’t require extra attention) related VS unrelated ERP dif shouldn’t be affected by T1
- related VS unrelated dif reduced at short SOAs
- suggests T1 processing interferes w/target meaning processing in T2
DETERMINING EEG/ERP SOURCE
- EEG obtained from head surface
- brain tissue conducts electricity well hence FPs conducted in all directions
- each head surface point reflects cortical activity originating from distant regions
- v difficult to precisely pinpoint brain regions where particular EEG/ERP activity originates
- researchers attempted using sophisticated maths/bio-physical modelling techniques to overcome this
- SO one can find plausible/reasonable/likely solutions BUT remain informed guesses
INVERSE EEG PROBLEM
- inferring cortical generators from known scalp potentials = v difficult
- mathematically cannot work out what kind of cortical activation pattern would result in EEG signal (too many possibilities)
EEG SOURCE ESTIMATION
- epileptic EEG distribution on scalp
- localisation based on EEG source estimation
- localisation by correlation w/concurrently acquired fMRI data
EEG EVALUATION
POSITIVES
- high temporal resolution; provide detailed temporal info about stimulus processing
- frequency analysis allows studying sleep/epilepsy
- time-course of particular component (peak) in ERP along w/scalp topography (map) seen as spatio-temporal signature of certain process (sets)
LIMITS
- limited spatial resolution; cannot localise activity in brain w/precision/confidence due to inverse problem complexity
