Electrophysiological Recordings of Brain Activity

Question 1

MICRO-ELECTRODE (SINGLE-CELL) RECORDINGS

Answer 1

• 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

Question 2

HUMAN MICRO-ELECTRODE RECORDINGS

Answer 2

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)

Question 3

MICRO-ELECTRODE RECORDINGS EVALUATION

Answer 3

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

Question 4

EEG

Answer 4

• change in voltage (electricity) recorded from scalp sensors

Question 5

EPSP (EXCITATORY POSTSYNAPTIC POTENTIAL) AT DENDRITE END

Answer 5

pre-synaptic neuron -> neurotransmitter -> Na+ chemically-grated channel -> dendrite end -> K+ passive channel

Question 6

EPSP x FP (RESULTING FIELD POTENTIAL)

Answer 6

• 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

Question 7

PHYSIOLOGICAL BASIS OF EEG

Answer 7

• 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)

Question 8

EEG POSSIBLE INFO EXTRACTION

Answer 8

• 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

Question 9

FREQUENCY ANALYSIS

Answer 9

• 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)

Question 10

EEG x SLEEP

Answer 10

0. Awake.
1. Sleep.
2. Sleep.
   - gradual slowing of EEG (lower frequencies) as sleep deepens
3. Sleep.
4. Sleep.
5. REM
   - fast (awake-like) EEG seen in REM (rapid eye movement sleep) during which most vividly recalled dreams believed to occur

Question 11

EEG FREQUENCY TO DETECT CONSCIOUS AWARENESS

Answer 11

• 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

Question 12

CRITERIA FOR VEGETATIVE STATE

Answer 12

• 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

Question 13

THE LANCET

Answer 13

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

Question 14

EEG x EPILEPSY

Answer 14

• abnormal/excessive post-synaptic potentials synchronisation in epilepsy = large amplitude discharges; observable during seizures (ictal activity)/between seizures (inter-ictal activity)

Question 15

ERPs (EVENT-RELATED POTENTIALS)

Answer 15

• 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)

Question 16

N400 COMPONENT AS SEMANTIC PROCESSING SIGNATURE

Answer 16

• 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

Question 17

AUTOMATIC WORD MEANING ACCESS

Answer 17

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

Question 18

DETERMINING EEG/ERP SOURCE

Answer 18

• 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

Question 19

INVERSE EEG PROBLEM

Answer 19

• 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)

Question 20

EEG SOURCE ESTIMATION

Answer 20

• epileptic EEG distribution on scalp
• localisation based on EEG source estimation
• localisation by correlation w/concurrently acquired fMRI data

Question 21

EEG EVALUATION

Answer 21

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