Electrophysiological recordings of brain activity

Question 1

What are micro-electrode recordings?

Answer 1

• highly invasive (requires surgery, usually done when surgery is being completed already)
• high temporal resolution: measure spikes and post-synaptic potentials
• high spatial resolution: measure activity at source and as it happens

Question 2

What are EEG/ERP?

Answer 2

• non-invasive
• measures post-synaptic potentials
• high temporal resolution
• low spatial resolution

Question 3

What occurs in micro-electrode recordings?

Answer 3

• electrical activity can be measured from brain tissue using thin electrodes inserted into tissue
• data is typically acquired from single brain cells
• firing rate is typically measured (frequency of action potentials a cell generates)

Question 4

What are the advantages and limitations of micro-electrode recordings?

Answer 4

• most direct and precise measure of brain activity
• use is restricted (of subjects and brain areas) due to invasiveness
• can be measured non-invasively with EEG

Question 5

What is the Halle Berry cell?

Quiroga et al, 2005

Answer 5

• recorded spike rates in the medial temporal lobe
• many showed selective responses to a specific item irrespective of view, size etc
• Halle Berry cell: cells responded to the face also responded to the name showing they aren’t coded only to visual appearance
• pattern was also found for famous buildings

Question 6

What occurs in electroencephalography (EEG)?

Answer 6

• is the change in voltage recorded from sensors on scalp
• excitatory postsynaptic potential (EPSP) at dendrite end
• EPSP and FP (field potential): temporary deficit of + charge develops in area of extracellular space where sodium enters neuron, temporary surplus of charge develops near soma where potassium exits cell

Question 7

What is the physiological basis of EEG?

Answer 7

• aren’t sensitive to action potentials due to the spatial extent of action potentials being too small and too short a time to be reflected in EEG, and shape of electrical fields doesn’t allow it
• most sensitive to activity in cortical tissue (nearest to scalp)
• since electrical fields diminish with distance the EEG is less sensitive to brain structures that are further down

Question 8

What is found by EEG during sleep?

Answer 8

• gradual slowing of EEG, lower frequencies the deeper sleep is
• fast EEG is seen in REM sleep though technically in state of paralysis as signals don’t reach the body from motor cortex
• eyes aren’t suppressed which is why they move

Question 9

How can EEG frequency detect conscious awareness?

Answer 9

• high-frequency (which is usually associated with greater cortical activity)
• high-frequency EEG over motor cortex to assess conscious awareness in those with vegetative state
• fMRI can be used but EEG is cheaper and more portable

Question 10

What is found by EEG in epilepsy?

Answer 10

• abnormal/excessive synchronisation of post-synaptic potentials in epilepsy results in large amplitude discharges
• such discharges can be observed during seizures or between them
• chaotic activity of large amounts of neurones
• distribution of the spike over the head surface

Question 11

What are event-related potentials (ERPs)?

Answer 11

• methodology of analysing EEG recordings by extracting from the EEG segments time-locked to specific stimuli/responses
• different stimuli are seperately averaged and then compared
• components: features in waveform, labelled by order or latency, size of peaks is related to stimuli/tasks

Question 12

How is the source of EEG and ERPs determined?

Answer 12

• difficult to pinpoint the regions where certain EEG/ERP activity originates due to brain tissue conducting electricity well so FPs are conducted in all directions
• EEG source estimation methods: epileptic EEG distribution on the scalp, localisation based on EEG source estimation, localisation by correlation with concurrently acquired fMRI data

Question 13

What are the strengths and weaknesses of EEG?

Answer 13

• EEG/ERP has high temporal resolution (can provide detailed temporal information about processing of stimulus)
• time-course of a certain component in ERP along with scalp topography can be seen as spatio-temporal ‘signature’ of certain/set of processes
• has limited spatial resolution though so can’t localise activity in the brain precisely