Lecture 8: Electrophysiological Recordings of Brain Activity Flashcards
What are the two electrophysiological brain imaging techniques?
Micro-electrode recording and EEG/ERP
What are the two metabolic brain imaging techniques?
PET and fMRI
What are micro-electrode recordings?
MER involves inserting a small, high-impedance electrode (a microelectrode) into the brain tissue (parenchyma) to record spontaneous and evoked neural activity.
MER helps determine the location and function of specific brain areas, particularly during deep brain stimulation (DBS) surgery, by identifying the electrical activity patterns characteristic of different structures.
What did R. Quian Quiroga and Colleagues (2005) record through micro-electrodes?
Recorded spike rates of cells in the medial temporal love (in and around the hippocampus) in patients undergoing monitoring for surgery
- Many showed selective responses to a specific item (e.g. face) irrespective of view, size, etc
What is the Halle Berry cell? Micro-electrode recordings
Some cells that responded to the face also responded to the name of that person, showing that they were not simply coding for visual appearance
What are the advantages of micro-electrode recordings?
- The most direct and precise measure of brain activity
What is a limitation of micro-electrode recordings?
It is invasive so its use is very restricted both in terms of the subjects available and brain areas that can be investigated in patients
What does EEG stand for?
electroencephalogram
What are EEGs?
The change of voltage (electricity) recorded from sensors on the scalp
What is Excitatory Postsynaptic Potential (EPSP) at the dendrite end?
a temporary depolarization of the postsynaptic membrane, caused by the flow of positively charged ions into the cell, increasing the likelihood of an action potential.
What is field potential?
Potential measured outside the neuron
What is EPSP and the resulting field potential?
A temporary deficit of positive charge develops in the area of extracellular space where sodium enters the neuron.
A temporary surplus of positive cage develops in the area near the soma where the potassium exits the cell
What is the physiological basis of EEG?
EEG is not sensitive to the spikes in action potentials
- This is because the spatial extent of action potentials is too small and the time is too short for them to be reflected in the EEG
EEG is most sensitive to the activity in cortical tissue, which is nearest to the surface of the scalp
- Because electrical fields diminish with distance, the EEG is less sensitive to the brain structures that are further down from the scalp
What information can you extract form an EEG?
How rapidly the EEG signal oscillates:
- Frequency or spectral analysis
- The frequency and shape can be used to inform us on sleep behaviour or detect pathologies (e.g. epilepsy)
- Segments of the EEG are associated with particular stimuli can be analysed separately: ERPS
What is frequency analysis?
- Frequency refers to the number of oscillations per unit of time (e.g 4 times per second is 4 Hz)
- EEG has a complex pattern of frequencies
- For example, several frequencies can be notices in the idealised wave form
What does sleep look like on an EEG?
Gradual slowing of the EEG (lower frequencies) as the sleep becomes deeper
Fast (awake-like) EEG is seen in the Rapid Eye Movement sleep, during which most vividly recalled dreams are believed to occur
How can we use EEG frequency to detect conscious awareness?
Higher frequency = greater cortical activity
What is the diagnostic criteria for ‘vegetative state’?
- No overt motor responses to commands
- No elaborate ‘voluntary’ or ‘willed’ behaviours from the upper or lower limbs
- No evidence of visual orientation
- No eye fixation greater than 5 seconds or tracking of visual or auditory stimuli
How did Cruse etl a. 2011 use EEG frequency to detect awareness?
Subjects: a group of patients in vegetative state and another of healthy control subjects
Each condition has its own set of auditory instructions
Cond1: Every time you hear a beep, try to imagine that you are squeezing your right-hand into a fist and then relaxing it
Cond2: Every time you hear a beep, try to imagine that you are wigling all the toes on both your feet, then relaxing them
What did Cruse et al., 2011 find?
3 out of the 16 patients activated the areas of the motor cortex
- strongly suggests the presence of conscious awareness in these patients
What is epilepsy like on an EEG?
Abnormal/ excessive synchronisation of post-synaptic potentials in epilepsy results in a large amplitude discharges
- such discharges can be observed during seizures or between seizures
What does ERP stand for?
Event-Related Potentials
How to ERPs work?
Methodology of analysing EEG recording by extracting from the EEG segments time locked to specific events (stimulus or responses)
What are the components of ERPs?
Features in the waveform (e.g. peaks)
- Labelled by order or latency
- The size of the peaks is related to stimuli
How to determine the source of EEG and ERPs?
EEG is obtained from the surface of the head
- brain tissue conducts electricity well
- Each point of the head surface reflects cortical activity
- difficult to precisely pinpoint the regions where the activity originates
What are the strengths of EEGs?
- High temporal resolution
- frequency analysis allows us to study things like sleep and epilepsy
What is a weakness of EEGs?
It has limited spatial resolution
- it cannot localise activity in the brain with precision of confidence
- Due to the complexity of the inverse problem
