Electrophysiological recordings of brain activity

Question 1

micro-electrode recordings overview

Answer 1

• Highly invasive: require surgery
• High temporal resolution - measure spikes and post-synaptic potentials
• High spatial res: measure activity at the source

Question 2

EEG/ERP

Answer 2

• Non-invasive (innocuous)
• Measures post-synaptic potentials
• High temporal res, since it measures fast electrical processes
• Low spatial resolution location of activity difficult to infer

Question 3

PET

Answer 3

• Moderately invasive: radioactivity is introduced into the body
• Measures indirect met correlates or neural activity (blood flow, glucose met)
• Can measure synaptic transmission (e.g. by labelling receptors)
• High spatial res precision
• Low temporal res because it measures slow processes

Question 4

fMRI

Answer 4

• Non-invasive (innocuous)
• Measures (indirect) met correlates of neural activity (blood flow, O consumption)
• High spatial res (highest among all techniques)
• Low temporal res because it measures slow processes

Question 5

micro-electrode (single cell) recordings

Answer 5

• Electrical activity can be measured from brain tissue using very thin electrodes (micro-electrodes) inserted into the tissue
• The technique is also referred to as single-cell recordings, because typically the data are acquired from single brain cells (neurons)
• Because these recordings are invasive (one needs to perform surgery to do them), they are performed in humans only when they undergo brain surgery
• What is typically measured is the firing rate (or spike rate)- the frequency of action potentials a cell generates
• Picks up action potentials

Question 6

micro-electrode recordings in humans - study

Answer 6

• R. Quian Quiroga and colleagues (2005, Nature, 35, 1102-1107) recorded spike rates of cells in the medial temporal lobe (in and around the Hippocampus (‘what area’)) in patients undergoing monitoring for surgery
• Many showed selective responses to a specific item (e.g. face) irrespective of view, size, etc
• Reactivity of cell to picture - irrespective of size of photo and angle of view of photo - recognition zone
• 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
• Other recognisable information
• This pattern of very selective responses was not seen only for faces, but also for other pictures, e.g. famous buildings and names of famous buildings

Question 7

micro-electrode recordings advantage

Answer 7

The most direct and precise measure of brain activity

Question 8

micro-electrode recordings disadvanatges

Answer 8

• However, because it is invasive, its use is very restricted both in terms of the subjects available and brain areas that can be investigated in patients
• Only done when someone has surgery for another reason

Question 9

EEG

Answer 9

The change in voltage (electricity) recorded from sensors on the scalp

Question 10

EPSP and the resulting Field Potential

Answer 10

• A temporary deficit of positive charge develops in the area of extracellular space where sodium enters the neuron
• A temporary surplus of positive charge develops in the area near the soma, where potassium exits the cell
• Field Potential- potential measured outside the neuron
• Electrical differential between inside and outside of cell
• Depolarisation - electrical current

Question 11

the physiological basis of EEG

Answer 11

• 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 brain structures that are further down from the scalp - e.g. not from thalamus
• However, it can still detect some activity from some such structures, e.g. the hippocampus

Question 12

what information can we extract from EEG?

Answer 12

1. One can examine how rapidly the EEG signal oscillates: this is referred as frequency or spectral analysis
2. The frequency and shape of EEG can be used to inform us on sleep behaviour or detect pathologies (e.g. epilepsy)
3. Segments of the EEG associated with particular stimuli can be analysed separately: Event-Related Potentials (ERPs)

Question 13

frequency

Answer 13

• 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 noticed in the idealised waveform here:
• Signal can come from more than one specific place
• Multiple frequencies embedded in same line

Question 14

EEG and sleep

Answer 14

• Gradual slowing of the EEG (lower frequencies) as the sleep becomes deeper - progressively more inactive
• Fast (awake-like) EEG is seen in the Rapid Eye Movement sleep, during which most vividly recalled dreams are believed to occur

Question 15

using EEG frequency to detect conscious awareness

Answer 15

• Typically, higher frequency activity in the EEG is associated with greater cortical activity
• Recently, researchers have recorded high-frequency EEG over the motor cortex to assess conscious awareness in patients with vegetative state
• The first brain measurement technique to be used for this purpose was fMRI
• But - EEG is much cheaper/more portable - can be easily deployed to the patient’s bedside

Question 16

diagnostic criteria for vegetative state

Answer 16

• 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

Question 17

Cruse et al. (2011) study

Answer 17

• Subjects: a group of patients in vegetative state and a group of healthy control subjects
• Two experimental conditions, separated into distinct blocks in time
• Each condition had its own set of auditory instructions
• Condition 1: “Every time you hear a beep, try to imagine that you are squeezing your right-hand into a fist and then relaxing it. Concentrate on the way your muscles would feel if you were really performing this movement. Try to do this as soon as you hear each beep.”
• Condition 2: “Every time you hear a beep, try to imagine that you are wiggling all of the toes on both your feet, and then relaxing them. Concentrate on the way your muscles would feel if you were really performing this movement. Try to do this as soon as you hear each beep.”

Question 18

Cruse et al. results

Answer 18

• Examination of the high frequencies in the EEG over the primary motor cortex showed that:
• 3 out of the 16 patients activated the areas of the motor cortex according to the instructions
• Blue- greater activity associated with the ‘squeeze hand condition’; red- greater activity associated with the ‘wiggle toes’ condition
• Strongly suggests presence of conscious awareness in these patients
• Some conscious response to instructions

Question 19

EEG and epilepsy

Answer 19

• Abnormal/excessive synchronisation of post-synaptic potentials in epilepsy results in large amplitude discharges
• Such discharges can be observed during seizures (ictal activity) or between seizures (inter-ictal activity).
• Can be used as form of diagnosis

Question 20

ERPs

Answer 20

• ERP refers to a methodology of analysing EEG recordings… - most commonly used
• …by extracting from the EEG segments time-locked to specific “events” (stimuli or responses) - full of noise

Question 21

ERPs averaging segments

Answer 21

• Different types of stimuli are separately averaged and then compared
• Show stimuli lots and lots of times
• Same segment lots of time from same stimulus - has lots of noise but diff for each segment
• Activation from stimulus remains the same every time
• Average EEG signal - thing left is related to stimulus - random noise eventually cancels out
• Potential in EEG related to particular event
• Can use this to look at what stimulus does

Question 22

ERP components

Answer 22

• ERP components: features in the waveform (e.g. peaks)
• Labelled by order (P1- first positive peak) or latency (P100- positive, at 100 ms after stimulus)
• The size (amplitude) of these peaks is related to stimuli/tasks (e.g. P1 and N1 reflect perceptual analysis and attention; P3 is associated with a decision about the stimulus, etc)
• Same thing may happen with different stimulus - e.g. pay attention to it
• P1 always occurs

Question 23

example - the N400 component as signature of semantic processing

Answer 23

• The N400 is not simply elicited by any anomaly in the sentence: e.g. it is not elicited by a word that is physically anomalous (font size, colour) or by syntactic anomaly
• It is specific to processing semantic information (meaning)
• It is not elicited only by semantic anomalies, but also by the degree of semantic congruity
• Its amplitude is inversely proportional to the predictability of the word in a sentence (see next slide)
• Lecture slides for diagrams and more in depth information*
• N400 reflects retrieving from the lexicon the word and its meaning
• It is larger in amplitude for words that are least expected

Question 24

when we read words do we access their meaning automatically?

Answer 24

• Lien et al. (2008) used the Psychological Refractory Period (PRP) paradigm: present two tasks in rapid succession
• On some trials the interval between the stimuli from the two tasks (SOA) is very short
• Task 1: tone task- discriminate between two tones
• Task 2: word task- decide if target word is related to context word

Question 25

do we access the meaning of words automatically?

Answer 25

• If meaning is accessed automatically (if this does not require extra attention), the related vs. unrelated difference should not be affected by Task 1 (tone discrimination)
• In Lien et al.’s study the related vs. unrelated difference was reduced at short SOAs
• This suggests that the processing in Task 1 does interfere with the processing of the target meaning in Task 2

Question 26

determining the source of EEG and ERPS

Answer 26

• The EEG is obtained from the surface of the head
• Brain tissue conducts electricity well, hence FPs are conducted is all directions
• Each point on the head surface reflects cortical activity originating from distant regions
• It is very difficult to precisely pinpoint the regions of the brain where particular EEG/ERP activity originates
• Researchers have attempted to use sophisticated mathematical and bio-physical modelling techniques to overcome this problem
• As a result one can find plausible (reasonable, likely) solutions, but these solutions remain informed guesses…
• Discussion as to how useful it is
• Inverse problem: inferring cortical generators from known scalp potentials.
• Solution: highly uncertain.
• Mathematically there is an infinity of cortical current distributions that could result in one scalp distribution
• Number of possibilities is huge

Question 27

EEG strengths

Answer 27

• EEG/ERP has high temporal resolution: it can provide detailed temporal information about the processing of a stimulus - which cognitive events happen when
• The time-course of a particular component (peak) in the ERP along with it scalp topography (map) can be seen as a spatio-temporal ‘signature’ of a certain process or set of processes
• However, it has limited spatial resolution (it cannot localise activity in the brain with precision or confidence), due to the complexity of the inverse problem

Question 28

why are EEGs not sensitive to APs?

Answer 28

• the spatial extent of action potentials is too small and the time too short for them to be reflected in the EEG
• of the shape of the electrical fields they elicit