TASK 4 - MEG

Question 1

MEG

Answer 1

= measures the magnetic signals generated by the brain

• detect the magnetic fields generated from weak electric impulses transmitted between brain cells
• right-hand rule: direction of the magnetic flux outside of the head is determined by the direction of the current within a group of neurones
• MEG used when interested in accurate time measurements (less distorted)
• -> EEG might be better for detecting deep sources of currents
• combination of MEG and/or EEG with fMRI to pinpoint where and when brain activations occur

Question 2

neural source

Answer 2

• electrical activity of neurones produces small magnetic fields that are perpendicular to the current
• femtotesla range: one ten-billionth of the size of the Earth’s steady magnetic field
• signals mainly arise from the fissures/grooves of the cortex (sulci)

Question 3

methods

- neuromagnetometer

Answer 3

• helmet-shaped: cover whole scalp, complete magnetic field pattern can be measured simultaneously without moving the instrument
• covers large area of head with 306 SQUIDs at 102 measuring sites

Question 4

methods

- SQUIDs

Answer 4

• magnetic fields are detected using highly sensitive superconducting quantum interference devices (= SQUIDs)
• transmitted to the SQUID by superconducting flux transformer
• sensors are immersed in liquid helium at -269C
• located close to the head (3cm) –> placing closer to the brain would increase the signal to noise ratio (if superconductors could work at higher liquid-nitrogen temperatures) –> less bulky insulation 1.5cm from the brain

Question 5

methods

- sensor units/measuring sites

Answer 5

• sensor units are equipped with three transformers that provide independent measurements of the magnetic field in the x, y, and z directions
  a) gradient coils: two transformers, record derivate of the radial magnetic field in the x and y directions
  b) agnetometer coil: measures the z-component

Question 6

methods

- flux transformers

Answer 6

• are housed in the concave base of a helium dewar flask that surrounds the head

Question 7

methods

- room

Answer 7

• room is made from several layers of aluminium and Mumetal (= alloy of mainly iron and nickel that has such a high magnetic permeability that external magnetic fields are trapped in it)
• try to yield the room inside from contamination of external magnetic fields (= noise)

Question 8

inverse problem

Answer 8

= identify which currents in the brain are responsible for particular MEG signals, using only information about the magnetic-field patterns

Question 9

inverse problem

- solving the problem

Answer 9

1. computer makes an initial guess where the dipole might be + uses five parameters to characterise the current dipole
   a) three parameters for position: 3D space
   b) one for its orientation: only currents that are tangential to the nearest point on the surface of the brain produce external magnetic fields
   c) one for its strength
2. computer calculates the external magnetic field that these dipoles would produce –> compares the computed field with the measured field
3. repeat calculation with the dipoles at different positions
4. until calculated and experimental results match as closely as possible
   - based on assumption that the brain is approx. spherical and its active areas can be adequately represented by a single or multiple dipoles

Question 10

multi-dipole models

Answer 10

= two or more regions are active, have time-varying source strengths

• magnetic field depends on the position and strength of the dipoles + extent to which the neurones in the different regions fire at the same time
• important for interpreting the complex field pattern that is produced

Question 11

minimum current estimate

Answer 11

• gives the most probable distribution of the currents in the brain calculated according to the concept of minimum norm
• another way of interpreting MEG measurements
• can be used without making any specific assumptions

Question 12

MEG

- advantages

Answer 12

• similar temporal resolution (better than a millisecond)

- superior spatial resolution than EEG/ERPs: magnetic signals are minimally distorted by organic tissue

Question 13

MEG

- disadvantages

Answer 13

• MEG device requires a room that is magnetically shielded from external magnetic fields: because magnetic fields generated by the brain are extremely weak
• detection only if that flow is oriented parallel to the surface of the skull
• -> some current that originate deep inside the brain are more radially oriented (use EEG)

Question 14

oscillations

Answer 14

• thalamic, thalamocortical + cortical networks are assumed to play a key role in rhythmical EEG (MEG) activities
• thalamus important for pacing of rhythmical activities
• thalamus oscillations activate firing of cortical neurones (implicated with alpha, beta, delta waves)

Question 15

oscillations

- delta band

Answer 15

= 1-4Hz; low frequency, high amplitude inhibitory rhythm

• sleep, proximity of brain lesions + tumours, during anaesthesia
• diminish with increasing age

Question 16

oscillations

- theta band

Answer 16

= 4-8Hz; low frequency, high amplitude

• mostly sleep
• 2 types during wakefulness
  1) widespread scalp distribution linked to decreased alertness (drowsiness) + impaired information processing
  2) frontal-midline distribution and associated with focussed attention, mental effort + effective stimulus processing
• generator: ACC; septo-hippocampal system, cingulate cortex

Question 17

oscillations

- alpha band

Answer 17

= 8-13Hz, amplitude between 10-45 μV
- relaxed wakefulness/ cortical “idling”/ cognitive inactivity
- greatest amplitude over posterior regions (esp. posterior occipito-temporal + parietal regions)
- alpha blockage/desynchronization: greatly diminished by eye opening, sudden alerting, mental concentration
–> attenuated when alertness decreases to level of drowsiness but then also decrease in frequency
Different alpha sub-bands are functionally dissociated
Lower alpha (8-10Hz) desynchronization (suppression) has been associated with stimulus and task-unspecific increases in attentional demands
Upper alpha (10-12Hz) desynchronization has been linked to processing of sensory-semantic information, better semantic memory performance & stimulus-specific expectancy

Question 18

oscillations

- beta band

Answer 18

= high frequency, small amplitude (10-20 μV)

• symmetric fronto-central distribution
• replaces alpha rhythm during cognitive activity
• increased excitatory activity coming with focused attention, diffuse arousal + vigilance

Question 19

oscillations

- gamma band

Answer 19

= high frequency, small amplitude

• attention, arousal, object recognition, top-down modulation of sensory processes
• directly associated with brain activation
• reflect large-scale integration of + synchrony among widely distributed neurons (esp. in states of diffusely increased vigilance)
• gamma bursts occur within periods of theta phase

Question 20

oscillations

- sampling

Answer 20

• extent to which the digital signal under investigation accurately reflects the physiological (analog) signal completely depends on the sampling rate
• sampling rate should be at least twice the highest frequency present in the signal under investigation

Question 21

oscillations

- spectral/frequency analysis

Answer 21

= estimate contribution of various frequencies to measured EEG signal

Question 22

oscillations

- time-frequency analysis

Answer 22

= shows when in time, frequency shifts occur

• short-time Fourier Transform (STFT): compute FFT-based time-dependent spectrum (= spectrogram)
• EEG signals viewed as composite sine waves with varying frequencies
• -> wavelet analyses: more adaptive approach, affording flexible resolution
• EEG signal viewed as shifted + scaled versions of a particular mathematical function

Question 23

applications

- attitudes

Answer 23

• P3: increases when a given stimulus represents a category different from the preceding stimuli
  a) not sensitive to subjects’ explicit reports about their attitudes (does not rely on self-reports)
• -> experiment: compared truthfully and falsely reported attitudes
  b) pictures of white faces more evaluatively discrepant from a negative context
• -> experiment: showed white faces, black faces + positive images in context of negative image
• -> white elicited larger P2 + N2 than black faces –> responses to white faces were more evaluatively discrepant from the negative context

Question 24

applications

- affective priming

Answer 24

• affective target words processed more quickly categorised when preceded by prime words of same valence (positive or negative)
• results from conflict during the response output stage, rather than spreading activation effects within semantic network
• under conditions where congruent trials were as likely/ more likely than incongruent trials, lateralised readiness potential (LRP) elicited by prime words showed that people activated response (before target appeared) suggested by the prime
• incongruent prime targets showed larger N2: trying to overcome initial prime-driven tendency to classify as congruent

Question 25

applications

- face perception

Answer 25

• intergroup attitudes + goals can affect the way we see faces
• N170: reflects initial process of recognising that an object is a conspecific (earliest stage of social cognition); might be sensitive to higher-level social/motivational factors
• -> experiment: whether social categories lead people to see ingroup and outgroup members’ faces differently
  a) no intergroup competition: favour own group members + are indifferent to outgroup members –> larger N170 for ingroup
  b) intergroup competition: enhanced processing of outgroup faces as they may be threatening/ otherwise noteworthy –> larger N170 for outgroup
• -> effects of individual differences, social goals and situational factors associated with the processing of ingroup and outgroup faces

Question 26

applications

- social categorisation

Answer 26

• gender and race are differentiated very quickly (before N170)
• -> experiment: oddball task in which images were (in-)consistent as a function of gender and race
• negative polarity between 65 and 165 ms in parietal regions –> represent attention to overall features of a face that are anticipated by top-down processes

Question 27

applications

- stereotyping

Answer 27

• people activate gender categories very early as they evaluate new information about a person
• P3/LPP amplitude enhanced for sentences containing definitional as well as stereotypical incongruencies (independent of overt judgement of grammatical + syntactical correctness)
• larger N400 when sentences violate gender stereotypes/social knowledge

Question 28

applications

- self-regulation

Answer 28

1. initial conflict monitoring: activity in dorsal ACC, monitors ongoing responses for conflict (plays a role in regulation of racial bias)
   - –> experiment: stereotype inhibition task
   - ERN always larger for incorrect than for correct responses –> related to ACC (ACC sensitive to unwanted stereotype-driven response tendencies)
   - occurred implicitly + operated independently of the regulative control process
2. Regulative mechanism: lateral PFC, which strengthens the influence of intentional responses (play a role in regulating racial bias)
   - -> experiment: compared truthfully and falsely reported attitudes
   - when people worry about coming across as racist, they are less sensitive to conflict monitoring
   - -> smaller ERN in response to conflict
   - people with liberal political ideology more sensitive to conflict monitoring
   - -> larger ERN