Scanning mechanisms Flashcards
What do electrodes in EEG do
pick up electrical signals from underlying neurons in order to make inferences about neural activity
What did Hans Berger discover in 1929
EEG - detected electrical activity in brain, with frequency of 8-13 hz. Described as alpha wave
What is minimum number of electrodes required to measure electrical activity
two
What is difference in temporal and spatial resolution between EEG and fMRI?
fMRI - low temporal, medium spatial resolution (where brain event happened)
EEG - very high temporal resolution, low spatial resolution (when brain event happened)
What is origin of electrical signal measured by EEG?
Summation of electrical potentials generated by millions of neurons
Where do EEG signals originate from
Cerebral cortex - tf only good for measuring activity in cortex
What neurons cause activity measured in EEGs?
Pyramidal neurons
Why can pyramidal neurons be measured in EEG?
- largely synchronised activity
- dendrites are well aligned so that electrical activity will sum together
- generally located near to scalp
Does EEG signal measure electrical activity from gyri or sulci?
Gyri [sulci measured by MEG]
What is primary limitation of EEG?
lack of spatial resolution
What are the three ways to measure the strength of an electrode?
- measure the potential difference between two different electrodes [eg voltage of F3 cf C7]
- measure electrode against ‘grounded’ electrode
- compare electrode against average of many electrodes [virtual reference]
What does one analyse in an EEG?
the periodic aspect of an EEG, and the transient aspects when presented with a stimuli
When is the periodic aspect of EEG analysed?
When subject is not stimulated and not asleep [ie with eyes closed]
What is Delta frequency range [EEG]
1-4 hz - slow wave or deep sleep
What is theta frequency range in EEG?
4-7 Hz - light sleep
What is alpha frequency range in EEG?
8-12 hz [eyes closed - no stimulation - & relaxed]
What is beta frequency range of EEG?
14-30 Hz - awake, alert, concentrating
What is gamma frequency range?
> 30 Hz - short term memory??
What is link between EEG and consciousness?
EEG measures activity and arousal - can therefore be used during operations to measure affect of anaesthetic
Why are reference electrodes essential in EEG?
To provide a point of comparison
What does BIS do?
measures dominant electrical signal in brain - used when measuring effect of anaesthetic. Gives range between 0-100.
What are clinical uses of EEG [apart from anaesthetic]
- check for life [zero electrical activity = death]
- monitor non-convulsive epileptic seizures
- confirm epileptic seizures over psychogenetic seizures
- interact with computers [eg wheelchairs]
- localise origin of epileptic seizure
What is iEEG?
intra-cranial EEG.
implanted electrodes in cranial area - typically used to determine where epilepsy originates.
more precise than EEG.
How can EEG signal be localised?
Extrapolate from electrodes across skull, work out how amplitude of signal varies
What are event related potentials?
[sometimes called evoked potentials]
Electrical activity generated by presentation of stimulus
How is evoked potential measured?
Through negative and positive polarities, and the latency of the waveforms
What is N1 component of evoked potential?
- occurs 100ms after presentation of stimulus
- occurs most strongly when stimulus is unexpected
- first studied by Pauline Davis
- disappears if subject generates stimulus
What is P3 component of evoked potential?
Endogenous potential linked to person’s reaction to stimulus, not to physical aspects of stimulus
[Endogenous = lack of expected stimulus]
Occurs when the unexpected happens, or when the expected doesn’t happen
What is N2pc component of evoked potential?
- Negative waveform that occurs 200ms after stimulus
- strongest over posterior cortex, contralateral to where observer is attending [hence pc]
Why does location of N2pc component matter?
Can tell where subject is attending [contralateral]
What are the two hypotheses re searching for objects
- Serial [all items attended in turn and serially processed]
- Parallel [all items attended simultaneously and processed in parallel
How did Woodman and Luck approach serial vs parallel hypotheses
Attended N2pc component - occurs contralaterally - so could assess where attention was.
Used coloured squares in hemisphere, asked subjects to look for square of particular colour, which 75% (C75) of time would appear in one hemisphere, 25% (C25) in another.
N2pc is strongest in contralateral hemisphere, when stimulus is expected (C75). When stimulus is unexpected (C25) attention switches (can be seen in crossing over of lines on EEG waveform)
Confirmed serial, not parallel, search processes
What are two theories of memory attention
- Theory of biased competition - any object in memory will automatically attract attention
- Neural theory of visual attention - an object in memory will only attract attention if it is subject of search
(tested by Carlisle and Woodman)
What was Carlile and Woodman’s methodology?
Used memory match distractors
Theory of biased competition - attention drawn to memory match distractor
Neural theory of visual attention - attention not drawn to memory match distractor
What were the predictions of the Theory of Biased Competition (Carlile and Woodman)
When memory match distractor is on same side as target, attention will be drawn to memory match distractor before target.
When memory match distractor is on different side to target, attention will be drawn to memory match distractor, then switch hemispheres to be drawn to target - tf N2pc will initially be strong contralateral to memory match distractor, then strong ipsilateral to memory match distractor
WHat were predictions of Neural theory of Visual Attention (Carlile and Woodman)
That memory match distractors would make no difference to N2pc
What were results of Carlile Woodman experiment?
Results supported Neural Theory of Visual Attentiveness [that there is no impact of memory objects on target search]
What is a brain lesion?
Damage that impairs functioning of a localised part of the brain
What is Broca’s area responsible for?
Speech production
What is Wernicke’s area responsible for?
Words [language comprehension]
Why is TMS good for studying effect of lesions?
- temporary and reversible
- can determine location
- can test before brain compensates
What is TMS?
Transcranial Magnetic Stimulation
Uses rapidly changing magnetic field to change electrical activity in brain
Can excite brain but more often disrupts or hinders processing and creates temporary lesion
What are basic principles TMS is based on?
Faraday’s principles of electromagnetic induction - a changing magnetic field will cause an electrical current to flow along a wire passing through the magnetic field
What is mechanical process of TMS?
Magnetic field flows around TMS coils, approx 2 Tesla for 1 ms
Magnetic field produces brief electrical current in brain, usually confined to cortex
What kinds of coils does TMS use?
Butterfly coils - two coils, each with opposing magnetic force, which produce electrical currents in opposite directions. Where coils meet, electrical currents converge and generate large localised current
What happens when TMS is applied over primary motor cortex?
Twitches
What can be generated when TMS is applied to primary visual cortex?
Phosphenes
What causes lesion from TMS?
Generally believed to be over-excitation of neurons. Random firing of neurons, masking the neurons that are firing correctly
[adding too much noise, stops communicaton]
Why use TMS over fMRI?
Just because an area lights up in fMRI doesn’t mean it is necessarily part of the brain that is working on particular task. TMS disables part of the brain - can see what is disabled as result.
Describe Amassian’s first experiment [TMS]
Used trigrams, applied TMS to primary visual cortex approx 60ms after exposure to trigrams, people couldn’t remember trigrams - hence TMS interfered w processing of info
What control experiments did Amassian do alongside primary visual cortex experiment?
Applied TMS to another part of the brain in order to rule out eg neck twitch interfering w processing
Applied very localised TMS & used retinatopical organisation of primary visual cortex to determine if just one letter of trigram could be blocked
How is primary visual cortex organised?
retinatopically - but inverted up/down and left/right
Describe Amassian’s second experiment
Presented two trigrams in quick succession. Normally second trigram would prevent first being seen. Applied TMS at time when second trigram was being processed, and first was seen.
Accuracy in reporting first trigram was greatest when time difference between presentation of second trigram and application of TMS was 100ms
What was unique about Amassian’s second experiment
Used TMS for positive effect
What are the two hypotheses as to effect of Amassian’s second experiment?
- Masking is caused because TMS increases noise in primary visual cortex
[can’t be right or the first trigram wouldn’t be reported] - Masking occurs somewhere else in the brain apart from primary visual cortex - application of TMS stops movement of second trigram beyond primary visual cortex, therefore can’t mask first.
What is important about Amassian’s second experiment?
- Shows TMS can be used for positive and negative effect
- Show how masking occurs and tf how stimuli are processed by brain
- Know that masking occurs somewhere after the primary visual cortex
- needed exact timing of TMS to show this
What is difference between fMRI and MRI?
fMRI shows activity of brain
MRI shows structure of brain
What is BOLD fMRI
Blood Oxygen Level Dependent fMRI
Neural activity requires oxygen. When part of brain is active, increase of blood supply to that part to give oxygen; overcompensation.
When neural activity increases, blood oxygen level increases.
How is BOLD fMRI measured?
Take two measures - one when brain is active, one when it’s not and subtract the second from first. Shows which parts of brain are activated for a task.
How is BOLD fMRI used in tracking objects?
Show subject 4 coloured dots among other black dots, ask them to track movement of dots when they turn black. In second experiment [control] ask them not to track movement of dots. By ssubtracting 2 from 1, can see which bits ofbrain are activated.
The protons of which atom are tracked in fMRI?
hydrogen
Why are the protons of the hydrogen atom used in fMRI?
Can act as bar magnets
What happens when hydrogen atoms are placed in magnetic field?
Try to align with magnetic field. Precesses [moves around magnetic field - rotating].
Frequency of precession is resonance frequency of proton
How does MEG measure neural activity?
measuring the changing magnetic field produced by the brain
What cells does MEG target?
post-synaptic currents of pyramidal cells of cortex
What are the detectors used in MEGs called?
SQUIDS
How is the background magnetic field in an MEG suppressed?
Shielding, gradient measures, electronically
How do gradient measures work in MEG background magnetic field suppression?
The strength of a magnetic field decreases most strongly close to the magnetic force; therefore record only the magnetic fields that vary rapidly with distance.
What are the two ways of using MEG?
temporal analysis [evoked recordings]
spatial analysis [dipole modeling]
What is an evoked recording in EEG or MEG?
a stimulus is presented to an observer and resulting change is measured
What is measured in an evoked recording in an MEG?
gradient of magnetic field [gradiometer]
absolute magnetic field strength [magnetometer]
What happens in MEG dipole modeling?
A computer analyses MEG signals to determine how many sources there are [diff waveforms], then calculates location of magnetic dipole that would lead to those magnetic fields
These dipoles are superimposed onto image of the brain
Times indicate when these sources become active relative to stimulus onset
What are limitations of dipole modeling?
- ill-posed inverse problem - multiple dipole arrangements could give rise to same magnetic fields
- sometimes neural activity is extended rather than localised
Differences between MEG and EEG
- MEG can only detect non-radially oriented magnetic dipoles, tf can’t detect neural activity at surface of brain or centre of brain [both typically radial]
Between MEG and EEG, which is better at location?
MEG [slightly]
When will a proton absorb energy?
When energy is at proton’s resonance frequency
What happens when radio frequency pulse is turned off? [fMRI]
stored energy is released; proton flips back
Do protons all flip at same time when radio frequency pulse is switched off? [fMRI]
No
What causes variations in precision frequency of protons? [fMRI]
The strength of magnetic field
What happens to resonance frequency signals in homogenous magnetic fields? [fMRI]
Add together
What happens to resonance frequency signals in inhomogenous magnetic fields [fMRI]?
cancel each other out
What’s the relationship between inhomogenous magnetic fields and the speed at which radio frequency signal decreases? [fMRI]
More inhomogenous = faster decrease of signal
What’s the relationship between blood oxygen level and the homogeneity of the magnetic field? [fMRI]
the higher the blood oxygen level, the higher the homogeneity, tf slower decrease in RF signal
What are four steps in fMRI process
- excite brain with RF pulse
- measure RF pulse emitted by brain
- measure how long RF pulse takes to decay [and tf infer neural activty in that part of brain]
- longer the decay rate, the greater the neural activity
How do you measure only one slice of brain in fMRI?
cause magnetic field to vary linearly, thereby varying the resonance frequency throughout the brain. RF pulse only resonates slice of brain where RF of protons matches RF of pulse
What does z axis represent [fMRI]
Slice of brain
What does x axis represent [fMRI]
phase encoding [magnetic field varied along x axis]
What does y axis represent [fMRI]
frequency specific readout [magnetic field varied along y axis]
Why is fMRI bad at temporal measurements?
haemodynamic lag
What is the distance within which fMRI can’t measure neural activity
1 mm
what is typical voxel measurement of fMRI?
3x3x3mm
What is safety of fMRI?
non-radioactive
magnet issues
What does a t-test measure?
compare the means of two conditions, to reject the null hypothesis that the means are the same
What is the Bonferroni Correction
when performing n t-tests, perform each t-test at 0.01/n level, so that overall level is only 1% chance of false positive
What is used to solve multiple comparisons problem in fMRI?
Bonferroni Correction
What is used to solve Non-independent sample selection problem in fMRI?
Using ROI on same subject, but in two different tests
What is used to solve over-interpretation of null results?
Not rule out brain area involvement in an activity even if result doesn’t show it
What did Kanwisher et al show?
Results are repeatable within a subject
Results are repeatable across subjects
FFA responds more strongly to faces than scrambled faces
FFA responds more strongly to faces than houses
FFA responds more strongly to faces than hands
What did Kanwisher et al identify
Fusiform Face Area
