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