Neuroscience Methods: TMS; EEG and ERP Flashcards
1
Q
what would be an ideal method for measuring brain activity?
A
- something that has spatial resolution on a cellular level
- something that has temporal resolution on a millisecond scale
- something that can study the whole brain simultaneously
- something that is non invasive
2
Q
what is cytoarchitectonics?
A
- an anatomical method based on segmenting the brain according to its appearance under the microscope
- has a fine spatial resolution
- Although the method is based on structural information only, the microscopic appearance in many cases reflects the predominant types of cells, which would reflect the function
3
Q
what do the cytoarchitectonic features of the brain areas correspond to?
A
The cytoarchitectonic features of the brain areas correspond with their main functions, output from the brain in case of motor cortex and input to the brain in case of primary somatosensory cortex.
4
Q
what is a disadvantage of cytoarchitectonic methods?
A
- unusual for cytoarchitectonic studies to include large numbers of brains, due the time-and labour-intensive process
- Only studies with larger numbers of participants allow to study inter-individual variation in cytoarchitectonics.
5
Q
what is the Brodmann area (BA) 4?
A
Primary motor cortex
6
Q
what is the Brodmann area 3,1,2?
A
primary somatosensory cortex
7
Q
what is the Brodmann area 17?
A
primary visual cortex
8
Q
what is the Brodmann area 41?
A
primary auditory cortex
9
Q
what does TMS stand for?
A
transcranial magnetic stimulation
10
Q
what are some features of TMS?
A
- the method has millisecond temporal resolution
- can resolve within cortical maps (e.g. motor homunculus in primary motor cortex)
- but TMS can only be applied to one single location at any given time point
11
Q
what is the process of TMS?
A
- simulator pressed above scalp, contains a coil wire
- a brief pulse of high electrical energy current fed through the coil
- results in a magnetic field with flux lines perpendicular to the plane of the coil
- the magnetic field induces electrical field perpendicular to the magnetic field
- electric field leads to neuronal excitation within the brain (trans-cranial)
12
Q
what are some advantages of TMS?
A
- non-invasive
- painless
- safe stimulation of human brain cortex
- temporal resolution in millisecond range
- short duration of experiment to reduce risk of plasticity
13
Q
what has TMS been used to study?
A
- Behaviour during experimentally contrlled “virtual brain lesions” which are fully reversible
- chronometry in brain activation
- functional connectivity.
14
Q
what happens when you stimulate the motor cortex TMS?
A
- activates corticospinal neurons trans-synaptically
- record motor EPs (surface EMG, target muscle relaxed)
- record silent period in contracted target muscles approx 150ms after motor cortex stimulus
15
Q
what happens when you stimulate the somatosensory cortex with TMS?
A
has excitatory effects e.g. the phosphenes; inhibitory effects: suppression of motion perception and letter identification
16
Q
what happens when you stimulate the auditory cortex?
A
interpretation of results is challenging
- loud coil clicks
17
Q
what happens when you stimulate the front cortex stimulation?
A
- effects subjects mood?
- potential for therapeutic use
18
Q
How are the effects of TMS measured?
A
measured as a peripheral response, as impaired or altered perception, as improved or impaired task performance, or as brain’s direct response (detected in EEG, PET and fMRI)
19
Q
TMS example 1: Chronometry (Hamilton and Pascual-Leone, 1998)
A
- Chronometry: the science of accurate measurement
- used single pulse TMS for mental chronometry
- ps were three early blind subjects
- Method: tactile stimuli in shape of braille or braille letters applied to tip of the right middle finger, two conditions were meaningful braille of nonsense patterns
- TMS coil placed over either left somatosensory cortex, or over left occipital cortex
- Results: TMS to somatosensory cortex has a strong inhibitory effect on detection and perception, and inhibition is strongest for TMS applied at 20ms after tactile stimulus
- Interpretation: Early blind participants occipital cortex is functionally important to processes braille by touch
- experiment shows crossmodal plasticity because the occipital cortex processes visual input in sighted individuals whereas here, in the early blind participants the occipital cortex has developed into an area of higher-order processing of tactile input
20
Q
TMS example 2: visual lesions (Cohen et al. 1997)
A
- early blind participants in one group and sighted participants who can read Braille in a control group
- Participants read Braille script and errors are recorded
- error rate in this task was similar between control and experimental condition
- A train stimuli were applied before the task, repetitive TMS can induce temporary inhibition of brain areas in a fully reversible way
- The interpretation of the differences between sighted and early blind individuals is that occipital cortex is essential to early blind participants’ but not to sighted participants’ performance, thereby illustrating cortical plasticity.
21
Q
what are some disadvantages of TMS?
A
- spatial undersampling (only one area at a time – although there is paired-pulse TMS with two different coils, assessing the effect of a conditioning stimulus from coil 1 on the response to a later test stimulus from coil 2
- only cortical areas accessible
- auditory cortex stimulation problematic (muscles)
- loud coil click, need “sham stimulation” as control condition
22
Q
How do EEGs measure the electrical activity of the brain?
A
- Electrodes are placed on the ps scalp, in a spatial pattern
- electrodes are evenly spaced to cover the whole scalp
- A typical setup comprises electrodes over frontal, temporal, parietal, and occipital cortices of both hemispheres.
23
Q
what is a beta rhythm?
A
- 13-30 Hz: most evident frontally, dominant rhythm when subject is alert, eyes open
24
Q
what is an alpha rhythm?
A
- 7-13 Hz
- occipital maximum
- dominant when subject is relaxed with eyes closed
- blocked by opening the eyes or by onset of mental effort (“Berger effect”)
25
what is a theta rhythm?
- 3-7 HZ
- "slow activity"
- rare in adults when awake but perfectly normal in children (up to 13 years) and sleep
26
what is a delta rhythm?
- less than 3 Hz
- dominant rhythm in infants (up to ~ 1 year) and stages 3 and 4 of sleep
27
what are spontaneous EEG and how are they used?
an EEG without experimentally controlled sensory stimuli and without task, containing ongoing oscillations in multiple frequency bands is used for example in sleep studies to assess sleep stage
28
what are event- related occilliations?
stimulus- or task-related changes in EEG oscillations, in terms of frequency or amplitude; temporal resolution tens to hundreds of milliseconds
29
what are event-related potentials?
waveforms defined in terms of latency relative to an event such as a sensory stimulus; obtained through time-locked averaging of EEG; temporal resolution tens of milliseconds
30
what is an example of an event related EEG oscilliation?
- lateralised occipital-parietal alpha oscillations during visual spatial attention, observed as event-related change to ongoing alpha oscillations
- The experiment evaluates EEG from left and right occipital electrode
- raw EEG signal obtained from left and right occipital electrodes will look similar to what is shown in the six-second EEG traces
- ongoing oscillations of 10 cycles per second (alpha) which disappear during periods of spatially directed attention (like the spontaneous alpha rhythm during eyes closed is diminished when eyes are opened)
- Both graphs show decrease (suppression) of alpha rhythm during the cue-target interval (relative to alpha rhythm at the start of the trial) when the participant expects the upcoming target
- The suppression is strongest for rightward directed attention at left occipital electrodes and for leftward directed attention at right occipital electrodes
- This pattern of results is as predicted from the neuroanatomy of the visual pathway: After a cue directing attention to the right hemifield, suppression of alpha oscillations is observed in the left occipito-parietal region of interest (ROI) and vice versa
31
How are ERPs recorded?
ERPs are recorded as averaged EEG epochs in two steps:
- record EEG trials, time-locked to the event of interest
- each trial contains ERP and voltage fluctuations that are not time-locked to the event
Under certain assumptions (!) averaging increases the signal-to-noise ratio of the ERP signal
32
what are exogenous ERPs?
automatic responses of the brain, controlled by physical properties of stimulus
33
what are endogenous ERPs?
reflect interaction between, subject and event
34
what are mesogenous ERPs?
semi-automatic but modulated by cognitive processes (attention, memory etc.)
35
what problem does topography highlight when using ERPs?
- topography highlights a difficulty in using ERs for localisation of neural sources
- The problem here is that ERP topography associated with bilateral temporal cortex responses cannot be distinguished from the topography associated with a midline frontocentral response
- That’s because source localization for ERPs means to determine neural generators whose activity results in scalp-recorded potential.
36
what is mismatch negativity (MMN)?
- This waveform is typically recorded in a passive auditory oddball paradigm with frequent "standard" tones and “deviant” tones that differ from standards in pitch, intensity, or duration.
- there is no instruction to attend to many auditory stimuli
37
how can mismatch negativity be used in studies with patients with schizophrenia?
- decrease in MMN amplitude (compared with healthy control group)
- attenuation stronger for duration deviants than for frequency deviants
- attenuated MMN also in first-degree relatives of schizophrenic patients who are at increased risk for schizophrenia, reflecting genetic vulnerability
38
how can mismatch negativity be used in children dyslexia?
- reduction in amplitude (frequency-deviant MMN)
- reduction correlated with the severity of dyslexia
