Method Flashcards
The development of human neuroimaging techniques
1930: Electroencephalography (EEG) was discovered
1970: First radiographic methods available
1980-1990: Positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) became available
1990-2000: U.S. president George H. W. Bush declared the “Decade of the Brain”
Ap recap
Post-synaptic potential is determined by integrating input of many synapses at the dendrites.
Action potential travels along the axon to all terminals.
-resting at -70mv, stimulus over threshold lead to depolarisation. follow by repolarisation also hyper polarisation
- We rarely get the chance to measure activity of neurons directly in humans
- We rely on techniques that measure activity “indirectly” and from ”the outside”
How can we measure neuron activity
- We (mostly) rely on methods that are non-invasive, meaning that we do not open the skull (or interfere with brain function)
- One class of neuroimaging methods available for human research detects frequencies in neural signals (i.e. the rate of change of the signal over time)
- 1 Hertz (Hz) = completing a full cycle (up and down) in one second
- Biological signals never contain just one frequency (as in artificial signals)
- Complex signals can be decomposed into frequency components, each has a particular frequency (e.g., 1 Hz, 2 Hz, 3 Hz, …)
- The amplitude describes how much it goes up and down
- The phase describes when it goes up and down
Methods that measure fast-changing electrical activity from outside the scalp
oMagnetoencephalography (MEG): Measures electrical activity through the magnetic fields produced by the electrical activity of neurons
oElectroencephalography (EEG): Measures small voltage fluctuations
picked up by sensitive scalp electrodes
hemodynamics
- The changes at the cell membrane leading up to the generation of action potentials and the neurotransmitter release at the synapse all require energy
- For this, oxygenated blood is transported to “active” brain regions, because oxygen is used there to produce the necessary energy
o functional Near-Infrared Spectroscopy (fNIRS): Optical imaging technique that uses light to study blood oxygenation through the skull
o Positron Emission Tomography (PET): Can measure the distribution of specific molecules in the blood (by radioactive labelling)
o functional Magnetic Resonance Imaging (fMRI): Measures local changes in blood oxygenation with high spatial resolution
How does neuron packing arragement affect signal mesurement
- Luckily for us, neurons with similar “interests” tend to “cluster” together
- This clustering happens at the level of small “columns” but also in larger “areas”, which serve (roughly) the same functions
The trade off
- We have to trade-off methods that have a high spatial resolution (e.g., fMRI) vs. methods that have a high temporal resolution (e.g., EEG)
- Finally, there are methods that are “causal”, because they rely on direct stimulation of the brain, meaning that we can study what effects this might have
- The most commonly used method is Transcranial Magnetic Stimulation (TMS)
Transcranial Magnetic Stimulation
transcranial Magnetic Stimulation is a “non-invasive” technique used to create “virtual cortical lesions”
- Studies on patients with real lesions have informed cognitive science for a long time as they allow studying what patients can’t do anymore
- E.g., Phineas Gage (1823-1860), an American railroad construction worker, who suffered a serious injury by an iron rod piercing his head and frontal cortex
- This led to severe changes in his personality
- Lesions can therefore tell us a lot about the functions of specific brain region
Why don’t we just rely on patients with natural lesions?
- Removing most parts of his hippocampus, parahippocampal gyrus and amygdala in famous patient H.M. led to severe anterograde amnesia
- In the same way, lesions in Broca and Wernicke areas have been linked to impairments of speech production and language comprehension, respectively
- However, there might not be enough patients with circumscribed lesions to study all cognitive functions
- Lesions in single, specialised areas are rare
- Recovery and brain plasticity might compensate for lesions over time è patients might become quite ‘special’ over time
How TMS work
TMS is a method to create a small, short-lived virtual lesion that is reversible
- TMS can be applied externally, using a coil placed on the scalp that produces a rapidly changing magnetic field to induce electrical currents in the brain
- These currents can depolarise neurons in a small, circumscribed area of cortex
- TMS-induced current causes neurons to fire randomly, acting as “neural noise”, thereby masking the neurons that are firing correctly
- In order to create the current pulse, which is required to generate the magnetic field, a capacitor is charged and then suddenly discharged
- In order to create a magnetic field strong enough for stimulation, very fast loading times (~100-200 μs) and short discharge durations (<1 ms) are required.
History of TMS
- Fritsch & Hitzig (1870) were the first to electrically stimulate the cortex of animals
- D’Arsonval (1896) discovered that the magnetic stimulation of the visual cortex can elicit “phosphenes”
- Magnusson & Stevens (1911) developed the first “head coil” covering the entire head
Barker, Jalinous & Freestone (1985) developed the current TMS technique, which had the great advantage of not being painful (mostly…)
rTMS
fast sequence of pulses instead of a single pulse (called: “repetitive TMS”, or rTMS in short)
coils
Different coils have been used, but the most common one is the ‘figure-eight’ coil
- The figure-eight coil generates magnetic fields generating offset current loops that circulate in opposite directions, allowing for high precision in the stimulation
- A more focal area of the cortex is stimulated using the figure-eight coil compared to the round coil (usually 3-4 mm radius, but up to 1 mm possible)
- The advantage is that the researcher knows which part of the cortex was affected
Applications of TMS in Biological Psychology Research
1) The injection of “neural noise” approach using single-pulse TMS
2) The “virtual lesion” approach using repetitive TMS
3) The “probing excitability” approach using single-pulse TMS
4) The “probing information transfer” approach using paired-pulse TMS
5) Using paired-pulse TMS to test for the decay of activity
6) Clinical applications of TMS
injection of “neural noise” approach
Using single-pulse TMS to disrupt cognitive processing
- If a single TMS pulse to a specific region of the cortex disrupts a cognitive function, this is a powerful demonstration of its causal involvement in this process
- Testing for causality is impossible using most other neuroimaging techniques, which usually rely on correlations
- One way of doing this is to interfere with the process of interest at exactly the time window during which the regions is required; e.g., to delay movements or to disrupt visual processing
- Regions do not stop working completely, but “neural noise” interferes with normal functioning
classical study of injection of “neural noise” approach
the researchers used 3 alphabetical letters as stimuli presented under difficult viewing conditions using illuminated frames/background Amassian et al., 1989
- Magnetic stimulation (i.e. TMS) was applied ~ 2 cm above the inion over visual cortex
- Effects on letter perception were investigated when varying the interval between visual stimuli and time point of TMS stimulation
It was found that during a critical period (40 – 120 ms) stimulation affected
detection performance
When shifting the stimulation site from left to right, perception of letters in
the contra-lateral visual field was predominantly impaired
- When moving the TMS stimulation from top to bottom at midline, and letters were displayed vertically, stimulation above the reference line suppressed
letters at the bottom of the display
- Stimulating below the centre was not possible (the bone was in the way)
The “virtual lesion” approach
repetitive TMS (rTMS) is used to interrupt or enhance cognitive processing
- It is also possible to inhibit cognitive functions for a longer period of time by applying repetitive TMS (rTMS)
- It can then be measured whether (and for how long) a specific cognitive task is impaired (usually slowing instead of total loss of function)
- There are strict safety guidelines for rTMS (Wassermann, 1998)
“probing excitability” approach using
single-pulse TMS
(usually over the motor system)
- For the motor system in particular, one option is to test how responsive (or “excitable”) the motor cortex is during a cognitive task
- The idea is that if the motor cortex is required for a cognitive task, then it should already be activated when single-pulse TMS is delivered
- The measure of interest is how strongly the motor cortex “reacts” to the pulse itself (any “disruption” is ignored), i.e. how strong its output is after being stimulated
- The excitability of the primary motor cortex can be measured by recording “motor evoked potentials” (MEPs) using the electromyogram (EMG) – the electrical activity of muscles
- One can then measure MEPs for each stimulation and compare average
MEPs between experimental conditions
“probing excitability” approach amd M1
- Is the primary cortex (M1) involved in the mental rotation of objects?
- Some neuroimaging studies found activation of M1 during mental rotation – which is odd as nothing is ‘really’ rotated!
- Maybe M1 involvement simply reflects in inner speech?
- Or do we use our brain in the same way for rotating real and imagined objects?
- Eisenegger and colleagues found that stimulation of M1 during mental rotation elicited stronger MEPs(Motor evoked potentials) as compared to baseline, reading aloud and reading silently
- Evidence that M1 is more excitable during mental rotation èmight be already activated, and hence, “involved” in the cognitive process
- Maybe the involvement of primary motor cortex (M1) can be explained by people imagining using their own hands for mental rotation? Kosslyn, Thompson, Wraga & Alpert, 2001; Kosslyn, Ganis, Thompson, 2001; Bode et al., 2007
- Then, M1 involvement might depend on how mental rotation is instructed?
- M1 could potentially depend on whether the stimuli automatically trigger the use of a particular strategy?
The “probing information transfer” approach
It is also possible to utilise two pulses – a technique used in the “probing information transfer” approach using paired-pulse TMS
- Uses two pulses, delivered in brief succession – the first pulse is usually sub-threshold while the second one is supra-threshold
- The question is how strongly the first pulse influences the effect of the second
Other paired-pulse TMS applications
Paired-pulse TMS is not exclusively used to test for transfer between two regions. It can also be used to test for the decay of induced activity within the same brain region.
- In Schizophrenia, abnormalities in inhibition in the motor cortex have been suggested. There is evidence that the cortical silence period (CSP) – a period of suppression of tonic motor activity that follows descending excitatory activity – is reduced
- The researchers produced the excitatory activity by a first (sub-threshold) TMS pulse to the left motor cortex, and measured the excitability by assessing the effect of a second (supra-threshold) pulse (via MEPs)
- The results show that, compared to controls, patients with and without medication showed stronger responses to the second pulse
- This result points to general deficits in motor inhibition, i.e. the induced “artificial” activity does not decay as quickly as it should
Measuring electrical activity via eeg: the basic
- Electrical currents flow from high to low voltage. We are interested in the current flow from the scalp to the ground, and the EEG measures these voltage signals
- EEG recorded at the scalp is non-invasive – however, it is also possible to record intracranial EEG by measuring activity directly at the exposed cortex
- Normal scalp EEG is cheap and (relatively) easy to conduct
The discovery of eeg
- Hans Berger (1873-1941) detected the first EEG signal in 1924 with electrodes attached to the scalp of a human (his wife) and reported the results in 1929
- Berger initially studied medicine because he was convinced that there is “psychic energy”, which might allow for telepathy
- He wanted to discover the objective activity in the brain and “psychic phenomena”, but he did not realise the basis and potential of his discovery at the time
- Initially, he used two electrodes – silver wires placed under the scalp – one attached to the front of the head and one to the rear, and recorded the potential (i.e. voltage) difference between them
- Later, he used sliver foil placed on the scalp
- Berger also first described the alpha rhythm – when people closed their eyes, the electrical signal varied with a characteristic frequency of 8-13 Hz
Measuring electrical activity via egg: nodes
- Electrode gel/paste is applied to the gap between scalp and electrode to decrease the impedance (i.e. get a better signal)
- Typical systems have 32, 64, 128, or even 256 head channels
- Some universal location of nodes: International 10-10, International 10-20, modified 10-10 for infant
- In addition, we need a ground and a reference channel, and we use other electrodes to measure eye movement and blinks
- The reference should be a neutral point (e.g., tip of the nose, mastoids), but some researchers reference to the average of all scalp electrodes