fNIRS & TMS
fNIRS
functional near-infrared spectroscopy
fNIRS quality
lower temporal resolution than EEG, but higher spatial resolution
higher temporal resolution than fMRI, but lower temporal resolution
Explain in short how fNIRS work
An optical, noninvasive neuroimaging technique that allows the measurement of blood tissue concentration of oxygenated (HbO2) and deoxygenated (HbR) hemoglobin following neuronal activation.
When a brain area is active and involved in the execution of a certain task, the brain’s metabolic demand for oxygen and glucose increases, increasing cerebral blood flow (CBF) in that
area. The increase in CBF in response to an increase in neuronal activity is called functional hyperemia.
The measurement is achieved by shining NIR light into the head, which will reach the brain tissue.
The interaction of the NIR light with the human tissue is either absorbed or scattered; these properties depend on whether the blood is oxygenated or not, and these differences can be measured.
three types of fNIRS
- Continuous wave fNIRS
Measures light attenuation with the modified Beer-Lambert law
cannot resolve absolute baseline concentrations of HbO2 and HbR because they cannot separate the contributions of absorption and scattering
- Time-domain fNIRS
quantifies the path length and amount of scattering of light
enabling the measurement of absolute HbO2 and HbR concentrations
- Frequency-domain fNIRS
obtains absolute concentrations, by seperating absorption and scattering
also obtaining absolute HbO2 and HbR concentrations
What is a channel?
The tissue interrogated by the light between a source and a detector.
two ways to configure fNIRS technology
- Fixed source-detector separations, providing a topographical representation of concentration changes over the cortical surface
- Diffuse Optical Tomography (DOT); uses much denser arrays of channels with multiple source-detector distances that sample overlapping brain volumes to provide a tomographical representation
What is the signal-to-noise ratio (SNR)?
A measure of the strength of a desired signal relative to the level of background noise.
A high SNR means the desired signal is easily distinguishable from the noise, while a low SNR means the signal is weak and may be obscured by noise
10 advantages of fNIRS
- safe
- tolerant to bodily movement
- portable
- suitable for all populations
- suitable for diverse setting (e.g. outside the lab)
- low cost
- compatible with other neuroimaging techniques
- silent
- can measure oxygenated AND deoxygenated hemoglobin
- widely available
5 disadvantages of fNIRS
- limited penetration depth (only cortical)
- no structural or anatomical information
- lack of standardization in analysis
- systemic blood flow changes introduce noise into the measured hemodynamic signals
Name some places fNIRS might provide good application
- Neurodevelopmental studies
- Atypical development
- Psychiatric conditions
- Motortasks
- Neurorehabilitation
- EF studies
- Social cognitive neuroscience
- Naturalistic environments
- Low-resource settings
- PFC studies
- Hyperscanning (multiple participants)
- Quiet environment
what is the most common analysis technique of fNIRS and what is the biggest limitation?
GLM relies on knowing the precise timeline of stimuli, This is described as a "behavior-first" approach.
What are the novel aspects of fNIRS?
- go outside a lab setting
- can be used in low-resource settings
- especially informative for the PFC
- exploring social neuroscience (interaction with others)
- Hyperscanning; chaining together recordings of two or more people
TMS
transcranial magnetic stimulation
TMS quality
low temporal and spatial resolution, but more capable of making inferences
In short, how does TMS work?
It works on the principles of electromagnetic induction; an electrical current passing through one coil can induce a current in a nearby coil. The current in the first coil produces a magnetic field that, in turn, causes current to flow in the second coil.
In TMS, that second coil is replaced by brain tissue, and the induced electric field elicits neuronal activity. The TMS machine delivers a large current in a short period of time,
which produces a magnetic field that induces an electric field sufficient to stimulate neurons or alter the resting membrane potentials in the underlying cortex.
What is chronometry?
The measurement of the timing and time course of mental events or information processing in the brain.
Allows researchers to move beyond simply localizing where brain activity occurs to understanding when specific cognitive processes are engaged and how they unfold over time.
three types of TMS
1. Single pulse TMS
inducing an electric field sufficient to stimulate neurons or change resting membrane potentials
2. repetitive TMS
Depending on the frequency and duration of the pulses, rTMS can either inhibit (create a "virtual lesion") or excite a targeted brain area
3. paired pulse TMS
This technique uses two coils to deliver pulses in rapid succession to the same or different brain areas, which relates to the broader use of TMS to examine dynamic connectivity and functional connections in the brain
What is Hebbian learning?
"Cells that fire together, wire together."
paired TMS -> If TMS causes neurons in a network to fire together repeatedly, the synaptic connections between them may strengthen (long-term potentiation, LTP), consistent with Hebbian learning.
Name some applications for TMS
- perception
- attention
- learning
- plasticity
- language
- disorders
- treatment
- connectivity
- brain lesions
Name 4 limitations of TMS
- can only reach cortical regions
- when trying to reach deeper cortical regions, might target the overlying regions as well
- hypotheses for spatial and temporal aspects are required
- effects connected sites
What is diaschisis?
a sudden change of function in a portion of the brain connected to a distant, but damaged, brain area. (rewiring).
- Results from TMS studies can sometimes challenge those from classical lesion studies