Task 7 fNIRS Flashcards
Basic principle of fNIRS
functional near-infrared spectroscopy
- changes in electrochemical activity and consequent change in blood levels affect properties –> way of measuring changes in HbO and Hb ratio
- oxygenated and deoxygenated Hb have different optical properties –> differences in NIR light absorption
optical windows of oxygenated and deoxygenated Hb
oxygenated Hb: 760
Deoxygenated Hb: 850
Method/setup of fNIRS
- placing sources of light and detectors on head surface:
- optode: light source (LED)
- light detector: receives reflected light
- photodetectors placed 2-7 cm away from optode
Mechanism of fNIRS
- Introduction of photons at scalp: pass through most of tissue but some are reflected by HbO2 and Hb
- 80% of photons follow curvilinear path from source to detector
- others get absorbed or are scattered –> not reaching detector - Changes in HbO2-Hb concentration cause different reflected light intensities
- quantification: Beer-Lambert Law: empirical description of optical attenuation in highly scattered medium - fNIRS measures alterations in intensity of attenuated light at different wavelengths:
- absorbance / reflectance changes at 2 different wavelengths –> changes in rel. concentration of Hbs can be calculated
fNIRS implementation
- Time-resolved system
- Frequency-domain system
–> provides information about phase and amplitude - Continuous wave spectroscopy measurement
- apply light to tissue at constant amplitude and measure attenuation of amplitude of incident light
- less information but can use LEDs rather than lasers
Strengths of fNIRS
- non-invasive
- safe
- cheap
- medium temporal resolution
- good spatial resolution
- low sensitivity to head movement
- measures 2 DVs: HbO2 and Hb
- good integration with other techniques
Limitations of fNIRS
- can’t do BS comparisons
- difficult to obtain baseline of HbO2 and Hb
- method is at an early developmental stage –> more cross-validation needed
Comparison to fMRI
- fMRI has higher spatial resolution: 1mm^3 vs. 1 cm^2
- fMRI takes picture of entire brain
- fNIRS is less invasive –> suitable for children, claustrophobics, etc.
- fNIRS is cheaper
- fNIRS can be integrated with EEG and TMS
Brain computer interfaces
basic principle: system that connects brain with a computer
BCI: method
- measuring brain activity with functional neuroimaging method
- extract specific features of the signal that can be controlled by BCI (e.g., activation level, connectivity measure, etc.)
- translate features into commands operating a device:
- rule-based algorithms
- specific temporal/spatial characteristic = specific output
- machine-learning algorithms –> improved accuracy of prediction/classification - command transferred to computer
- neurofeedback: overall task performance is relayed back to individual as sensory feedback
- allows users to regulate state of specific brain function –> achievement of better performance
BCI (EEG)
- computer detects P300 elicited when matrix elements containing the chosen alphabetic character are presented
BCI (MEG)
- real-time BCI
- can classify covert spatial attention by using modulatory property of posterior alpha rhythms –> 90% classification accuracy for motor and motor imagery tasks
- better spatial resolution and SNR than EEG
BCI (fMRI)
- receive state of own brain activity online –> voluntarily control region-specific activation
- automated interpretation and classification: extraction of spatial and temporal features of activation maps with machine-learning algorithms
- variable accuracy (50-90%)
- identification of areas that are consistently active for given task improves classification
BCI (fNIRS)
- left vs. right motor imagery
- use hemodynicamic response corresponding to P300
Direct vs. indirect encoding
Direct encoding: think what you say/do (challenging)
Indirect encoding: use activity where we know that it can be differentiated from another one
