Introduction

Neuroimaging techniques have been and continue to be a dominating avenue of science since their introduction in the late nineteenth century. These techniques have diversified in today’s world, and many different kinds of imaging tools exist, each having its own strengths and weaknesses, such as spatial/temporal resolution, invasiveness, and cost.

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Electroencephalography (EEG)

Among these techniques is Electroencephalography (EEG), which is a physiological imaging technique that utilizes electrodes to measure the brain’s electrical activity non-invasively. It possesses very competent temporal resolution (within milliseconds) but does not have great spatial resolution (five to ten centimetres) due to the thickness of the skull and meninges through which the device needs to pass in order to record (Warbrick, 2022). This also means that deeper brain structures cannot always be measured with an EEG since the electrodes will only pick up signals that are so deep.

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Functional Near-Infrared Spectroscopy (fNIR)

Another advancement in neuroimaging is Functional Near-Infrared Spectroscopy (fNIR), which is used to detect blood oxygenation changes within the brain non-invasively by utilizing light. By shining light that is near infrared in wavelength, it is possible to detect how much of the light is absorbed by oxygenated and deoxygenated blood, and this difference will tell us which areas of the brain are most active relative to the others. In terms of the capabilities of fNIR, it has decent spatial resolution (around one centimetre) and poor temporal resolution (within seconds) (Li et al., 2022).

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Hybrid Imaging Technique

Since these imaging techniques both happen to be non-invasive due to their operation on the outside of the head, they could be compatible in terms of how they are used. The strengths and weaknesses of these imaging techniques are reversed in comparison to one another, and by fusing these techniques into one tool, it may be possible to create an ideal form of brain imaging. The integration of EEG and fNIR together not only seems possible but also beneficial to the overall design of the hybrid device.

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Device Design and Functionality

Both techniques work by placing the device attachments on the outside of the patient’s head. For EEG, electrodes are placed on the head, whereas fNIR uses visual sensors called optodes. The sensors can coexist on the head since it would be possible to house each pair of electrodes and optodes in the same capsules, which saves space for additional sensors. These techniques do not interfere with each other when they are in use; this is due to their modes of measurement, where one is measuring electrical signals and the other is measuring optical signals.

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Data Synchronization and Visualization

It is possible for the EEG and fNIR to acquire their data simultaneously since they are performing different actions on the brain that do not interfere with each other. They do, however, still differ in their respective temporal resolutions, with EEG having great temporal resolution and fNIR having a much slower temporal resolution. A potential method to make the collection of data closer in time is to set a clock that signals the fNIR to acquire at its normal rate while the EEG collects on a slower rate to better match data across time.

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Challenges and Considerations

There are some computational errors that may arise when fusing two imaging techniques into one modality. Both EEG and fNIR gather data and display them as varying amounts of digital activity for the researcher to view and gather information from. Having two datasets feed into one system poses the risk of overloading and stalling the database that collects the imaging results (Ahn and Jun, 2021). There is also the concern of aligning the data collection temporally, as data will be collected at two different rates, and the computer will need to be capable of recognizing this.

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Applications and Implications

This application of dual neuroimaging has relevance in research fields that are looking into the relationship of two processes within the brain, and in this case, research that is concerned with the relationship between electrical signals in the brain and the hemodynamic response seen in those same areas. By seeing how these two forms of data imply about a specific part of the brain, researchers can look into a further depth of insight regarding neural activity during events or certain external stimuli acting on the patient.

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Conclusion

Overall, a hybridized EEG-fNIR device would benefit both the research and clinical worlds, and it is worth looking into as the domain of neuroimaging is constantly evolving and improving as time goes on.

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References

  1. Ahn, S., & Jun, S. C. (2021). Multi-modal integration of EEG-fNIRS for brain-computer interfaces — current limitations and future directions. Frontiers in Human Neuroscience, 15, 645869. https://doi.org/10.3389/fnhum.2021.645869
  2. Fazli, S., Mehnert, J., Steinbrink, J., Curio, G., Villringer, A., Müller, K.-R., & Blankertz, B. (2012). Enhanced performance by a hybrid NIRS – EEG brain computer interface. NeuroImage, 59(1), 519–529. https://doi.org/10.1016/j.neuroimage.2011.07.084
  3. Flanagan, K., & Saikia, M. J. (2023). Consumer-grade electroencephalogram and functional near-infrared spectroscopy neurofeedback technologies for mental health and well-being. Sensors, 23(20), 8482. https://doi.org/10.3390/s23208482
  4. LaRocco, J., Le, M. D., & Paeng, D.-G. (2020). A systemic review of available low-cost EEG headsets used for drowsiness detection. Frontiers in Neuroinformatics, 14, Article 553352. https://doi.org/10.3389/fninf.2020.553352
  5. Li, R., Yang, D., Fang, F., Hong, K.-S., Reiss, A. L., & Zhang, Y. (2022). Concurrent fNIRS and EEG for brain function investigation: A systematic, methodology-focused review. Sensors, 22(15), 5865. https://doi.org/10.3390/s22155865
  6. Muñoz, V., Muñoz-Caracuel, M., Angulo-Ruiz, B. Y., & Gómez, C. M. (2023). Neurovascular coupling during auditory stimulation: Event-related potentials and fNIRS hemodynamic. Brain Structure and Function, 228, 1943–1961. https://doi.org/10.1007/s00429-023-02698-9
  7. Pinti, P., Aichelburg, C., Gilbert, S., Hamilton, A. F. D., Hirsch, J., Burgess, P. W., & Tachtsidis, I. (2018). The present and future use of functional near-infrared spectroscopy (fNIRS) for cognitive neuroscience. Annals of the New York Academy of Sciences, 1464(1), 5-29. https://doi.org/10.1111/nyas.13948
  8. Su, W.-C., Dashtestani, H., Miguel, H. O., Condy, E., Buckley, A., Park, S., Perreault, J. B., Nguyen, T., Zeytinoglu, S., Millerhagen, J., Fox, N., & Gandjbakhche, A. (2023). Simultaneous multimodal fNIRS-EEG recordings reveal new insights in neural activity during motor execution, observation, and imagery. Scientific Reports, 13, 31609. https://doi.org/10.1038/s41598-023-31609-5
  9. Warbrick, T. (2022). Simultaneous EEG – fMRI: What Have we Learned and What Does the Future Hold? Sensors, 22(6), 2262. https://doi.org/10.3390/s22062262