Introduction

Functional Near-Infrared Spectroscopy (fNIRS) is an emerging neuroimaging technique that allows non-invasive measurement of cortical brain activity by detecting changes in blood oxygenation. Using near-infrared light, it monitors hemodynamic responses associated with neuronal activation. In recent years, fNIRS has attracted increasing interest in psychiatry, neurology, and cognitive neuroscience, offering a practical alternative to established methods like fMRI and PET.

How fNIRS Works

• Basic Principle: Neuronal activity increases regional cerebral blood flow, altering concentrations of oxygenated hemoglobin (HbO) and deoxygenated hemoglobin (HbR).

• Optical Measurement: Near-infrared light (650–950 nm) penetrates the scalp and skull, with detectors measuring absorption changes that correspond to HbO and HbR fluctuations.

• Output: By comparing light absorption at multiple wavelengths, fNIRS estimates cortical activation patterns in real time.

Advantages of fNIRS

1. Non-Invasive and Safe: No radiation exposure, safe for children and vulnerable populations.

2. Portable: Unlike fMRI, fNIRS systems can be compact and even wearable, enabling bedside and classroom studies.

3. Cost-Effective: Lower equipment and maintenance costs compared to MRI or PET.

4. Tolerance of Movement: fNIRS is more resilient to motion artifacts, allowing study of naturalistic behaviors (e.g., conversation, classroom learning).

5. High Temporal Resolution: While not as precise as EEG, fNIRS captures dynamic changes in cortical activity with better temporal resolution than fMRI.

Limitations

1. Limited Depth Penetration: fNIRS mainly measures activity in superficial cortical regions (up to 2–3 cm deep), missing subcortical structures.

2. Lower Spatial Resolution: Compared to fMRI, spatial mapping is coarser.

3. Signal Contamination: Extracerebral signals (from scalp blood flow) can affect accuracy.

4. Standardization Issues: Diverse devices and protocols make cross-study comparisons difficult.

Applications in Psychiatry and Neurosciences

1. Attention-Deficit/Hyperactivity Disorder (ADHD)

• fNIRS studies have revealed reduced prefrontal activation during working memory and inhibitory control tasks.

• Promising as a potential biomarker to complement clinical diagnosis and track treatment response.

2. Depression

• Patients often show hypofrontality—reduced oxygenated hemoglobin response in the dorsolateral prefrontal cortex during cognitive tasks.

• Used to evaluate treatment effects of antidepressants, electroconvulsive therapy, and repetitive transcranial magnetic stimulation (rTMS).

3. Schizophrenia

• Abnormalities in prefrontal oxygenation patterns during verbal fluency and executive function tasks.

• Potential for use in differentiating schizophrenia from mood disorders.

4. Neurodevelopmental Disorders

• Applied to autism spectrum disorder (ASD), highlighting atypical prefrontal and temporal lobe responses during social and language processing.

5. Neurorehabilitation

• Used to assess brain activation during motor and cognitive rehabilitation post-stroke.

• Provides real-time feedback for neurofeedback-based interventions.

Future Directions

• Multimodal Integration: Combining fNIRS with EEG, fMRI, or eye-tracking to improve accuracy and interpretability.

• Wearable fNIRS: Development of lightweight, wireless devices for home and school monitoring.

• Machine Learning: Algorithms to analyze large fNIRS datasets, aiding in biomarker discovery for psychiatric and neurological disorders.

• Clinical Translation: Movement from research tool to standardized clinical application requires consensus on protocols and normative data.

Conclusion

fNIRS represents a promising bridge between neuroscience research and clinical application. Its portability, safety, and cost-effectiveness make it particularly attractive for psychiatry, where repeated, ecologically valid assessments are needed. While limitations in depth and spatial resolution remain, ongoing technological and analytic advances suggest that fNIRS could play a central role in the future of brain-based diagnostics and interventions.

Dr. Srinivas Rajkumar T
MD (Psychiatry), AIIMS New Delhi
Consultant Psychiatrist – Child, Adult & Geriatric Psychiatry
Apollo Clinic, Velachery, Chennai

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