The lack of clinical laboratory tests is a major obstacle in the reliable diagnosis and quantitative treatment assessment and prevention of psychiatric disorders and in the development of patient-centric psychiatric practices. Optical topography has been approved as an insurance-covered auxiliary laboratory test for differential diagnosis of depressive state by Ministry of Health, Labour and Welfare in Japan since 2014. Near-infrared spectroscopy (NIRS), theoretical basis of optical topography, is one of functional neuroimaging techniques that has been increasingly employed in psychology and psychiatry. Because NIRS can detect only cerebral cortex reactivities with low spatial resolution and may suffer from contaminating signals from skin and skull, its data should be interpreted as a global index of cerebral cortex reactivities. Within these limitations, the advantages of NIRS over fMRI such as complete non-invasiveness, small measurement apparatus, high time resolution, and natural examination setting lead it to one of the preferred methods in studies of brain substrates of psychiatric disorders. Two-thirds of the original articles on NIRS application in psychiatry have been published by Japanese researchers. NIRS examination of major depressive disorder, bipolar disorder, and schizophrenia using a verbal fluency task of only three minutes demonstrated diagnosis-specific characteristics of frontal lobe function. These characteristics have been established as suggesting potential diagnosis of bipolar disorder or schizophrenia in clinically diagnosed major depressive disorder. In order to establish the application of NIRS as clinically useful laboratory tests in psychiatry, auxiliary nature of NIRS examination for differential diagnosis should be properly recognized both by patients and psychiatrists.
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