ReviewApplication of functional near-infrared spectroscopy in psychiatry
Introduction
Two decades ago, the introduction of functional near-infrared spectroscopy (fNIRS) into the field of neuroscience created new opportunities for investigating not only healthy, but also abnormal functional processes within the human cerebral cortex. Starting with basic validation studies in the early and middle 1990s (e.g., Colier et al., 1995, Duncan et al., 1996, Hoshi and Tamura, 1993, Kato et al., 1993), fNIRS has been increasingly applied in psychological studies ever since, particularly focusing on visual (e.g., Herrmann et al., 2008a, Kato et al., 1993, Obrig et al., 2002), motor (e.g., Gratton et al., 1995, Hirth et al., 1996, Plichta et al., 2006), language (e.g., Herrmann et al., 2003, Quaresima et al., 2002, Watanabe et al., 1998), and cognitive paradigms (e.g., Herrmann et al., 2005, Hoshi and Tamura, 1997, Schroeter et al., 2002) in adult subjects, as well as perceptual and language tests in infants (e.g., Bartocci et al., 2000, Kotilahti et al., 2005, Kusaka et al., 2004, Zaramella et al., 2001). Within the last decade, the scope of fNIRS research has been increasingly extended towards abnormal changes of cerebral hemodynamics during functional activation studies. To our knowledge, to date, at least 115 original research articles have employed fNIRS to investigate psychiatric research questions, ranging among over 900 research articles that applied fNIRS in brain activation studies in general (see Fig. 1). This development clearly emphasizes the increasing relevance of measuring cerebral hemodynamics in the human brain using fNIRS. Fig. 1 illustrates how the number of publications in the field of human brain research in general, as well as psychiatric neuroscience in particular, has increased especially within the past ten years.
Beside its versatile applicability, the methodological advancement of fNIRS strongly contributed to this trend. While early psychiatric neuroscience studies had to rely on relatively simple single-, two- or four-channel systems (Fallgatter et al., 1997, Hock et al., 1996, Hock et al., 1997), the extension to multi-channel solutions up to the recent introduction of a 256-channel system (NIRx Medical Technologies LLC, Glen Head, NY) has allowed for the investigation of topographic research questions. Moreover, substantial progress has been made concerning data analysis (Abdelnour and Huppert, 2009, Akgul et al., 2005, Cui et al., 2010, Plichta et al., 2007, Schroeter et al., 2004), artifact control and detection (Izzetoglu et al., 2005, Jang et al., 2009, Sato et al., 2006) and spatial registration of fNIRS data (Cutini et al., 2011, Singh et al., 2005, Tsuzuki et al., 2007, Tsuzuki et al., 2012).
Despite some remaining limitations (i.e., restricted depth and spatial resolution; confounding influence of extracranial signals and anatomical parameters; see discussion), fNIRS exhibits a number of important advantages making its application attractive for both neuroscience in general and psychiatric research in particular. First, the combinability of fNIRS with other brain imaging methods led to a strong increase in studies implementing simultaneous applications of f NIRS together with Doppler sonography (Hirth et al., 1997, Tachtsidis et al., 2008), fMRI (Heinzel et al., 2013a, Kennan et al., 2002, Lee et al., 2008, Strangman et al., 2002), EEG (e.g., Koch et al., 2008, Obrig et al., 2002), PET (Rostrup et al., 2002), and SPECT (Schytz et al., 2009). Within the combination of different imaging modalities, fNIRS can thus strongly contribute to a holistic understanding of functional characteristics underlying neuropsychiatric illnesses. As another important advantage, fNIRS enables cortical hemodynamic assessments under circumstances where other methods fail (e.g., during real-life social interaction or whole body movements). Likewise, its easy applicability and high ecological validity make fNIRS particularly suitable for psychiatric patients who may be afraid of tight surroundings (e.g., in MRI/PET scanners) or show motor restlessness (e.g., in attention-deficit/hyperactivity disorder [ADHD]) that interferes with motion-sensitive imaging methods such as MRI, EEG, MEG or PET. Taken together, the benefit of fNIRS in psychiatric research not only arises from its relative insensitivity to movement artifacts, but is further linked to its easy applicability and high versatility. Allowing for frequent measurement repetitions, fNIRS can be easily used for longitudinal studies that become more and more important for the investigation of the development and treatment of psychiatric disorders.
This review will provide a comprehensive overview of state of the art fNIRS research in psychiatry, particularly focusing on applications regarding the phenomenological characterization, treatment and etiology of psychiatric disorders as well as recent progress concerning diagnostics and classification (see Fig. 2). Published studies available in databases (pubmed, google scholar) before December 2012 were considered. Finally, research perspectives and promising future applications of fNIRS in psychiatry will be addressed.
Section snippets
Cortical alterations in psychiatric syndromes (Topic 1)
A large number of studies aimed at investigating differences between various groups of psychiatric patients and healthy control samples, in order to describe abnormal brain activity patterns potentially underlying the etiopathogenesis of a disease. In this respect, previous studies have mainly focused on schizophrenic (n = 20), affective (unipolar and bipolar depression; n = 15) and anxiety disorders (n = 9), as well as ADHD (n = 4), borderline personality disorder (n = 2), eating disorders (n = 2), and
fNIRS in the assessment of life-time brain function development (Topic 2)
About fifteen years ago, researchers started to use fNIRS to elucidate developmental aspects of cortical functions in psychiatric patients in order to complement pure phenomenological descriptions of altered brain activity. These studies include examinations at early stages of psychological anomalies in children, research on normal vs. pathological aging (e.g., Alzheimer's disease), and studies investigating certain effects of age and aging-related diseases on specific brain functions.
Using fNIRS to investigate treatment effects (Topic 3)
Aiming at the investigation of pharmacological effects, fNIRS has been used to perform either psychiatric treatment studies or directly examine the effects of pharmacological agents on neural activation in experimental challenges.
Imaging genetics (Topic 4)
Etiologically, psychiatric disorders are strongly influenced by genetic factors with estimated heritabilities of up to 60% and more (e.g., Faraone and Doyle, 2000), while the influence of specific common alleles in such non-Mendelian polygenetic disorders is as expected quite low (e.g., International Schizophrenia Consortium et al., 2009). The “imaging genetics” approach aims at connecting the genetic level and the neural network domain with respect to the pathogenesis of mental disorders.
Discussion
In this review, we present the range of fNIRS applications in psychiatric settings that relate to different major research questions (i.e. phenomenology, life-time development, treatment effects and genetic influences). Currently, the number of available studies illustrates the huge potential of the technique in psychiatric contexts, while also allowing us to assess previous shortcomings and deduce potential future applications and improvements in experimental designs as well as data analysis
Conflict of interest statement
The authors declare that they have no actual or potential conflicts of interest concerning this work.
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