Near infrared spectroscopy (NIRS) can be used to measure changes in oxygenated and de-oxygenated haemoglobin using near-infrared light (Siesler, Ozaki, Kawata & Heise, 2008). NIRS involves delivering infrared light into the head via a NIRS source. Infrared light from the NIRS source is absorbed or scattered as it enters tissue inside the head, such as the brain. The concentration of de-oxygenated haemoglobin and oxygenated haemoglobin affects how the infrared light is absorbed. As a result, changes in the oxygenation of haemoglobin will affect how much infrared light is absorbed by the brain. The extent of scattering and absorption is measured by a NIRS detector, which is placed near the NIRS source. This provides a mean of non-invasively assessing motor and cognitive activities, which are conventionally assessed using functional magnetic resonance imaging (fMRI) (Hoshi, 2005).
NIRS can also assess changes in haemoglobin oxygenation whilst participant’s are moving their entire body, which cannot be achieved with fMRI. For example, recent evidence has shown that fNIRS can be used to ‘encourage’ subjects to recruit the anterior prefrontal cortex (aPFC) in a neurofeedback procedure whilst they completed a movement imagery task (Ota et al., 2020). The use of NIRS as a means of neurofeedback improved performance in a movement rehabilitation task. Moreover, NIRS also enabled neuroimaging during a movement task, revealing M1 and S1 and premotor oxygenation changes (Ota et al., 2020). This is an exciting means of usIng NIRS as a neurofeedback technique to improve rehabilitation outcomes, which are measured by a participant’s ability to move.
Despite its high spatial resolution, the combination of functional magnetic resonance imaging with TMS is very challenging. For instance, TMS coils introduce artefact is MRI measurements (eg. Bungert et al., 2010; Weiskopf et al., 2009), not to mention the difficulty associated with constructing coils that withstand the strong magnetic field in the MR environment. NIRS is an alternative measure that can be used to measure changes in oxygenation and avoid many of the difficulties associated with combining TMS and fMRI. NIRS can also be used in populations that are difficult to assess with fMRI, such as children or can simply exist as a chapter alternative. NIRS relies on an optical signal, where the scattering and absorption of light emitted by the NIRS source is measured by a NIRS detector. As a result NIRS is capable of measuring changes in the concentration of oxygenated an de-oxygenated haemoglobin can be measured during the application of brain stimulation in brain sites underneath and proximal to the site of stimulation. Recent experiments have applied repetitive TMS at different frequencies and reveal that increases in oxygenated haemoglobin when TMS applied at 2Hz and 5 Hz but not at 1 Hz (Cao et al., 2013). Given the the extraction of artefact free signals during brain stimulation can be challenging (e.g. Rogasch et al., 2017; Dowsett & Herrmann, 2016; Kohli & Casson, 2019), NIRS is a promising means of assessing the consequences of non-invasive brain stimulation.
A number of effects of TMS have been revealed on NIRS measurements. Groiss et al. (2013) applied four pulses of TMS to M1 and measured the consequences of these pulses using NIRS above M1, the prefrontal cortex and the primary sensory cortex bilaterally. NIRS revealed a decrease in in oxygenated haemoglobin compared to sham within M1, immediately beneath the site of stimulation (Groiss et al., 2013). Decreases in oxygenated haemoglobin were also observed within premotor cortex, a site functionally and anatomically connected to M1 (Groiss et al., 2013). To conclude, NIRS is a promising approach that has already been demonstrated to be sensitive to TMS, which can be used in the future to reveal causal evidence for the involvement of cortical regions in neurophysiological processes.
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