1.4.1 Aim of the Study and Research Questions
Previous NIRS studies investigating the hemodynamic response of ME and MI of swallowing found that during ME there is an increase in frontal areas in HbO and in HbR, whereas during MI there is a decrease in HbO and an increase in HbR, probably reflecting inhibition of active swallowing. The strongest activations could be found over the IFG for both ME and MI (Kober, Bauernfeind, et al., 2015; Kober, Grössinger, et al., 2019; Kober &
Wood, 2014). Further neurofeedback studies showed that it is possible to upregulate HbR in the IFG during MI of swallowing. Hence, the usage of MI based neurofeedback could be a promising tool in treatment of patients suffering from dysphagia. As jaw movements result in massive temporal muscle activity (Schecklmann et al., 2017), it is assumed that execution of swallowing also leads to strong motion artifacts in the signal. So far, studies investigating neuronal correlates of swallowing with NIRS, relied on visual inspection of the signal and manual rejection of obviously visible motion artifacts (Kober, Bauernfeind, et al., 2015;
Kober & Wood, 2014, 2018). Manual rejection of artifacts is highly subjective and some motion artifacts are hard to distinguish from the HRF in the signal (Brigadoi et al., 2014;
Jahani et al., 2018; Yücel, Selb, Cooper, et al., 2014). Therefore, brain activation patterns found during ME of swallowing (Kober, Bauernfeind, et al., 2015; Kober & Wood, 2014) could still have been affected by artifacts, which questions the comparability between ME and MI of swallowing. In order to use MI of swallowing for the treatment of dysphagia, due to neural plasticity, it is important to ensure that MI leads to comparable brain activation patterns as ME of swallowing (Faralli et al., 2013; Ruffino et al., 2017; Szynkiewicz et al., 2018;
Huijuan Yang, Guan, Ang, Wang, & Yu, 2012). The latest advancements in NIRS, such as SD-channels and correction techniques, enable to gain a better insight into the real
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hemodynamic response in signals containing massive motion artifacts (Brigadoi et al., 2014;
Chen et al., 2020; Cooper et al., 2012; Yücel et al., 2021). Therefore, the first aim of this study is to validate earlier findings (Kober, Bauernfeind, et al., 2015; Kober & Wood, 2014) with a newer NIRS system using additional short distance detectors and wavelet filtering.
Previous NIRS studies investigating swallowing (Kober, Bauernfeind, et al., 2015;
Kober & Wood, 2014, 2018) handled motion artifacts with visual detection and manual rejection. In recent years, more and more methods addressing the problem of motion artifacts have been proposed. As motion artifact correction is always data dependent (Brigadoi et al., 2014) and there is no established standard procedure, it is important to specifically investigate appropriate correction method for swallowing. For real speech, wavelet filtering appears to be a promising approach (Brigadoi et al., 2014). Considering that both, jaw movements and swallowing lead to similar muscular activity and therefore create similar motion artifacts (Schecklmann et al., 2017), wavelet filtering could present a suitable approach for swallowing as well. Furthermore, SD channels showed to be a valuable addition for signals confounded with a lot of motion (Brigadoi & Cooper, 2015; Gagnon et al., 2014). The second aim of the study is, therefore, to investigate the effects of wavelet filtering and the application of SD-channels on the NIRS signal assessed during executing swallowing movements and if these artifact correction methods could present benefits over conventional manual approaches.
So far, little is known about influences leading to interindividual differences in brain activation patterns, elicited by MI. As swallowing can only be imagined via a kinesthetic MI approach, influences of visual strategies on activated brain regions (Guillot et al., 2009) can be ruled out for MI of swallowing. Besides the effects of different strategies, individuals with high MI ability have been found to produce more focused activation in motor related brain areas during MI than novices (Guillot et al., 2008; Seiler et al., 2017; Van der Meulen et al., 2014). Additionally, enhanced activation in premotor areas has been associated with higher subjective ratings of the imagination quality (Moriuchi et al., 2020; Zabicki et al., 2019) and psychological factors such as motivational aspects, as well as an individual’s body awareness seem to influence the neuronal activation during MI (Tinaz et al., 2018). Although outlining first insights into correlates of brain activation during MI, previous studies differed in types of movements that had been imagined. For the use of MI in neurorehabilitation, it is important to identify individuals that profit most from the approach (Stephan Frederic Dahm, 2020; Guillot et al., 2008). Thus, the third aim of this study is to explore relationships between brain
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activation patterns elicited by MI of swallowing and possible correlates such as kinesthetic motor imagery ability, subjective quality of imagination, motivation and body awareness.
1.4.2 Hypotheses
Hemodynamic Response of ME and MI of Swallowing
Based on the previous results from Kober & Wood (2014) it is expected that during ME there will be an increase in HbO and in HbR, whereas during MI there will be a decrease in HbO and an increase in HbR. The strongest signal changes are expected over the IFG for both tasks.
Influence of Correction Methods and SD-Detectors on NIRS Signal during Swallowing Wavelet filtering has been found to be effective in a task involving comparable motion artifacts than swallowing (Brigadoi et al., 2014). As motion artifact correction is data
dependent, and so far, no standard procedure for swallowing is established, no clear
hypothesis can be made. However, it is expected that the usage of a correction method such as Wavelet filtering and regressing SD-channels out of the NIRS signal contribute to a clearer NIRS signal compared to manual artifact rejection only. As motion artifacts lead to stronger activation changes in the NIRS signal, a clearer signal should be reflected by lower HbO and HbR levels.
Correlates of the Hemodynamic Response of MI of Swallowing
Considering the lack of literature investigating interindividual differences in brain activation elicited by MI, the third hypothesis is of an explorative nature. As earlier studies indicated possible relations between MI ability, motivation, subjectively perceived quality of MI and body awareness with neuronal activation patterns, correlations between those
variables will be further explored.
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