Infrared Spectroscopy Lab Report

[Endike Baur]

PolyanilinePANI), a conducting polymer, was prepared by the oxidation of aniline with ammonium peroxydisulfate in various aqueous media. When the polymerization was carried out in the solution of strong (sulfuric) acid, a granular morphology of PANI was obtained. In the solutions of weak (acetic or succinic) acids or in water, PANI nanotubes were produced. The oxidation of aniline under alkaline conditions yielded aniline oligomers. Fourier transform infrared (FTIR) spectra of the oxidation products differ. A group of participants from 11 institutions in different countries recorded the FTIR spectra of PANI bases prepared from the samples obtained in the solutions of strong and weak acids and in alkaline medium within the framework of an IUPAC project. The aim of the project was to identify the differences in molecular structure of PANI and aniline oligomers and to relate them to supramolecular morphology, viz. the nanotube formation. The assignment of FTIR bands of aniline oxidation products is reported.

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Near-infrared spectroscopy (NIRS) is a new and increasingly widespread brain imaging technique, particularly suitable for young infants. The laboratories of the McDonnell Consortium have contributed to the technological development and research applications of this technique for nearly a decade. The present paper provides a general introduction to the technique as well as a detailed report of the methodological innovations developed by the Consortium. The basic principles of NIRS and some of the existing developmental studies are reviewed. Issues concerning technological improvements, parameter optimization, possible experimental designs and data analysis techniques are discussed and illustrated by novel empirical data.

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Although novel findings have been discovered using fMRI and EEG to address the aforementioned challenges, certain limitations of the two neuroimaging technologies have hindered the investigation of neural coupling during natural verbal communication. fMRI, for example, requires subjects to lie down motionlessly in a noisy scanning environment. Simultaneous scanning of multiple individuals engaged in a face-to-face communication is impractical for fMRI based setups. EEG, on the other hand, is able to provide a more naturalistic environment. However, EEG is susceptible to muscle induced artifacts during vocalization, and is therefore less suitable for studying speaker-listener interactions15. Furthermore, the localization of sources from the EEG signal requires higher-density recordings and additional computation to solve the inverse problem16,17,18.

In this study, we propose using functional near-infrared spectroscopy (fNIRS) to investigate speaker-listener coupling as an effective complement to the existing studies. fNIRS is an optical brain imaging technology for monitoring the concentration changes of oxygenated hemoglobin (ΔHbO) and deoxygenated hemoglobin (ΔHbR) in the cortex. By utilizing portable and wearable sensors, fNIRS provides an imaging solution with high ecological validity for studying cortical hemodynamic changes in real-life contexts19. Furthermore, fNIRS has been used in natural environments, including on mobile individuals outdoors20, consistent with the neuroergonomic and mobile brain/body imaging approaches21,22. fNIRS has been adopted to study brain-to-brain coupling during a cooperation-competition game23 and a finger-tapping imitation task24. fNIRS has demonstrated usefulness for studying single brain activations during social interactions in a natural setting with traditional block design25. At present, studying brain-to-brain coupling during natural verbal communication using fNIRS has not been demonstrated.

A second objective of this study is to compare the fNIRS recorded in this study with the fMRI BOLD response recorded in the previous fMRI study14. It has been known that fNIRS and fMRI BOLD signals, both of which are based on the neurovascular coupling phenomenon, are correlated in various cognitive tasks30,31,32. However, previous studies primarily adopted block designs with simple stimulation to investigate the mean activity induced by trigger-averaging a condition over time. The relationship between fNIRS and BOLD during natural verbal communication is yet to be shown. To this end, we hypothesized that fNIRS biomarkers (ΔHbO and ΔHbR) of our listeners are correlated with the fMRI BOLD responses of the previously tested listeners14 during the comprehension of only the same English story E2 (which all of these listeners heard) and not for the other English story E1 and Turkish stories T1 and T2 (which the previously tested fMRI participants did not hear). The convergence of fNIRS and fMRI for comprehension of the same story serves as a further validation that fNIRS can be used to investigate brain-to-brain coupling during natural verbal communication.

Another interesting observation is that significant inter-subject correlations were found primarily between prefrontal areas in the speaker and parietal areas in the listeners. This result supports a previous EEG study in which the coupling between speaker and listener was found to be mainly between different channels13. In that study, listeners watched the video playback of speakers who were telling either a fairytale or the plot of their favorite movie or book. A canonical correlation analysis between the EEG of the speakers and listeners showed that coupling was mainly limited to non-homologous channels although source localization was not applied. To our best knowledge, our study presents the first fNIRS-based evidence that the brain-to-brain coupling between speaker and listener was mainly between non-homologous brain areas.

In the current study, we further compared the fMRI and fNIRS signal time courses when two groups of subjects were listening to the same audio recording of a real-life story. The neural activation of one group was recorded with fNIRS. The neural activation of the other group (the fMRI group) was recorded with fMRI in a previous study14. We first analyzed the two datasets separately with inter-subject multilevel GLM and found similar patterns of coupling between listeners in the two modalities. We then investigated the correlation between fMRI and fNIRS signals and found that ΔHbO and BOLD were significantly correlated, despite the fact that they were collected from different subjects in different recording environments, and with different techniques (fMRI vs. fNIRS). Furthermore, the fMRI voxels that were significantly correlated with fNIRS optodes were not randomly distributed but came from brain areas usually considered to be related to listening comprehension1. When the fNIRS and fMRI signals corresponding to different stories were compared for control purposes, no significant correlation was found, as expected. Significant BOLD-ΔHbR correlations were found but to a much lesser extent compared to ΔHbO (Fig. 5a). One possible explanation is the superior reliability of ΔHbO across the listeners compared to ΔHbR as shown in Fig. 1.

The current study, however, is only the first step toward studying brain coupling during natural communication using fNIRS. While fNIRS has obvious disadvantages relative to fMRI, which include coarser spatial resolution and an inability to measure signal beneath the cortical surface, it also has some advantages over fMRI. The advantages of fNIRS over fMRI include: (1) lower cost; (2) easier setup; (3) higher temporal resolution, (4) greater ecological validity, which can allow face-to-face communication (in fMRI setups subjects cannot see each other); (5) greater ease of connecting two systems simultaneously to test bi-directional dialogue-based communication; and (6) ease of use in real-life clinical communicative contexts. In recent studies, fNIRS has been used in extreme settings such as in aerospace applications41,42 and with mobile participants walking outdoors20. In the context of the current study, the true advantages of fNIRS can be exploited when the neural activation of two or more subjects is studied during face-to-face conversations in a natural context such as a classroom.

Despite these promising results, our study was limited in certain aspects. First, for the fNIRS-fMRI comparison, the spatial resolution and coverage of our fNIRS system were limited, so fMRI-fNIRS correlations were estimated between all possible voxel-optode pairs within large cortical areas or the whole brain. Second, our data from fNIRS and fMRI were not recorded simultaneously and involved different participants, so any possible between-subject differences should be taken into account when interpreting the results. Future studies, preferably with concurrent fNIRS and fMRI, can validate our findings and deepen our understanding of the fNIRS-fMRI relationship for complex natural stimuli. Potential future uses of this approach would include everyday settings such as classrooms, where we could investigate communication between a teacher and students, or in business meetings, across a speaker and attendees. However, due to the nature of the hemodynamic response that fNIRS measures, rapid communication may be difficult to analyze at short timescales (e.g., at the word-level). Future studies can investigate the temporal and spatial limits and requirements of fNIRS-based speaker-listener coupling.

fNIRS data were recorded from the listeners throughout the audio playbacks. The playback sequence always began with E2 (English story, Pie-man), and order of the remaining stories (E1, T1, and T2) was counterbalanced across subjects. Before each story playback, short samples of scrambled audio were played to the subjects so they could adjust the volume of the headphones they were wearing. Before the start and after the end of the audio story playback, there was a 15-s fixation period for stabilizing the signal. Immediately after each playback, subjects were asked to write a detailed report of the story they just heard to verify if they understood the story. Figure 6, below, shows the timeline of a story session.

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