In cases of severe hearing loss or deafness the hearing ability can be restored in parts, for example, by using a cochlear implant (CI; for a review see , ). Therefore, the CI mimics the function of the inner ear. The challenge is to objectively measure and verify whether the stimulation of the CI reaches the auditory cortex (AC) and induces the desired neural activity. This is primarily caused by the fact that the use of traditional neuroimaging modalities (i.e. EEG, MEG, fMRI) is limited due to electrical artifacts and ferromagnetic components of the CI. Therefore other, also repeatedly and frequently usable methods, have to be investigated. Functional near-infrared spectroscopy (fNIRS; for a review see ), which can be used stand alone or in combination with other modalities (multimodal recording), seems therefore a promising approach. fNIRS is an non-invasive optical technique for the assessment of cerebral oxygenation (changes of oxygenated (oxy-Hb) and deoxygenated hemoglobin (deoxy-Hb) concentration) which strongly correlates with cortical (neuronal) activity , . Recently fNIRS was proposed as a novel neuroimaging approach in CI recipients (for a first review see ). However, the existing literature is limited and mainly focusing on higher-level auditory processing (for an overview see ) or speech perception . Therefore further, basic research on lower-level auditory perception  is necessary to investigate the future use of fNIRS on CI users and the application in the field of auditory diagnostics.
In the present work we report on first pilot measurements of cortical activation patterns in normal-hearing adults with the target to generate normative data of AC activation.
2.1 Participants and experimental paradigm
The initial investigations were carried out on a group of four normal hearing subjects (three male, one female; aged 25.0 ± 4.9 years). The subjects performed a passive listening task consisting of six different auditory stimuli (1 kHz sine wave, amplitude modulated by 4 and 10 Hz sine waves; modified from ). Stimuli were presented spatially (binaural “Bin” & loudness matched  monaural left “MonL” and right “MonR”) with two different intensities (“HIGH-”: 70dB and “LOW-”: 40 dB sound pressure level, SPL). For each subject 2 (Intensity: “HIGH-”, “LOW-”) × 3 (Spatial: “Bin”, “MonL”, “MonR”) × 10 trials plus 10 additional trials without stimulation (“Silence”) were recorded in a random order (total 70 trials). Each trial consisted of an auditory stimuli presented for 10 s (“Silence”: no stimulus within this time) followed by a fixed pause of 20 s taking into account the time needed for the hemodynamic response to return back to baseline . In addition an inter-trial interval randomly varying in duration from 0 to 5 s was included to reduce temporal adjustment of the subjects to the paradigm. During the experiment the subjects lay comfortably in a semi-supine position (about 30 degrees elevation of head, Figure 1). No attempt was made to control the subjects thought content. Subjects were instructed to avoid head movements and were told to restrict movements like swallowing to the pause and inter-trial interval.
2.2 Data acquisition and processing
For fNIRS recordings a multichannel system (Imagent™, ISS Inc., Champaign, USA) was used. The system consists of 12 detectors and 16 emitters (38 channels, details see Figure 1) and enables to measure the changes of (de)oxy-Hb in the unit of mm*mm. To allow a probabilistic reference to cortical areas underlying the measurement channels, a procedure was used which projects topographical data based on skull landmarks into a 3D reference frame (MNI-space, Montreal Neurological Institute) optimized for fNIRS analysis . For each fNIRS channel position (Figure 1C), a set of MNI coordinates (x, y and z) was calculated.
For data analysis the following steps were performed: After a visual inspection of the raw data, trials containing artifacts were excluded. After high- (0.01 Hz) and low- (0.09 Hz) pass filtering global influences  were reduced by a spatial filtering approach (details see ). Mean task-related changes referred to a 5 s baseline interval prior to the task were calculated. To generate topographic distributions (de)oxy-Hb values (mean over 10–12 s; end of task) were interpolated and plotted corresponding to their spatial positions.
Subsequent first results of the measured cortical patterns (only oxy-Hb) for binaural stimulation with two different intensities (“HIGH-Bin” & “LOW-Bin”) are presented. In general, all subjects exhibited pronounced and spatially localized pattern, either for one or for both (40 dB in 3, 70 dB in four subjects) stimulus intensities. Oxy-Hb increases could be found in both hemispheres and were mainly related to channels overlaying Brodmann areas (BA) 21 & 22, 44 & 45 (Broca’s area) and 47. Comparing both intensities, significant differences (in at least three channels) in oxy-Hb responses were found in three subjects. As an example Figure 2 depicts the recorded patterns of a subject with highly pronounced contrast.
Comparing both stimulus intensities (“HIGH-Bin”–“LOW-Bin” contrast, Figure 3) this subject displays significant (p < 0.05) bilateral, but slightly more left lateralized (6 vs. 4 channels), oxy-Hb contrast increases in channels overlaying BA 21 & 22, 44 & 45 and 47. In parallel significant decreases in the oxy-Hb contrast are localized to channels overlaying BA 1 & 2 and 40.
4 Discussion and conclusion
Already with the pilot investigations it was possible to achieve promising initial results. We found pronounced activation patterns in cortical areas which are related with auditory processing. Based on these results, detailed measurements on intensity related AC activation patterns are in work. Also investigations on the comparison of binaural and monaural stimulus presentation are in progress. Future investigations also include the measurement of brain activity patterns in hearing impaired subjects and patients with cochlear implants and the comparison with normal hearing subjects.
Research funding: This work is supported by the Cluster of Excellence “Hearing4all” (EXC 1077/1). Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent has been obtained from all individuals included in this study. Ethical approval: The research related to human use complies with all the relevant national regulations, institutional policies and was performed in accordance the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.
Krueger B, Joseph G, Rost U, Strauss-Schier A, Lenarz T, Buechner A. Performance groups in adult cochlear implant users: speech perception results from 1984 until today. Otol Neurotol. 2008;29:509–12. Google Scholar
Kral A, O’Donoghue GM. Profound deafness in childhood. N Engl J Med. 2010;363:1438–50. Google Scholar
Ferrari M, Quaresima V. A brief review on the history of human functional near-infrared spectroscopy (fNIRS) development and fields of application. NeuroImage. 2012;63:921–35. Google Scholar
Malonek D, Grinvald A. Interactions between electrical activity and cortical microcirculation revealed by imaging spectroscopy: implications for functional brain mapping. Science. 1996;272:551–4. Google Scholar
Wolf M, Wolf U, Toronov V, Michalos A, Paunescu LA, Choi J, et al. Different time evolution of oxyhemoglobin and deoxyhemoglobin concentration changes in visual and motor cortices during functional stimulation: a near-infrared spectroscopy study NeuroImage. 2002;16:704–12. Google Scholar
Saliba J, Bortfeld H, Levetin D, Oghalai J. Functional near-infrared spectroscopy for neuroimaging in cochlear implant recipients. Hear Res. 2016;(in press). Google Scholar
Chen LC, Sandmann P, Thorne JD, Herrmann CS, Debener S. Association of concurrent fNIRS and EEG signatures in response to auditory and visual stimuli. Brain Topogr. 2015;28:710–25. Google Scholar
Fastl H. Binaural hearing. In: Fastl H, Zwicker E, editors. Psychoacoustics: facts and models. Berin: Springer Series in Information Sciences; 2007. p. 293–313. Google Scholar
Singh AK, Okamoto M, Dan H, Jurcak V, Dan I. Spatial registration of multichannel multi-subject fNIRS data to MNI space without MRI. NeuroImage. 2005;27:842–51. Google Scholar
Bauernfeind G, Boeck C, Wriessnegger SC, Müller-Putz GR. Physiological noise removal from fNIRS signals. Biomed Tech (Berl). 2013;58:1–2. Google Scholar
Bauernfeind G, Wriessnegger SC, Daly I, Müller-Putz GR. Separating heart and brain: on the reduction of physiological noise from multichannel functional near-infrared spectroscopy (fNIRS) signals. J Neural Eng. 2014;11:5601. Google Scholar
About the article
Published Online: 2016-09-30
Published in Print: 2016-09-01
Citation Information: Current Directions in Biomedical Engineering, Volume 2, Issue 1, Pages 229–232, ISSN (Online) 2364-5504, DOI: https://doi.org/10.1515/cdbme-2016-0051.
©2016 Günther Bauernfeind et al., licensee De Gruyter.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0