## 1 Introduction  
Most listening situations include surrounding masking sounds and background noise. Nevertheless, most of the time, listeners with normal hearing (NH) are able to understand speech in such adverse listening conditions. However, hearing impaired individuals who use cochlear implants (CIs) exhibit remarkable deterioration in speech recognition in noisy conditions, with high performance variability [1, 2, 3, 4, 5, 6, 7]. This may lead to significant communication problems for CI users in many real-life listening situations [8] and may have a negative effect on linguistic and neurocognitive development [3, 9]. Multiple behavioral studies have been conducted to assess the factors that contribute to the variability in speech-in-noise (SIN) perception observed in CI users. These included NH listeners who listened via acoustic simulations of CI hearing (vocoders) and/or actual CI users (e.g., [7, 10]). In general, the suggested contributing factors include spectral and temporal resolution of the implant itself, age of hearing loss onset, use of residual hearing, mode of communication, period of auditory deprivation, auditory nerve survival, and reorganization of the central auditory system [11, 12, 13, 14, 15, 16, 17]. Neuroimaging can supplement behavioral findings in assessing the neural mechanisms underlying speech understanding of the degraded auditory signal transmitted via a CI device [18, 19, 20]. In the current study, we applied an emerging optical imaging technique, functional near-infrared spectroscopy (fNIRS), to compare neural activation patterns to speech in quiet and noisy conditions between three groups: prelingually deafened CI users, that is, individuals who became hearing impaired before their perceptual or spoken language development was completed, NH individuals who listened to spectrally degraded (vocoded) speech, and NH individuals who listened to natural speech stimuli. Commonly used neuroimaging techniques have certain disadvantages in the assessment of the cortical activation to auditory signals in CI individuals. For example, functional magnetic resonance imaging (fMRI) may have limitations in auditory research due to the impact of extraneous scanner noise [20; 21]. Additionally, artifacts associated with the CI magnet may interfere with the imaging of the temporal areas of the brain. Magnetic artifacts are also problematic in magnetoencephalography (MEG). Electroencephalography (EEG) data may also be limited by implant-related electrical artifacts, although several studies have presented new analysis techniques to overcome this problem [22; 23]. Positron emission tomography (PET) has been successfully used in some studies to visualize the brains of CI users [24; 25; 26]. However, this technique requires the injection of radioactive isotopes. An optical neuroimaging technique such as fNIRS does not have these limitations. fNIRS uses near-infrared light to noninvasively image hemodynamic responses to neuronal activity [27; 28; 29]. Specifically, changes in optical absorption are recorded across the scalp over time and converted to relative changes in the concentrations of oxygenated and deoxygenated hemoglobin, which are then mapped to the specific volumes of the activated underlying cerebral cortex regions. Because the equipment is mobile, quiet, and tolerates some movement, it is well suited for testing speech processing while participants are awake and performing various tasks [30; 31; 32; 33]. Importantly, owing to its optical nature, fNIRS is compatible with the magnetic and electronic components of CI devices, making it an ideal imaging modality for assessing brain activity in CI individuals [34; 35; 36; 37; 31; 38].