Investigating the Effect of Data Length on the Performance of Frequency-Domain fNIRS Functional Connectivity Measures

Resting-state functional connectivity is a promising tool for understanding and characterizing brain network architecture. However, obtaining uninterrupted long recording of resting-state data is challenging in many clinically relevant populations. Moreover, the interpretation of connectivity results may heavily depend on the data length and functional connectivity measure used. We compared the performance of three frequency-domain connectivity measures: magnitude-squared, wavelet and multitaper coherence; and the effect of data length ranging from 3 to 9 minutes. Performance was characterized by distinguishing two groups of channel pairs with known different connectivity strengths. While all methods considered improved the ability to distinguish the two groups with increasing data lengths, wavelet coherence performed best for the shortest time window of 3 minutes. Knowledge of which measure is more reliably used when shorter fNIRS recordings are available could make the utility of functional connectivity biomarkers more feasible in clinical populations of interest.

Brain Functional Connectivity Networks do not Return to Resting-state During Control Trials in Block Design Experiments. Annu Int Conf IEEE Eng Med Biol Soc 2023;2023:1-4. [PMID: 38083712 DOI: 10.1109/embc40787.2023.10340143] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Many studies on morphology analysis show that if short inter-stimulus intervals separate tasks, the hemodynamic response amplitude will return to the resting-state baseline before the subsequent stimulation onset; hence, responses to successive tasks do not overlap. Accordingly, popular brain imaging analysis techniques assume changes in hemodynamic response amplitude subside after a short time (around 15 seconds). However, whether this assumption holds when studying brain functional connectivity has yet to be investigated. This paper assesses whether or not the functional connectivity network in control trials returns to the resting-state functional connectivity network. Traditionally, control trials in block-design experiments are used to evaluate response morphology to no stimulus. We analyzed data from an event-related experiment with audio and visual stimuli and resting state. Our results showed that functional connectivity networks during control trials were more similar to that of tasks than resting-state networks. In other words, contrary to task-related changes in the hemodynamic amplitude, where responses settle after a short time, the brain's functional connectivity networks do not return to their intrinsic resting-state network in such short intervals.

A Parametric Model for Characterizing Time-Variant Single Trials of Block-Design fNIRS Experiments

Block-design is a popular experimental paradigm for functional near-infrared spectroscopy (fNIRS). Traditional block-design analysis techniques such as generalized linear modeling (GLM) and waveform averaging (WA) assume that the brain is a time-invariant system. This is a flawed assumption. In this paper, we propose a parametric Gaussian model to quantify the time-variant behavior found across consecutive trials of block-design fNIRS experiments. Using simulated data at different signal-to-noise ratios (SNRs), we demonstrate that our proposed technique is capable of characterizing Gaussian-like fNIRS signal features with ≥3dB SNR. When used to fit recorded data from an auditory block-design experiment, model parameter values quantitatively revealed statistically significant changes in fNIRS responses across trials, consistent with visual inspection of data from individual trials. Our results suggest that our model effectively captures trial-to-trial differences in response, which enables researchers to study time-variant brain responses using block-design fNIRS experiments.

Connections between spatially distant primary language regions strengthen with age during infancy, as revealed by resting-state fNIRS

Objective.Hearing is an important sensory function that plays a key role in how children learn to speak and develop language skills. Although previous neuroimaging studies have established that much of brain network maturation happens in early childhood, our understanding of the developmental trajectory of language areas is still very limited. We hypothesized that typical development trajectory of language areas in early childhood could be established by analyzing the changes of functional connectivity in normal hearing infants at different ages using functional near-infrared spectroscopy.Approach.Resting-state data were recorded from two bilateral temporal and prefrontal regions associated with language processing by measuring the relative changes of oxy-hemoglobin (HbO) and deoxy-hemoglobin (HbR) concentrations. Connectivity was calculated using magnitude-squared coherence of channel pairs located in (a) inter-hemispheric homologous and (b) intra-hemispheric brain regions to assess connectivity between homologous regions across hemispheres and two regions of interest in the same hemisphere, respectively.Main results.A linear regression model fitted to the age vs coherence of inter-hemispheric homologous test group revealed a significant coefficient of determination for both HbO (R²= 0.216,p= 0.0169) and HbR (R²= 0.206,p= 0.0198). A significant coefficient of determination was also found for intra-hemispheric test group for HbO (R²= 0.237,p= 0.0117) but not for HbR (R²= 0.111,p= 0.0956).Significance.The findings from HbO data suggest that both inter-hemispheric homologous and intra-hemispheric connectivity between primary language regions significantly strengthen with age in the first year of life. Mapping out the developmental trajectory of primary language areas of normal hearing infants as measured by functional connectivity could potentially allow us to better understand the altered connectivity and its effects on language delays in infants with hearing impairments.

Contralateral dominance to speech in the adult auditory cortex immediately after cochlear implantation

Sensory deprivation causes structural and functional changes in the human brain. Cochlear implantation delivers immediate reintroduction of auditory sensory information. Previous reports have indicated that over a year is required for the brain to reestablish canonical cortical processing patterns after the reintroduction of auditory stimulation. We utilized functional near-infrared spectroscopy (fNIRS) to investigate brain activity to natural speech stimuli directly after cochlear implantation. We presented 12 cochlear implant recipients, who each had a minimum of 12 months of auditory deprivation, with unilateral auditory- and visual-speech stimuli. Regardless of the side of implantation, canonical responses were elicited primarily on the contralateral side of stimulation as early as 1 h after device activation. These data indicate that auditory pathway connections are sustained during periods of sensory deprivation in adults, and that typical cortical lateralization is observed immediately following the reintroduction of auditory sensory input.

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Auditory activity was present on the contralateral side directly after implantation

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Visual-evoked cross-modal activity was also present on the contralateral side

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Monaural auditory stimulation elicited bilateral activity in listeners with two CIs

Effects of degraded speech processing and binaural unmasking investigated using functional near-infrared spectroscopy (fNIRS)

The present study aimed to investigate the effects of degraded speech perception and binaural unmasking using functional near-infrared spectroscopy (fNIRS). Normal hearing listeners were tested when attending to unprocessed or vocoded speech, presented to the left ear at two speech-to-noise ratios (SNRs). Additionally, by comparing monaural versus diotic masker noise, we measured binaural unmasking. Our primary research question was whether the prefrontal cortex and temporal cortex responded differently to varying listening configurations. Our a priori regions of interest (ROIs) were located at the left dorsolateral prefrontal cortex (DLPFC) and auditory cortex (AC). The left DLPFC has been reported to be involved in attentional processes when listening to degraded speech and in spatial hearing processing, while the AC has been reported to be sensitive to speech intelligibility. Comparisons of cortical activity between these two ROIs revealed significantly different fNIRS response patterns. Further, we showed a significant and positive correlation between self-reported task difficulty levels and fNIRS responses in the DLPFC, with a negative but non-significant correlation for the left AC, suggesting that the two ROIs played different roles in effortful speech perception. Our secondary question was whether activity within three sub-regions of the lateral PFC (LPFC) including the DLPFC was differentially affected by varying speech-noise configurations. We found significant effects of spectral degradation and SNR, and significant differences in fNIRS response amplitudes between the three regions, but no significant interaction between ROI and speech type, or between ROI and SNR. When attending to speech with monaural and diotic noises, participants reported the latter conditions being easier; however, no significant main effect of masker condition on cortical activity was observed. For cortical responses in the LPFC, a significant interaction between SNR and masker condition was observed. These findings suggest that binaural unmasking affects cortical activity through improving speech reception threshold in noise, rather than by reducing effort exerted.

Reducing false discoveries in resting-state functional connectivity using short channel correction: an fNIRS study

Significance: Functional near-infrared spectroscopy (fNIRS) is a neuroimaging tool that can measure resting-state functional connectivity; however, non-neuronal components present in fNIRS signals introduce false discoveries in connectivity, which can impact interpretation of functional networks. Aim: We investigated the effect of short channel correction on resting-state connectivity by removing non-neuronal signals from fNIRS long channel data. We hypothesized that false discoveries in connectivity can be reduced, hence improving the discriminability of functional networks of known, different connectivity strengths. Approach: A principal component analysis-based short channel correction technique was applied to resting-state data of 10 healthy adult subjects. Connectivity was analyzed using magnitude-squared coherence of channel pairs in connectivity groups of homologous and control brain regions, which are known to differ in connectivity. Results: By removing non-neuronal components using short channel correction, significant reduction of coherence was observed for oxy-hemoglobin concentration changes in frequency bands associated with resting-state connectivity that overlap with the Mayer wave frequencies. The results showed that short channel correction reduced spurious correlations in connectivity measures and improved the discriminability between homologous and control groups. Conclusions: Resting-state functional connectivity analysis with short channel correction performs better than without correction in its ability to distinguish functional networks with distinct connectivity characteristics.

Development of an Australian behavioural method for assessing listening task difficulty at high speech intelligibility levels

OBJECTIVE

To develop and validate an Australian version of a behavioural test for assessing listening task difficulty at high speech intelligibility levels.

DESIGN

In the SWIR-Aus test, listeners perform two tasks: identify the last word of each of seven sentences in a list and recall the identified words after each list. First, the test material was developed by creating seven-sentence lists with similar final-word features. Then, for the validation, participant's performance on the SWIR-Aus test was compared when a binary mask noise reduction algorithm was on and off.

STUDY SAMPLE

All participants in this study had normal hearing thresholds. Nine participants (23.8-56.0 years) participated in the characterisation of the speech material. Another thirteen participants (18.4-59.1 years) participated in a pilot test to determine the SNR to use at the validation stage. Finally, twenty-four new participants (20.0-56.9 years) participated in the validation of the test.

RESULTS

The results of the validation of the test showed that recall and identification scores were significantly better when the binary mask noise reduction algorithm was on compared to off.

CONCLUSIONS

The SWIR-Aus test was developed using Australian speech material and can be used for assessing task difficulty at high speech intelligibility levels.

Spectral features of cortical auditory evoked potentials inform hearing threshold and intensity percepts in acoustic and electric hearing

Objective. Stimulus-elicited changes in electroencephalography (EEG) recordings can be represented using Fourier magnitude and phase features (Makeiget al(2004Trends Cogn. Sci.8204-10)). The present study aimed to quantify how much information about hearing responses are contained in the magnitude, quantified by event-related spectral perturbations (ERSPs); and the phase, quantified by inter-trial coherence (ITC). By testing if one feature contained more information and whether this information was mutually exclusive to the features, we aimed to relate specific EEG magnitude and phase features to hearing perception.Approach.EEG responses were recorded from 20 adults who were presented with acoustic stimuli, and 20 adult cochlear implant users with electrical stimuli. Both groups were presented with short, 50 ms stimuli at varying intensity levels relative to their hearing thresholds. Extracted ERSP and ITC features were inputs for a linear discriminant analysis classifier (Wonget al(2016J. Neural. Eng.13036003)). The classifier then predicted whether the EEG signal contained information about the sound stimuli based on the input features. Classifier decoding accuracy was quantified with the mutual information measure (Cottaris and Elfar (2009J. Neural. Eng.6026007), Hawelleket al(2016Proc. Natl Acad. Sci.11313492-7)), and compared across the two feature sets, and to when both feature sets were combined.Main results. We found that classifiers using either ITC or ERSP feature sets were both able to decode hearing perception, but ITC-feature classifiers were able to decode responses to a lower but still audible stimulation intensity, making ITC more useful than ERSP for hearing threshold estimation. We also found that combining the information from both feature sets did not improve decoding significantly, implying that ERSP brain dynamics has a limited contribution to the EEG response, possibly due to the stimuli used in this study.Significance.We successfully related hearing perception to an EEG measure, which does not require behavioral feedback from the listener; an objective measure is important in both neuroscience research and clinical audiology.

The use of broad vs restricted regions of interest in functional near-infrared spectroscopy for measuring cortical activation to auditory-only and visual-only speech

As an alternative to fMRI, functional near-infrared spectroscopy (fNIRS) is a relatively new tool for observing cortical activation. However, spatial resolution is reduced compared to fMRI and often the exact locations of fNIRS optodes and specific anatomical information is not known. The aim of this study was to explore the location and range of specific regions of interest that are sensitive to detecting cortical activation using fNIRS in response to auditory- and visual-only connected speech. Two approaches to a priori region-of-interest selection were explored. First, broad regions corresponding to the auditory cortex and occipital lobe were analysed. Next, the fNIRS Optode Location Decider (fOLD) tool was used to divide the auditory and visual regions into two subregions corresponding to distinct anatomical structures. The Auditory-A and -B regions corresponded to Heschl's gyrus and planum temporale, respectively. The Visual-A region corresponded to the superior occipital gyrus and the cuneus, and the Visual-B region corresponded to the middle occipital gyrus. The experimental stimulus consisted of a connected speech signal segmented into 12.5-sec blocks and was presented in either an auditory-only or visual-only condition. Group-level results for eight normal-hearing adult participants averaged over the broad regions of interest revealed significant auditory-evoked activation for both the left and right broad auditory regions of interest. No significant activity was observed for any other broad region of interest in response to any stimulus condition. When divided into subregions, there was a significant positive auditory-evoked response in the left and right Auditory-A regions, suggesting activation near the primary auditory cortex in response to auditory-only speech. There was a significant positive visual-evoked response in the Visual-B region, suggesting middle occipital gyrus activation in response to visual-only speech. In the Visual-A region, however, there was a significant negative visual-evoked response. This result suggests a significant decrease in oxygenated hemoglobin in the superior occipital gyrus as well as the cuneus in response to visual-only speech. Distinct response characteristics, either positive or negative, in adjacent subregions within the temporal and occipital lobes were fairly consistent on the individual level. Results suggest that temporal regions near Heschl's gyrus may be the most advantageous location in adults for identifying hemodynamic responses to complex auditory speech signals using fNIRS. In the occipital lobe, regions corresponding to the facial processing pathway may prove advantageous for measuring positive responses to visual speech using fNIRS.

No Evidence That Music Training Benefits Speech Perception in Hearing-Impaired Listeners: A Systematic Review

As musicians have been shown to have a range of superior auditory skills to non-musicians (e.g., pitch discrimination ability), it has been hypothesized by many researchers that music training can have a beneficial effect on speech perception in populations with hearing impairment. This hypothesis relies on an assumption that the benefits seen in musicians are due to their training and not due to innate skills that may support successful musicianship. This systematic review examined the evidence from 13 longitudinal training studies that tested the hypothesis that music training has a causal effect on speech perception ability in hearing-impaired listeners. The papers were evaluated for quality of research design and appropriate analysis techniques. Only 4 of the 13 papers used a research design that allowed a causal relation between music training and outcome benefits to be validly tested, and none of those 4 papers with a better quality study design demonstrated a benefit of music training for speech perception. In spite of the lack of valid evidence in support of the hypothesis, 10 of the 13 papers made claims of benefits of music training, showing a propensity for confirmation bias in this area of research. It is recommended that future studies that aim to evaluate the association of speech perception ability and music training use a study design that differentiates the effects of training from those of innate perceptual and cognitive skills in the participants.

Applications of Phenomenological Loudness Models to Cochlear Implants

Cochlear implants electrically stimulate surviving auditory neurons in the cochlea to provide severely or profoundly deaf people with access to hearing. Signal processing strategies derive frequency-specific information from the acoustic signal and code amplitude changes in frequency bands onto amplitude changes of current pulses emitted by the tonotopically arranged intracochlear electrodes. This article first describes how parameters of the electrical stimulation influence the loudness evoked and then summarizes two different phenomenological models developed by McKay and colleagues that have been used to explain psychophysical effects of stimulus parameters on loudness, detection, and modulation detection. The Temporal Model is applied to single-electrode stimuli and integrates cochlear neural excitation using a central temporal integration window analogous to that used in models of normal hearing. Perceptual decisions are made using decision criteria applied to the output of the integrator. By fitting the model parameters to a variety of psychophysical data, inferences can be made about how electrical stimulus parameters influence neural excitation in the cochlea. The Detailed Model is applied to multi-electrode stimuli, and includes effects of electrode interaction at a cochlear level and a transform between integrated excitation and specific loudness. The Practical Method of loudness estimation is a simplification of the Detailed Model and can be used to estimate the relative loudness of any multi-electrode pulsatile stimuli without the need to model excitation at the cochlear level. Clinical applications of these models to novel sound processing strategies are described.

Comparing fNIRS signal qualities between approaches with and without short channels

Functional near-infrared spectroscopy (fNIRS) is a non-invasive technique used to measure changes in oxygenated (HbO) and deoxygenated (HbR) hemoglobin, related to neuronal activity. fNIRS signals are contaminated by the systemic responses in the extracerebral tissue (superficial layer) of the head, as fNIRS uses a back-reflection measurement. Using shorter channels that are only sensitive to responses in the extracerebral tissue but not in the deeper layers where target neuronal activity occurs has been a ‘gold standard’ to reduce the systemic responses in the fNIRS data from adults. When shorter channels are not available or feasible for implementation, an alternative, i.e., anti-correlation (Anti-Corr) method has been adopted. To date, there has not been a study that directly assesses the outcomes from the two approaches. In this study, we compared the Anti-Corr method with the ‘gold standard’ in reducing systemic responses to improve fNIRS neural signal qualities. We used eight short channels (8-mm) in a group of adults, and conducted a principal component analysis (PCA) to extract two components that contributed the most to responses in the 8 short channels, which were assumed to contain the global components in the extracerebral tissue. We then used a general linear model (GLM), with and without including event-related regressors, to regress out the 2 principal components from regular fNIRS channels (30 mm), i.e., two GLM-PCA methods. Our results found that, the two GLM-PCA methods showed similar performance, both GLM-PCA methods and the Anti-Corr method improved fNIRS signal qualities, and the two GLM-PCA methods had better performance than the Anti-Corr method.

Objective measurement of tinnitus using functional near-infrared spectroscopy and machine learning

Chronic tinnitus is a debilitating condition which affects 10-20% of adults and can severely impact their quality of life. Currently there is no objective measure of tinnitus that can be used clinically. Clinical assessment of the condition uses subjective feedback from individuals which is not always reliable. We investigated the sensitivity of functional near-infrared spectroscopy (fNIRS) to differentiate individuals with and without tinnitus and to identify fNIRS features associated with subjective ratings of tinnitus severity. We recorded fNIRS signals in the resting state and in response to auditory or visual stimuli from 25 individuals with chronic tinnitus and 21 controls matched for age and hearing loss. Severity of tinnitus was rated using the Tinnitus Handicap Inventory and subjective ratings of tinnitus loudness and annoyance were measured on a visual analogue scale. Following statistical group comparisons, machine learning methods including feature extraction and classification were applied to the fNIRS features to classify patients with tinnitus and controls and differentiate tinnitus at different severity levels. Resting state measures of connectivity between temporal regions and frontal and occipital regions were significantly higher in patients with tinnitus compared to controls. In the tinnitus group, temporal-occipital connectivity showed a significant increase with subject ratings of loudness. Also in this group, both visual and auditory evoked responses were significantly reduced in the visual and auditory regions of interest respectively. Naïve Bayes classifiers were able to classify patients with tinnitus from controls with an accuracy of 78.3%. An accuracy of 87.32% was achieved using Neural Networks to differentiate patients with slight/ mild versus moderate/ severe tinnitus. Our findings show the feasibility of using fNIRS and machine learning to develop an objective measure of tinnitus. Such a measure would greatly benefit clinicians and patients by providing a tool to objectively assess new treatments and patients' treatment progress.

Interpreting the Effect of Stimulus Parameters on the Electrically Evoked Compound Action Potential and on Neural Health Estimates

Variations in the condition of the neural population along the length of the cochlea can degrade the spectral and temporal representation of sounds conveyed by CIs, thereby limiting speech perception. One measurement that has been proposed as an estimate of neural survival (the number of remaining functional neurons) or neural health (the health of those remaining neurons) is the effect of stimulation parameters, such as the interphase gap (IPG), on the amplitude growth function (AGF) of the electrically evoked compound action potential (ECAP). The extent to which such measures reflect neural factors, rather than non-neural factors (e.g. electrode orientation, electrode-modiolus distance, and impedance), depends crucially upon how the AGF data are analysed. However, there is currently no consensus in the literature for the correct method to interpret changes in the ECAP AGF due to changes in stimulation parameters. We present a simple theoretical model for the effect of IPG on ECAP AGFs, along with a re-analysis of both animal and human data that measured the IPG effect. Both the theoretical model and the re-analysis of the animal data suggest that the IPG effect on ECAP AGF slope (IPG slope effect), measured using either a linear or logarithmic input-output scale, does not successfully control for the effects of non-neural factors. Both the model and the data suggest that the appropriate method to estimate neural health is by measuring the IPG offset effect, defined as the dB offset between the linear portions of ECAP AGFs for two stimuli differing only in IPG.

Perception and prediction of loudness in sound coding strategies using simultaneous electric stimulation

Cochlear Implant (CI) sound coding strategies based on simultaneous stimulation lead to an increased loudness percept when compared to sequential stimulation using the same current levels. This is due to loudness summation as a result of channel interactions. Studying the loudness perception evoked by dual-channels compared to single-channels can be useful to optimize sound coding strategies that use simultaneous current pulses. Fourteen users of HiRes90k implants and one user of a CII implant loudness balanced single-channel to dual-channel stimuli with varying distance between simultaneous channels. In this study each component of a dual channel was a virtual channel, which shared current across two adjacent electrodes. Balancing was performed at threshold and comfortable level, for two spatial references (apical and basal) and for dual-channels with different relative current ratios. Increasing distance between dual-channels decreased the amount of current compensation in the dual-channel required to reach equal loudness to a single channel component by an average of 0.24 dB / mm without a significant difference between threshold and most comfortable level. If the components of the dual-channels were not at equal loudness, the loudness summation was reduced with respect to the equal loudness case. The results were incorporated into an existing loudness model by McKay et al. (2003). The predictions from the adapted model were evaluated by comparing the loudness evoked by simultaneous and sequential sound coding strategies. The application of the adapted model resulted in a deviation between predicted and actual behavioral loudness balancing adjustments in electrical level between simultaneous and sequential processing strategies of 0.24 dB on average.

The Effects of Frequency Response on Speech Perception for Cochlear Implant Users

The aim of this study was to investigate the effects on speech perception of manipulating filter gains in a cochlear implant speech processor. Five implantees, who use the CI22 implant and Spectra processor manufactured by Cochlear Ltd, participated. Four experimental maps were created that were identical to their clinical map except for the profile of gains across the filters. Experimental gain profiles had rising or falling gains across the frequency range, or emphasized or de-emphasized the middle frequencies, relative to the clinical map. Perception of CNC (consonant-vowel-consonant) words at 70 dB SPL was significantly better with the clinical map than with all experimental maps, whereas at the lower level (60 dB SPL) there was minimal difference between the maps, with the low-frequency emphasis map giving significantly better scores than the high-frequency emphasis map. Perception of sentences at 70 dB SPL with a signal-to-noise ratio of +10 dB was better with the high-frequency emphasis map than with the low-frequency emphasis map. None of these best-conditions, however, were statistically better than the clinical map. The results highlighted the importance of signal audibility for speech perception with cochlear implants.

Benefits of Syllabic Input Compression for Users of Cochlear Implants

Ten users of multielectrode cochlear implants participated in an evaluation of the perceptual effects of input-signal compression. A syllabic compressor was introduced into the microphone circuit of Spectra-22 or SPrint sound processors. The post-compression gain was adjusted to provide similar loudness for speech at an average level of 65 dBA with compression either enabled or disabled. Sentence recognition was measured at three levels. Averaged across all listeners, statistically significant score increases were obtained at each level with compression enabled (45 dBA: 19.6 percentage points, p < .0001; 55 dBA: 16.6 percentage points, p < .0001; 70 dBA: 3.1 percentage points, p = .031). A test of speech intelligibility in noise showed no significant effect of compression. Generally, participants in the trial reported improved perception of low-level sounds with compression, although a few disliked the increased loudness of some background noises. Some participants suggested that the ability to enable or disable compression with a manual switch would be helpful. Overall, the results show that input compression can improve the performance of these sound processors for users of cochlear implants, especially when listening to speech at low levels.

Optimizing Frequency-to-Electrode Allocation in Cochlear Implants

This study tested the hypothesis that speech perception of cochlear implant users could be improved by increasing the number of electrodes allocated to frequencies below 2.6 kHz, with correspondingly fewer electrodes allocated above this point. Eight users of the Nucleus CI22 implant participated and wore experimental maps in which the widths of the analysis filters were altered to provide either two or three extra low-frequency channels. Speech perception was tested following periods of take-home experience. Information transmission analysis of vowel and consonant confusions appear to support our hypothesis, showing that vowel first formant information was significantly better perceived with the experimental map and that consonant information was not significantly different. The results of CNC word testing appear to contradict this result, showing that the experimental map degraded both vowel and consonant perception. Overall, the experimental map had a small and variable effect on speech perception. These results do not support our hypothesis and indicate that further research is needed to investigate the possible effects of narrowing the filter bandwidth in low frequencies, as done in this experiment.

Cortical Pitch Response Components Correlate with the Pitch Salience of Resolved and Unresolved components of Mandarin Tones

Cortical pitch responses (CPRs) are generated at the initiation of pitch-bearing sounds. CPR components have been shown to reflect the pitch salience of iterated rippled noise with different temporal periodicity. However, it is unclear whether features of the CPR correlate with the pitch salience of resolved and unresolved harmonics of speech when the temporal periodicity is identical, and whether CPRs could be a neural index for auditory cortical pitch processing. In this study, CPRs were recorded to two speech sounds: a set including only resolved harmonics and a set including only unresolved harmonics. Speech-shaped noise preceding and following the speech was used to temporally discriminate the neural activity coding the onset of acoustic energy from the onset of time-varying pitch. Analysis of CPR peak latency and peak amplitude (Na) showed that the peak latency to speech sounds with only resolved harmonics was significantly shorter than for sounds with unresolved harmonics (p = 0.01), and that peak amplitude to sounds with only resolved harmonics was significantly higher than for sounds with unresolved harmonics (p <; 0.001). Further, the CPR peak phase locking value in response to sounds with only resolved harmonics was significantly higher than to sounds with only unresolved harmonics (p <; 0.001). Our findings suggest that the CPR changes with pitch salience and that CPR is a potentially useful indicator of auditory cortical pitch processing.

Audio-visual integration in cochlear implant listeners and the effect of age difference

This study aimed to investigate differences in audio-visual (AV) integration between cochlear implant (CI) listeners and normal-hearing (NH) adults. A secondary aim was to investigate the effect of age differences by examining AV integration in groups of older and younger NH adults. Seventeen CI listeners, 13 similarly aged NH adults, and 16 younger NH adults were recruited. Two speech identification experiments were conducted to evaluate AV integration of speech cues. In the first experiment, reaction times in audio-alone (A-alone), visual-alone (V-alone), and AV conditions were measured during a speeded task in which participants were asked to identify a target sound /aSa/ among 11 alternatives. A race model was applied to evaluate AV integration. In the second experiment, identification accuracies were measured using a closed set of consonants and an open set of consonant-nucleus-consonant words. The authors quantified AV integration using a combination of a probability model and a cue integration model (which model participants' AV accuracy by assuming no or optimal integration, respectively). The results found that experienced CI listeners showed no better AV integration than their similarly aged NH adults. Further, there was no significant difference in AV integration between the younger and older NH adults.

Cortical Speech Processing in Postlingually Deaf Adult Cochlear Implant Users, as Revealed by Functional Near-Infrared Spectroscopy

An experiment was conducted to investigate the feasibility of using functional near-infrared spectroscopy (fNIRS) to image cortical activity in the language areas of cochlear implant (CI) users and to explore the association between the activity and their speech understanding ability. Using fNIRS, 15 experienced CI users and 14 normal-hearing participants were imaged while presented with either visual speech or auditory speech. Brain activation was measured from the prefrontal, temporal, and parietal lobe in both hemispheres, including the language-associated regions. In response to visual speech, the activation levels of CI users in an a priori region of interest (ROI)—the left superior temporal gyrus or sulcus—were negatively correlated with auditory speech understanding. This result suggests that increased cross-modal activity in the auditory cortex is predictive of poor auditory speech understanding. In another two ROIs, in which CI users showed significantly different mean activation levels in response to auditory speech compared with normal-hearing listeners, activation levels were significantly negatively correlated with CI users’ auditory speech understanding. These ROIs were located in the right anterior temporal lobe (including a portion of prefrontal lobe) and the left middle superior temporal lobe. In conclusion, fNIRS successfully revealed activation patterns in CI users associated with their auditory speech understanding.

Assessing hearing by measuring heartbeat: The effect of sound level

Functional near-infrared spectroscopy (fNIRS) is a non-invasive brain imaging technique that measures changes in oxygenated and de-oxygenated hemoglobin concentration and can provide a measure of brain activity. In addition to neural activity, fNIRS signals contain components that can be used to extract physiological information such as cardiac measures. Previous studies have shown changes in cardiac activity in response to different sounds. This study investigated whether cardiac responses collected using fNIRS differ for different loudness of sounds. fNIRS data were collected from 28 normal hearing participants. Cardiac response measures evoked by broadband, amplitude-modulated sounds were extracted for four sound intensities ranging from near-threshold to comfortably loud levels (15, 40, 65 and 90 dB Sound Pressure Level (SPL)). Following onset of the noise stimulus, heart rate initially decreased for sounds of 15 and 40 dB SPL, reaching a significantly lower rate at 15 dB SPL. For sounds at 65 and 90 dB SPL, increases in heart rate were seen. To quantify the timing of significant changes, inter-beat intervals were assessed. For sounds at 40 dB SPL, an immediate significant change in the first two inter-beat intervals following sound onset was found. At other levels, the most significant change appeared later (beats 3 to 5 following sound onset). In conclusion, changes in heart rate were associated with the level of sound with a clear difference in response to near-threshold sounds compared to comfortably loud sounds. These findings may be used alone or in conjunction with other measures such as fNIRS brain activity for evaluation of hearing ability.

Auditory Brainstem Representation of the Voice Pitch Contours in the Resolved and Unresolved Components of Mandarin Tones

Accurate perception of voice pitch plays a vital role in speech understanding, especially for tonal languages such as Mandarin. Lexical tones are primarily distinguished by the fundamental frequency (F0) contour of the acoustic waveform. It has been shown that the auditory system could extract the F0 from the resolved and unresolved harmonics, and the tone identification performance of resolved harmonics was better than unresolved harmonics. To evaluate the neural response to the resolved and unresolved components of Mandarin tones in quiet and in speech-shaped noise, we recorded the frequency-following response. In this study, four types of stimuli were used: speech with either only-resolved harmonics or only-unresolved harmonics, both in quiet and in speech-shaped noise. Frequency-following responses (FFRs) were recorded to alternating-polarity stimuli and were added or subtracted to enhance the neural response to the envelope (FFR_ENV) or fine structure (FFR_TFS), respectively. The neural representation of the F0 strength reflected by the FFR_ENV was evaluated by the peak autocorrelation value in the temporal domain and the peak phase-locking value (PLV) at F0 in the spectral domain. Both evaluation methods showed that the FFR_ENV F0 strength in quiet was significantly stronger than in noise for speech including unresolved harmonics, but not for speech including resolved harmonics. The neural representation of the temporal fine structure reflected by the FFR_TFS was assessed by the PLV at the harmonic near to F1 (4th of F0). The PLV at harmonic near to F1 (4th of F0) of FFR_TFS to resolved harmonics was significantly larger than to unresolved harmonics. Spearman's correlation showed that the FFR_ENV F0 strength to unresolved harmonics was correlated with tone identification performance in noise (0 dB SNR). These results showed that the FFR_ENV F0 strength to speech sounds with resolved harmonics was not affected by noise. In contrast, the response to speech sounds with unresolved harmonics, which were significantly smaller in noise compared to quiet. Our results suggest that coding resolved harmonics was more important than coding envelope for tone identification performance in noise.

Temporal Coding of Voice Pitch Contours in Mandarin Tones

Accurate perception of time-variant pitch is important for speech recognition, particularly for tonal languages with different lexical tones such as Mandarin, in which different tones convey different semantic information. Previous studies reported that the auditory nerve and cochlear nucleus can encode different pitches through phase-locked neural activities. However, little is known about how the inferior colliculus (IC) encodes the time-variant periodicity pitch of natural speech. In this study, the Mandarin syllable /ba/ pronounced with four lexical tones (flat, rising, falling then rising and falling) were used as stimuli. Local field potentials (LFPs) and single neuron activity were simultaneously recorded from 90 sites within contralateral IC of six urethane-anesthetized and decerebrate guinea pigs in response to the four stimuli. Analysis of the temporal information of LFPs showed that 93% of the LFPs exhibited robust encoding of periodicity pitch. Pitch strength of LFPs derived from the autocorrelogram was significantly (p < 0.001) stronger for rising tones than flat and falling tones. Pitch strength are also significantly increased (p < 0.05) with the characteristic frequency (CF). On the other hand, only 47% (42 or 90) of single neuron activities were significantly synchronized to the fundamental frequency of the stimulus suggesting that the temporal spiking pattern of single IC neuron could encode the time variant periodicity pitch of speech robustly. The difference between the number of LFPs and single neurons that encode the time-variant F0 voice pitch supports the notion of a transition at the level of IC from direct temporal coding in the spike trains of individual neurons to other form of neural representation.

Intensity Discrimination and Speech Recognition of Cochlear Implant Users

The relation between speech recognition and within-channel or across-channel (i.e., spectral tilt) intensity discrimination was measured in nine CI users (11 ears). Within-channel intensity difference limens (IDLs) were measured at four electrode locations across the electrode array. Spectral tilt difference limens were measured with (XIDL-J) and without (XIDL) level jitter. Only three subjects could perform the XIDL-J task with the amount of jitter required to limit use of within-channel cues. XIDLs (normalized to %DR) were correlated with speech recognition (r = 0.67, P = 0.019) and were highly correlated with IDLs. XIDLs were on average nearly 3 times larger than IDLs and did not vary consistently with the spatial separation of the two component electrodes. The overall pattern of results was consistent with a common underlying subject-dependent limitation in the two difference limen tasks, hypothesized to be perceptual variance (how the perception of a sound differs on different presentations), which may also underlie the correlation of XIDLs with speech recognition. Evidence that spectral tilt discrimination is more important for speech recognition than within-channel intensity discrimination was not unequivocally shown in this study. However, the results tended to support this proposition, with XIDLs more correlated with speech performance than IDLs, and the ratio XIDL/IDL also being correlated with speech recognition. If supported by further research, the importance of perceptual variance as a limiting factor in speech understanding for CI users has important implications for efforts to improve outcomes for those with poor speech recognition.

Brain Plasticity and Rehabilitation with a Cochlear Implant

The functional changes that occur in the brain due to deafness may affect the way the auditory system processes sound after cochlear implantation. Brain plasticity plays a crucial role in the success of cochlear implantation to facilitate or develop spoken language in profoundly deaf individuals. The functional plasticity that occurs in postlingually deaf adults during periods of deafness can both support and hinder speech understanding with a cochlear implant, depending on the nature and degree of functional changes. Evidence so far suggests that the strategies people use to communicate while deaf may influence whether the functional changes are adaptive or maladaptive. In the case of children with congenital deafness, evidence is very strong for a sensitive period in which auditory input must be restored if subsequent oral language is to be developed successfully. Successful oral language use and speech understanding in individuals implanted after 7 years of age depends strongly on the pre-implant use of hearing aids and auditory-verbal communication. Future research should focus on how to harness our growing knowledge of brain plasticity to optimize the outcomes of cochlear implantation in each individual.

The effect of presentation level and stimulation rate on speech perception and modulation detection for cochlear implant users

In order to improve speech understanding for cochlear implant users, it is important to maximize the transmission of temporal information. The combined effects of stimulation rate and presentation level on temporal information transfer and speech understanding remain unclear. The present study systematically varied presentation level (60, 50, and 40 dBA) and stimulation rate [500 and 2400 pulses per second per electrode (pps)] in order to observe how the effect of rate on speech understanding changes for different presentation levels. Speech recognition in quiet and noise, and acoustic amplitude modulation detection thresholds (AMDTs) were measured with acoustic stimuli presented to speech processors via direct audio input (DAI). With the 500 pps processor, results showed significantly better performance for consonant-vowel nucleus-consonant words in quiet, and a reduced effect of noise on sentence recognition. However, no rate or level effect was found for AMDTs, perhaps partly because of amplitude compression in the sound processor. AMDTs were found to be strongly correlated with the effect of noise on sentence perception at low levels. These results indicate that AMDTs, at least when measured with the CP910 Freedom speech processor via DAI, explain between-subject variance of speech understanding, but do not explain within-subject variance for different rates and levels.

Perceptual Spaces Induced by Cochlear Implant All-Polar Stimulation Mode

It has been argued that a main limitation of the cochlear implant is the spread of current induced by each electrode, which activates an inappropriately large range of sensory neurons. To reduce this spread, an alternative stimulation mode, the all-polar mode, was tested with five participants. It was designed to activate all the electrodes simultaneously with appropriate current levels and polarities to recruit narrower regions of auditory nerves at specific intracochlear electrode positions (denoted all-polar electrodes). In this study, the all-polar mode was compared with the current commercial stimulation mode: the monopolar mode. The participants were asked to judge the sound dissimilarity between pairs of two-electrode pulse-train stimuli that differed in the electrode positions and were presented in either monopolar or all-polar mode with pulses on the two electrodes presented either sequentially or simultaneously. The dissimilarity ratings were analyzed using a multidimensional scaling technique and three-dimensional stimulus perceptual spaces were produced. For all the conditions (mode and simultaneity), the first perceptual dimension was highly correlated with the position of the most apical activated electrode of the electrical stimulation and the second dimension with the position of the most basal electrode. In both sequential and simultaneous conditions, the monopolar and all-polar stimuli were significantly separated by a third dimension, which may indicate that all-polar stimuli have a perceptual quality that differs from monopolar stimuli. Overall, the results suggest that both modes might successfully represent spectral information in a sound processing strategy.

Clinical validation of a precision electromagnetic tremor measurement system in participants receiving deep brain stimulation for essential tremor

Tremor is characterized commonly through subjective clinical rating scales. Accelerometer-based techniques for objective tremor measurement have been developed in the past, yet these measures are usually presented as an unintuitive dimensionless index without measurement units. Here we have developed a tool (TREMBAL) to provide quantifiable and objective measures of tremor severity using electromagnetic motion tracking. We aimed to compare TREMBAL's objective measures with clinical tremor ratings and determine the test-retest reliability of our technique. Eight participants with ET receiving deep brain stimulation (DBS) therapy were consented. Tremor was simultaneously recorded using TREMBAL and video during DBS adjustment. After each adjustment, participants performed a hands-outstretched task (for postural tremor) and a finger-nose task (for kinetic tremor). Video recordings were de-identified, randomized, and shown to a panel of movement disorder specialists to obtain their ratings. Regression analysis and Pearson's correlations were used to determine agreement between datasets. Subsets of the trial were repeated to assess test-retest reliability. Tremor amplitude and velocity measures were in close agreement with mean clinical ratings (r  >  0.90) for both postural and kinetic tremors. Test-retest reliability for both translational and rotational components of tremor showed intra-class correlations  >0.80. TREMBAL assessments showed that tremor gradually improved with increasing DBS therapy-this was also supported by clinical observation. TREMBAL measurements are a sensitive, objective and reliable assessment of tremor severity. This tool may have application in clinical trials and in aiding automated optimization of deep brain stimulation.

Electrode Selection and Speech Understanding in Patients With Auditory Brainstem Implants

OBJECTIVES

The objective of this study was to evaluate whether speech understanding in auditory brainstem implant (ABI) users who have a tumor pathology could be improved by the selection of a subset of electrodes that were appropriately pitch ranked and distinguishable. It was hypothesized that disordered pitch or spectral percepts and channel interactions may contribute significantly to the poor outcomes in most ABI users.

DESIGN

A single-subject design was used with five participants. Pitch ranking information for all electrodes in the patients' clinic maps was obtained using a pitch ranking task and previous pitch ranking information from clinic sessions. A multidimensional scaling task was used to evaluate the stimulus space evoked by stimuli on the same set of electrodes. From this information, a subset of four to six electrodes was chosen and a new map was created, using just this subset, that the subjects took home for 1 month's experience. Closed-set consonant and vowel perception and sentences in quiet were tested at three sessions: with the clinic map before the test map was given, after 1 month with the test map, and after an additional 2 weeks with their clinic map.

RESULTS

The results of the pitch ranking and multidimensional scaling procedures confirmed that the ABI users did not have a well-ordered set of percepts related to electrode position, thus supporting the proposal that difficulty in processing of spectral information may contribute to poor speech understanding. However, none of the subjects benefited from a map that reduced the stimulation electrode set to a smaller number of electrodes that were well ordered in place pitch.

CONCLUSIONS

Although poor spectral processing may contribute to poor understanding in ABI users, it is not likely to be the sole contributor to poor outcomes.

The perception of complex pitch in cochlear implants: A comparison of monopolar and tripolar stimulation

Cochlear implant listeners typically perform poorly in tasks of complex pitch perception (e.g., musical pitch and voice pitch). One explanation is that wide current spread during implant activation creates channel interactions that may interfere with perception of temporal fundamental frequency information contained in the amplitude modulations within channels. Current focusing using a tripolar mode of stimulation has been proposed as a way of reducing channel interactions, minimising spread of excitation and potentially improving place and temporal pitch cues. The present study evaluated the effect of mode in a group of cochlear implant listeners on a pitch ranking task using male and female singing voices separated by either a half or a quarter octave. Results were variable across participants, but on average, pitch ranking was at chance level when the pitches were a quarter octave apart and improved when the difference was a half octave. No advantage was observed for tripolar over monopolar mode at either pitch interval, suggesting that previously published psychophysical advantages for focused modes may not translate into improvements in complex pitch ranking. Evaluation of the spectral centroid of the stimulation pattern, plus a lack of significant difference between male and female voices, suggested that participants may have had difficulty in accessing temporal pitch cues in either mode.

The Feasibility of Using Acoustic Markers of Speech for Optimizing Patient Outcomes during Randomized Amplitude Variation in Deep Brain Stimulation: A Proof of Principle Methods Study

Background

Deep brain stimulation (DBS) is an effective treatment for reducing symptoms of tremor. A common and typically subjectively determined adverse effect of DBS is dysarthria. Current assessment protocols are driven by the qualitative judgments of treating clinicians and lack the sensitivity and objectivity required to optimize patient outcomes where multiple stimulation parameters are trialed.

Objective

To examine the effect of DBS on speech in patients receiving stimulation to the posterior sub-thalamic area (PSA) via randomized manipulation of amplitude parameters.

Methods

Six patients diagnosed with tremor receiving treatment via DBS of the PSA were assessed in a double-blinded, within-subjects experimental protocol. Amplitude (i.e., voltage or current) was randomly adjusted across 10 settings, while speech samples (e.g., sustained vowel, counting to 10) were recorded to identify the patient-specific settings required for optimal therapeutic benefit (reduced tremor) with minimal adverse effects (altered speech). Speech production between stimulation parameters was quantified using acoustic analysis.

Results

Speech changed as a response to DBS but those changes were not uniform across patients nor were they generally in line with changes in amplitude with the exception of reduced vocal control and increased mean silence length in two patients. Speech outcomes did not correlate with changes in tremor.

Conclusion

Intra-individual changes in speech were detected as a response to modified amplitude; however, no clear pattern was observed across patients as a group. The use of objective acoustic measures allows for quantification of speech changes during DBS optimization protocols, even when those changes are subtle and potentially difficult to detect perceptually.

Effect of Pulse Rate and Polarity on the Sensitivity of Auditory Brainstem and Cochlear Implant Users to Electrical Stimulation

To further understand the response of the human brainstem to electrical stimulation, a series of experiments compared the effect of pulse rate and polarity on detection thresholds between auditory brainstem implant (ABI) and cochlear implant (CI) patients. Experiment 1 showed that for 400-ms pulse trains, ABI users’ thresholds dropped by about 2 dB as pulse rate was increased from 71 to 500 pps, but only by an average of 0.6 dB as rate was increased further to 3500 pps. This latter decrease was much smaller than the 7.7-dB observed for CI users. A similar result was obtained for pulse trains with a 40-ms duration. Furthermore, experiment 2 showed that the threshold difference between 500- and 3500-pps pulse trains remained much smaller for ABI than for CI users, even for durations as short as 2 ms, indicating the effect of a fast-acting mechanism. Experiment 3 showed that ABI users’ thresholds were lower for alternating-polarity than for fixed-polarity pulse trains, and that this difference was greater at 3500 pps than at 500 pps, consistent with the effect of pulse rate on ABI users’ thresholds being influenced by charge interactions between successive biphasic pulses. Experiment 4 compared thresholds and loudness between trains of asymmetric pulses of opposite polarity, in monopolar mode, and showed that in both cases less current was needed when the anodic, rather than the cathodic, current was concentrated into a short time interval. This finding is similar to that previously observed for CI users and is consistent with ABI users being more sensitive to anodic than cathodic current. We argue that our results constrain potential explanations for the differences in the perception of electrical stimulation by CI and ABI users, and have potential implications for future ABI stimulation strategies.

Loudness of time-varying stimuli with electric stimulation

McKay, Henshall, Farrell, and McDermott [J. Acoust. Soc. Am. 113, 2054-2063 (2003)] developed a practical method to estimate the loudness of periodic electrical signals presented through a cochlear implant. In the present work, this method was extended to time-varying sounds based on two models of time-varying loudness for normal listeners. To fit the model parameters, loudness balancing data was collected with six cochlear implant listeners. The pulse rate of a modulated pulse train was adjusted to equalize its loudness to a reference stimulus. The stimuli were single-electrode time-limited pulse bursts, repeated at a rate of 50 Hz, with on-times varying between 2 and 20 ms. The parameters of two different models of time-varying loudness were fitted to the results. For each model, parameters defining the time windows over which the electrical pulses contribute to instantaneous loudness were optimized. In each case, a good fit was obtained with the loudness balancing results. Therefore, the practical method was successfully extended to time-varying sounds by combining it with existing models of time-varying loudness for acoustic stimulation.

Interpulse interval discrimination within and across channels: comparison of monopolar and tripolar mode of stimulation

Perception of temporal patterns is crucial to speech understanding and music perception in normal hearing, and is fundamental in the design and implementation of processing strategies for cochlear implants. Two experiments described here investigated the effect of stimulation mode (monopolar versus tripolar) on interpulse interval discrimination using single-electrode stimulation (experiment 1) and dual-electrode stimulation (experiment 2). Experiment 1 required participants to discriminate stimuli containing different interpulse intervals and experiment 2 required listeners to discriminate between two dual-electrode stimuli that had the same temporal pattern on each electrode, but differed in inter-electrode timing. The hypotheses were that (i) stimulation mode would affect the ability to distinguish interpulse interval patterns on a single electrode and (ii) the electrode separation range in which subjects were sensitive to inter-electrode timing would be more restricted in tripolar than in monopolar stimulation. Results in nine cochlear implant users showed that mode did not have a significant mean effect on either the ability to discriminate interpulse intervals in single-electrode stimulation or the range of electrode separation in dual-electrode stimulation in which participants were sensitive to inter-electrode timing. In conclusion, tripolar stimulation did not show any advantage in delivering temporal information within or across channels in this group.

Estimating confidence intervals for information transfer analysis of confusion matrices

A non-parametric bootstrapping statistical method is introduced and investigated for estimating confidence intervals resulting from information transfer (IT) analysis of confusion matrices. Confidence intervals can be used to statistically compare ITs from two or more confusion matrices obtained in an experiment. Information transfer is a nonlinear analysis and does not satisfy many of the assumptions of a parametric method. The bootstrapping method accurately estimated IT confidence intervals as long as the confusion matrices contained a sufficiently large number of presentations per stimulus category, which is also a condition for reduced bias in IT analysis.

Auditory responses to electric and infrared neural stimulation of the rat cochlear nucleus

In an effort to improve the auditory brainstem implant, a prosthesis in which user outcomes are modest, we applied electric and infrared neural stimulation (INS) to the cochlear nucleus in a rat animal model. Electric stimulation evoked regions of neural activation in the inferior colliculus and short-latency, multipeaked auditory brainstem responses (ABRs). Pulsed INS, delivered to the surface of the cochlear nucleus via an optical fiber, evoked broad neural activation in the inferior colliculus. Strongest responses were recorded when the fiber was placed at lateral positions on the cochlear nucleus, close to the temporal bone. INS-evoked ABRs were multipeaked but longer in latency than those for electric stimulation; they resembled the responses to acoustic stimulation. After deafening, responses to electric stimulation persisted, whereas those to INS disappeared, consistent with a reported "optophonic" effect, a laser-induced acoustic artifact. Thus, for deaf individuals who use the auditory brainstem implant, INS alone did not appear promising as a new approach.

Place specificity measured in forward and interleaved masking in cochlear implants

Interleaved masking in cochlear implants is analogous to acoustic simultaneous masking and is relevant to speech processing strategies that interleave pulses on concurrently activated electrodes. In this study, spatial decay of masking as the distance between masker and probe increases was compared between forward and interleaved masking in the same group of cochlear implant users. Spatial masking patterns and the measures of place specificity were similar between forward and interleaved masking. Unlike acoustic hearing where broader tuning curves are obtained in simultaneous masking, the type of masking experiment did not influence the measure of place specificity in cochlear implants.

Place specificity of monopolar and tripolar stimuli in cochlear implants: the influence of residual masking

This experiment investigated whether place specificity of neural activity evoked by cochlear implant stimulation is improved in tripolar compared to monopolar mode using a forward masking protocol addressing some limitations of previous methods of measurement and analysis. The amount of residual masking (masking remaining at long masker-probe delays) was also measured, and its potential influence on the specificity measures was evaluated. The masker stimulus comprised equally loud interleaved mono- or tripolar stimulation on two electrodes equidistant from a central probe electrode in an apical and basal direction, reducing the influence of off-site listening. The effect of masker-probe distance on the threshold shift of the tripolar probe was analyzed to derive a measure of place specificity. On average, tripolar maskers were more place specific than monopolar maskers, although the mean effect was small. There was no significant effect of masker level on specificity or on the differences observed between modes. The mean influence of residual masking on normalized masking functions was similar for the two modes and, therefore, did not influence the comparison of specificity between the modes. However, variability in amount of residual masking was observed between subjects, and therefore should be considered in forward masking studies that compare place specificity across subjects.

Electrically evoked compound action potential artifact rejection by independent component analysis: technique validation

The electrically-evoked compound action potential (ECAP) is the synchronous whole auditory nerve activity in response to an electrical stimulus, and can be recorded in situ on cochlear implant (CI) electrodes. A novel procedure (ECAP-ICA) to isolate the ECAP from the stimulation artifact, based on independent component analysis (ICA), is described here. ECAPs with artifact (raw-ECAPs) were sequentially recorded for the same stimulus on 9 different intracochlear recording electrodes. The raw-ECAPs were fed to ICA, which separated them into independent sources. Restricting the ICA projection to 4 independent components did not induce under-fitting and was found to explain most of the raw-data variance. The sources were identified and only the source corresponding to the neural response was retained for artifact-free ECAP reconstruction. The validity of the ECAP-ICA procedure was supported as follows: N₁ and P₁ peaks occurred at usual latencies; and ECAP-ICA and artifact amplitude-growth functions (AGFs) had different slopes. Concatenation of raw-ECAPs from multiple stimulus currents, including some below the ECAP-ICA threshold, improved the source separation process. The main advantage of ECAP-ICA is that use of maskers or alternating polarity stimulation are not needed.

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Novel technique to cancel artifact in electrically-evoked compound action potentials.

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No maskers or alternate polarity are required.

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ECAPs output from the procedure are compared to forward-masking technique.

Voice gender differences and separation of simultaneous talkers in cochlear implant users with residual hearing

Perception of a target voice in the presence of a competing talker, of same or different gender as the target, was investigated in cochlear implant users, in implant-alone and bimodal (acoustic hearing in the non-implanted ear) conditions. Recordings of two male and two female talkers acted as targets and maskers, to investigate whether bimodal benefit increased for different compared to same gender target/maskers due to increased ability to perceive and utilize fundamental frequency and spectral-shape differences. In both listening conditions participants showed benefit of target/masker gender difference. There was an overall bimodal benefit, which was independent of target/masker gender difference.

Beneficial acoustic speech cues for cochlear implant users with residual acoustic hearing

This study investigated which acoustic cues within the speech signal are responsible for bimodal speech perception benefit. Seven cochlear implant (CI) users with usable residual hearing at low frequencies in the non-implanted ear participated. Sentence tests were performed in near-quiet (some noise on the CI side to reduce scores from ceiling) and in a modulated noise background, with the implant alone and with the addition, in the hearing ear, of one of four types of acoustic signals derived from the same sentences: (1) a complex tone modulated by the fundamental frequency (F0) and amplitude envelope contours; (2) a pure tone modulated by the F0 and amplitude contours; (3) a noise-vocoded signal; (4) unprocessed speech. The modulated tones provided F0 information without spectral shape information, whilst the vocoded signal presented spectral shape information without F0 information. For the group as a whole, only the unprocessed speech condition provided significant benefit over implant-alone scores, in both near-quiet and noise. This suggests that, on average, F0 or spectral cues in isolation provided limited benefit for these subjects in the tested listening conditions, and that the significant benefit observed in the full-signal condition was derived from implantees' use of a combination of these cues.

Forward masking as a method of measuring place specificity of neural excitation in cochlear implants: a review of methods and interpretation

This paper reviews the psychophysical forward masking methods that have been used to investigate place specificity in cochlear implantees. These experiments are relevant for investigating whether the individual variability in outcomes for people using the same device can be explained by individual variations in frequency resolution or whether place specificity is affected by different modes of stimulation (such as bipolar, monopolar or tripolar) in the same person. Unfortunately, there has been no consensus about the methods used to derive electrical forward masking functions, or in the way that they are interpreted in relation to place specificity. Here, the different methods are critically examined to provide insight into the optimal methods that should be used to measure and interpret spatial forward masking functions in electric hearing. It is shown that, in order to separate the temporal effects of masking decay from the place-specificity information, different analyses of the functions are needed depending on whether a fixed-probe or fixed-masker method is employed. The effects of unit of measurement on specificity measures and the effects of subject listening strategy on the forward masked functions are also discussed.

Interhemispheric transfer time in patients with auditory hallucinations: an auditory event-related potential study

Central auditory processing in schizophrenia patients with a history of auditory hallucinations has been reported to be impaired, and abnormalities of interhemispheric transfer have been implicated in these patients. This study examined interhemispheric functional connectivity between auditory cortical regions, using temporal information obtained from latency measures of the auditory N1 evoked potential. Interhemispheric Transfer Times (IHTTs) were compared across 3 subject groups: schizophrenia patients who had experienced auditory hallucinations, schizophrenia patients without a history of auditory hallucinations, and normal controls. Pure tones and single-syllable words were presented monaurally to each ear, while EEG was recorded continuously. IHTT was calculated for each stimulus type by comparing the latencies of the auditory N1 evoked potential recorded contralaterally and ipsilaterally to the ear of stimulation. The IHTTs for pure tones did not differ between groups. For word stimuli, the IHTT was significantly different across the 3 groups: the IHTT was close to zero in normal controls, was highest in the AH group, and was negative (shorter latencies ipsilaterally) in the nonAH group. Differences in IHTTs may be attributed to transcallosal dysfunction in the AH group, but altered or reversed cerebral lateralization in nonAH participants is also possible.