Physiological methods as indexes of listening effort measurement: an integrative literature review

Guijo, Laura Mochiatti; Cardoso, Ana Cláudia Vieira

doi:10.1590/1982-021620182044018

ABSTRACT

Purpose: to review the scientific literature and present existing instruments and methods for the objective assessment of the listening effort in normal hearing individuals worldwide.

Methods: a literature integrative review whose purpose was to gather and summarize the scientific knowledge regarding the objective methods theme for measuring the listening effort, developed through the search of articles in specialized national and international journals, in the English and Portuguese languages, available in the databases: PUBMED, Cochrane Library, LILACS and SCIELO.

Results: 18 articles which used physiological methods to measure the listening effort in individuals with normal hearing were reviewed. The main findings described in those articles refer to the author(s) and purpose(s) of the research, country where the research was conducted, casuistry, physiological method used and results.

Conclusion: there is no consensus among the researchers about the best physiological method to measure this parameter, that is, this effort in the speech perception tasks, although the level of skin conductance is considered the most accurate measure to date.

Keywords: Hearing; Cognition; Heart Rate; Pupil; Galvanic Skin Response

RESUMO

Objetivo: revisar a literatura científica e apresentar os instrumentos e métodos existentes para a avaliação objetiva do esforço auditivo em indivíduos normo-ouvintes no cenário mundial.

Métodos: esta foi uma revisão integrativa de literatura cujo intuito foi reunir e resumir o conhecimento científico a respeito da temática métodos objetivos para mensuração do esforço auditivo, desenvolvida por meio da busca de artigos em periódicos especializados, nacionais e internacionais, nos idiomas inglês e português, disponíveis nas bases de dados: PUBMED, Biblioteca Cochrane, LILACS e SCIELO.

Resultados: foram revisados 18 artigos nos quais os métodos fisiológicos foram empregados para mensurar o esforço auditivo em indivíduos normo-ouvintes. Os principais achados dos artigos revisados se referem aos autore(s) e objetivo(s) da pesquisa, país onde a pesquisa foi conduzida, casuística, método fisiológico empregado e, resultados.

Conclusão: não existe consenso entre os pesquisadores quanto ao melhor método fisiológico para mensurar este parâmetro, ou seja, este esforço nas tarefas de percepção de fala, embora o nível de condutância da pele tenha sido considerado a medida mais precisa até o momento.

Descritores: Audição; Cognição; Frequência Cardíaca; Pupila; Resposta Galvânica da Pele

Introduction

In recent years, the areas of cognitive psychology and audiology have been dedicating to find the most appropriate method to measure listening effort, using different instruments to quantify this phenomenon both through subjective procedures and objective procedures. In the subjective assessment, questionnaires¹^,², classification scales³ or self-report methods have been used. Although these methods are easy and quick to apply, the use of an objective method to quantify such cognitive effort in clinical settings would be invaluable in obtaining more accurate and reliable results.

In the literature, there are different approaches to estimate listening effort objectively. One of the measurement methods employed by researchers who study this topic is pupillometry⁴^-⁶. In these studies, the speech stimulus used to the listening effort assessment were sentences presented in different signal-to-noise ratios, the expected response was the repetition of these sentences and the increase in pupil diameter was interpreted as an increase of the listening effort.

Other studies used different physiological measures to investigate and quantify the listening effort, including heart rate variability (HRV), skin conductance and temperature, and electromyographic (EMG) activity. The authors stated that these measures could be used as possible indexes of listening effort⁷.

As well as in pupillometry studies, physiological data were recorded during a speech recognition task and, as far as the task demand increased, electromyographic activity and skin conductance also increased.

Authors⁸^-¹⁰ reported that changes in cardiac measurements and skin conductance are commonly noticed when mental demands of the tasks are increased. Other authors¹¹^,¹² determined changes in breath, electromyography, electroencephalography activity and skin temperature during listening effort assessment, but these measurements were less consistent to quantify this effort.

A review study on listening effort¹³ concluded that the lack of uniformity in the methods employed for the assessment of this parameter is due to the different assumptions of the professionals involved in the area. Therefore, these authors¹³ recommended that researchers consider these assumptions in interpreting their data and, whenever possible, make predictions based on current theoretical knowledge to increase understanding about the underlying mechanisms of listening effort.

Although the international literature presents several studies that measure the listening effort objectively, there is still no consensus in relation to the best index to assess this parameter.

Knowing the research and the instruments of objective assessment of the listening effort that have been used in the national and international scope is the first step for the training of the audiologist who works in this area and this domain will allow the choice of the best assessment method. In this way, the purposes of this article were to review the scientific literature and present the existing instruments and methods for the objective assessment of the listening effort in normal hearing individuals throughout the world.

Methods

As living beings were not used, it was not necessary to approve this work by the Research Ethics Committee.

The present article presents as a method the integrative literature review, whose purpose is to gather and summarize the scientific knowledge produced in respect of the thematic investigated, allowing the evaluation and synthesis of the available evidence, collaborating for the development of this subject¹⁴. This review was performed according to the following stages: identification of the research question; search in the literature through the establishment of criteria, such as: keywords, inclusion and exclusion criteria; evaluation and analysis of the studies included in the bibliographic review; review presentation and knowledge synthesis¹⁵.

In order to guide the search and discussion of the studies, the following question was formulated: "What existing methods have been used to assess listening effort objectively in normal hearing individuals on the world scenario?". In order to obtain answers to this question, a collection of articles was performed in specialized national and international journals, both in English and Portuguese languages, available in the following databases: US National Library of Medicine National Institutes of Health (PUBMED), Cochrane Library, Biblioteca Vitual em Saúde - Literatura Latino-Americana e do Caribe em Ciências da Saúde (LILACS) and Scientific Electronic Library Online (SCIELO), specifically in each base and without limiting the search period.

In order to collect the articles, it was defined terms related to the "listening effort", found in the Descriptors in Health Sciences (Descritores em Ciências da Saúde - DeCS), in the Medical Subject Headings (MeSH) and, keywords related to the topic, combined among them with the use of the Boolean operators AND and OR. The terms chosen for the search were used in an isolated and crossed way (Figure 1). In all searches the "word" filter was used.

Figure 1:
Relation of words and descriptors of topics used in the literature search

Initially, the inclusion or exclusion of the articles was based on the information contained in the title and in the abstract, but if this information was not explicit in relation to the researched topic, the article was read for the fulfilment of the following inclusion criteria: articles that used objective methods to assess the listening effort in subjects with normal hearing. All articles about listening effort that used only self-report methods, questionnaires, subjective scales and dual-task paradigms were excluded; studies on assessment of listening effort in populations with hearing loss; studies with animals; articles to which only abstract and/or abstract were available, and literature review articles.

The analysis of the studies was initially based on the reading of the titles and abstracts, followed by the full reading of the articles that fit the selection criteria. Each article was evaluated by one of the authors and, in case of uncertainty, there was a consensus among all authors with respect the inclusion of the article.

Publications that were classified as eligible for this review were read by at least one of the authors. The main characteristics of the selected studies and the physiological indexes used to measure the listening effort were organized in a figure format.

For the data organization, the articles that resulted from each crossed term were computed, which were repeated in relation to the previous search and that were related to the present study.

In the analysis of the selected studies, the following data were considered: authors and purpose(s) of the research, country where the research was conducted, casuistry, physiological method employed and results.

Literature Review

From the search carried out in the PUBMED, Cochrane library, LILACS and SCIELO databases, 206 articles were found, and each article was presented in one or more databases. In the reading of the abstracts of these articles, those that had previously been selected in other databases, those that did not fit the inclusion criteria, and the journals to which complete articles were not available during the search period were excluded.

From this total of 206 articles, the abstracts were read and 56 articles were discarded because the participants had hearing loss, 16 because the texts were unavailable, 24 because they were studies whose assessment method of the listening effort was subjective, 69 did not address the listening effort topic, 18 due to duplicity, 4 were literature review studies and 1 was an experimental study developed with animals. After this detailed analysis, 18 articles were selected that met the criteria established for this review.

The selection process of the studies included in this literature review is illustrated in the figure format (Figure 2).

Figure 2:
Flowchart of the selection of reviewed and analyzed articles

The synthesis of the articles that presents information about the authors and purpose(s) of the research, country where the research was conducted, casuistry, physiological method employed and results of the studies is shown in Figure 3.

Figure 3:
Synthesis of selected studies for the analysis of physiological methods of listening effort assessment in normal hearing individuals (n = 18)

The current article presents an overview of the studies that used physiological methods to assess listening effort objectively in subjects with normal hearing and their comprehensiveness on the world scenario. It should be emphasized that this review was conducted according to the guidelines for an integrative literature review, since it was not intended to evaluate the quality of the included studies, but describe the previous publications in order to provide an overview of the studies developed so far about the topic listening effort, and the measurement of this parameter in an objective way.

Specifically, the purposes of this review were to present the existing physiological indexes that are used to measure the listening effort in normal hearing individuals in the world scenario and to provide a synthesis of the results obtained.

Based on the analyzed studies, it was possible to infer that among the physiological indexes used for the listening effort measurement and presented in the reviewed articles, the level of skin conductance was demonstrated to be the most accurate and promising measurement in the quantification of this effort⁷^,²⁰. Authors stated that the increase in skin conductance occurred due to excitation of the sympathetic nervous system as a consequence of the increase in the demand for the performing of the speech perception task²².

Physiological indexes such as heart rate, skin temperature⁷ and pupillometry¹⁷ did not undergo significant changes that proved the listening effort employed in the speech perception task in normal individuals.

However, a study reported that pupil dilation, noticed during the application of speech perception tests, especially in adverse situations, reflects the auditory and cognitive processes required for the resolution of the listening task²⁸.

Another study investigated by means of the pupillometry if the processing effort of musicians and non-musicians is different. The results demonstrated that the musicians dispensed less listening effort when performing a speech perception task in different signal-to-noise ratios during the pupillometry recording probably due to their musical abilities¹⁹.

In the majority of the reviewed studies the population assessed was composed of young adults, being only a study conducted with a child population¹⁷.

Some authors state the relevance of conducting new research to determine the physiological aspects²¹^,³² and the cognitive demands involved in speech processing in different signal-to-noise ratios, with acoustic stimuli that present distinct characteristics and, considering the emotional responses of the individuals assessed²¹^,²².

It is worth mentioning that two studies measured the listening effort both subjectively and objectively, and found that the difference of the methods used in the assessment represented differences in the cognitive processes involved¹⁶^,²⁰.

Some studies measured the listening effort of normal hearing individuals with the use of long-latency auditory potentials²⁶^,³⁰^,³² and the authors noticed that these measures were able to detect differences in the auditory cortical pathways when individuals performed the necessary listening effort to perform speech perception tasks, and even understand speech under distinct listening conditions such as in silence and in noise. A study that used the Event-Related Auditory Evoked Potential²⁶ as a listening effort index aimed to compare the performance of young and old adults. The analysis of the findings showed that the amplitudes of Novelty P3 and the delayed positive potential were higher in young people than in the elderly, and that with the increase of the task demand, the Novelty P3 presented more robust effects in the young people and the delayed positive potential in the elderly people.

The present literature review demonstrates that the physiological methods used to the listening effort measurement seem to be sensitive to different experimental conditions, and for this reason, new research that adopt other study designs in this measurement is required.

In addition, it is evident that studies in this area are relatively recent, since of the 18 selected articles, 72.2%¹⁶^-²⁸ are from the last five years. It should also be noted that the analysis of the articles revealed the lack of national studies that measured the listening effort using physiological indexes as a method of measuring this parameter in both normal hearing individuals and hearing-impaired individuals. It is also worth noting that in the international literature 50% of the studies were conducted in the United States⁷^,¹⁷^,²⁰^-²²^,²⁹^-³², 22,2% in Netherlands¹⁸^,²⁴^,²⁷^,²⁸^),,11,1% in Australia⁶^,²³ and 5.5% respectively in Denmark¹⁹, Germany²⁵, and Switzerland²⁶.

In front of the relevance of studying and researching the cognitive processes involved in speech perception and its effort to understand it, it would be essential to continue the investigations that contribute to the discovery of the most accurate physiological index for the listening effort measurement, in order to benefit individuals with hearing loss in their speech comprehension process in daily situations.

Conclusion

This study presented a literature review about the physiological indexes most commonly used for the listening effort measurement of normal-hearing individuals, and it was concluded that there is no consensus among researchers with respect to the best physiological method to measure this auditory parameter, that is, this effort in the speech perception tasks, although the skin conductance level was considered the most accurate measure for this measurement.

References

¹ Gatehouse S, Noble W. The speech, spatial and qualities of hearing scale (SSQ). Int J Audiol. 2004;43(2):85-99.
² Gatehouse S, Akeroyd M. Two-eared listening in dynamic situations. Int J Audiol. 2006;45(Suppl 1):S120-4.
³ Humes LE. Dimensions of hearing aid outcome. J Am Acad Audiol. 1999;10(1):26-39.
⁴ Goldwater BC. Psychological significance of pupillary movements. Psychol Bull. 1972;77(5):340-55.
⁵ Kramer SE, Kapteyn TS, Festen JM, Kuik DJ. Assessing aspects of auditory handicap by means of pupil dilatation. Audiology. 1997;36(3):155-64.
⁶ Zekveld AA, Kramer SE, Festen JM. Pupil response as an indication of effortful listening: the influence of sentence intelligibility. Ear Hear. 2010;31(4):480-90.
⁷ Mackersie CL, Cones H. Subjective and psychophysiological indexes of listening effort in a competing-talker task. J Am Acad Audiol. 2011;22(2):113-22.
⁸ Fournier LR, Wilson GF, Swain CR. Electrophysiological, behavioral, and subjective indexes of workload when performing multiple tasks: manipulations of task difficulty and training. Int J Psychophysiol. 1999;31(2):129-45.
⁹ Wilson GF, Russell CA. Real-time assessment of mental workload using psychophysiological measures and artificial neural networks. Hum Factors. 2003;45(4):635-44.
¹⁰ Richter M, Friedrich A, Gendolla GHE. Task difficulty effects on cardiac activity. Psychophysiology. 2008;45(5):869-75.
¹¹ Backs RW, Seljos KA. Metabolic and cardiorespiratory measures of mental effort: The effects of level of difficulty in a working memory task. Int J Psychophysiol. 1994;16(1):57-68
¹² Veltman JA, Gaillard AWK. Physiological workload reactions to increasing levels of task difficulty. Ergonomics. 1998;41(5):656-69.
¹³ McGarrigle R, Munro KJ, Dawes P, Stewart AJ, Moore DR, Barry JG et al. Listening effort and fatigue: what exactly are we measuring? A British Society of Audiology Cognition in Hearing Special Interest Group 'white paper'. Int J Audiol. 2014;53(7):433-40.
¹⁴ Mendes KDD, Silveira RCCP, Galvão CM. Revisão integrativa: método de pesquisa para a incorporação de evidências na saúde e na enfermagem. Texto & Contexto Enferm. 2008;17(4):758-64.
¹⁵ Whittemore R, Knafl K. The integrative review: updated methodology. J Adv Nurs. 2005;52(5):546-53.
¹⁶ Miles K, Mc Mahon C, Boisvert I, Ibrahim R, de Lissa P, Graham P et al. Objective assessment of listening effort: coregistration of pupillometry and EEG. Trends Hear. 2017;21:1-13.
¹⁷ McGarrigle R, Dawes P, Stewart AJ, Kuchinsky SE, Munro KJ. Measuring listening-related effort and fatigue in school aged children using pupillometry. J Exp Child Psychol. 2017;161:95-112.
¹⁸ Wagner AE, Toffanin P, Baskent D. The timing and effort of lexical access in natural and degraded speech. Front. Psychol. 2016;7(398):1-14.
¹⁹ Bianchi F, Santurette S, Wendt D, Dau T. Pitch discrimination in musicians and non-musicians: effects of harmonic resolvability and processing effort. J. Assoc. Res. Otolaryngol. 2016;17(1):69-79.
²⁰ Wendt D, Dau T, Hjortkjær J. Impact of background noise and sentence complexity on processing demands during sentence comprehension front. Psychol. 2016;7(345):1-12.
²¹ Francis AL, MacPherson MK, Chandrasekaran B, Alvar AM. Autonomic nervous system responses during perception of masked speech may re?ect constructs other than subjective listening effort. Front. Psychol. 2016;7(263):1-15.
²² Mackersie CL, Calderon-Moultrie N. Autonomic nervous system reactivity during speech repetition tasks: heart rate variability and skin conductance. Ear Hear. 2016;37(1):118S-25S.
²³ McMahon, Boisvert I, de Lissa P, Granger L, Ibrahim R, Lo CY et al. Monitoring alpha oscillations and pupil dilation across a performance-intensity function. Front. Psychol. 2016;7(745):1-12.
²⁴ Wagner A, Pals C, Blecourt CM, Sarampalis A, Baskent D. Does signal degradation affect top-down processing of speech? In: Van Dijk P, Baskent D, Gaudrain E, Kleine E, Wagner A, Lanting C (orgs). Physiology, psychoacoustics and cognition in normal and impaired hearing. Switzerland: Springer Open, 2016. p. 297-306.
²⁵ Damian A, Corona-Strauss FI, Hannemann R, Strauss DJ. Towards the assessment of listening effort in real life situations: mobile EEG recordings in a multimodal driving situation. IEEE. 2015;2015(1):8123-6.
²⁶ Bertoli S, Bodmer D. Effects of age and task difficulty on ERP responses to novel sounds present during a speech perception in noise test. Clin Neurophysiol. 2016;127(1):360-8.
²⁷ Koelewijna T, Kluivera H, Shinn-Cunninghamb BG, Zekvelda AA, Krame SE. The pupil response reveals increased listening effort when it is difficult to focus attention. Hear Res. 2015;323:81-90.
²⁸ Zekveld AA, Heslenfeld DJ, Joshrude IS, Versfeld NJ, Kramer SE. The eye as a window to the listening brain neural correlates of pupil size as a measure of cognitive listening load. Neuroimage. 2014;101:76-86.
²⁹ Bernarding C, Strauss DJ, Hannemann R, Corona-Strauss FI. Quantification of listening effort correlates in the oscillatory EEQ activity: a feasibility study. IEEE. 2012;2012(1):4615-8.
³⁰ Bernarding C, Strauss DJ, Latzel M, Hannemann R, Chalupper J, Corona-Strauss FI. Simulations of hearing loss and hearing aid: effects on electrophysiological correlates of listening effort. IEEE. 2011;2011(1):2319-22.
³¹ Zekveld AA, Kramer SE, Festen JM. Pupil response as an indication of effortful listening: the influence of sentence intelligibility. Ear Hear. 2010;31(4):480-90.
³² Bernarding C, Corona-Strauss FI, Latzel M, Strauss DJ. Auditory streaming and listening effort: an event related potential study. IEEE. 2010;2010(1):6817-20.

Publication Dates

Publication in this collection
Jul-Aug 2018

History

Received
20 Mar 2018
Accepted
25 June 2018

This is an open-access article distributed under the terms of the Creative Commons Attribution License

[1] ¹ Gatehouse S, Noble W. The speech, spatial and qualities of hearing scale (SSQ). Int J Audiol. 2004;43(2):85-99.

[2] ² Gatehouse S, Akeroyd M. Two-eared listening in dynamic situations. Int J Audiol. 2006;45(Suppl 1):S120-4.

[3] ³ Humes LE. Dimensions of hearing aid outcome. J Am Acad Audiol. 1999;10(1):26-39.

[4] ⁴ Goldwater BC. Psychological significance of pupillary movements. Psychol Bull. 1972;77(5):340-55.

[5] ⁵ Kramer SE, Kapteyn TS, Festen JM, Kuik DJ. Assessing aspects of auditory handicap by means of pupil dilatation. Audiology. 1997;36(3):155-64.

[6] ⁶ Zekveld AA, Kramer SE, Festen JM. Pupil response as an indication of effortful listening: the influence of sentence intelligibility. Ear Hear. 2010;31(4):480-90.

[7] ⁷ Mackersie CL, Cones H. Subjective and psychophysiological indexes of listening effort in a competing-talker task. J Am Acad Audiol. 2011;22(2):113-22.

[8] ⁸ Fournier LR, Wilson GF, Swain CR. Electrophysiological, behavioral, and subjective indexes of workload when performing multiple tasks: manipulations of task difficulty and training. Int J Psychophysiol. 1999;31(2):129-45.

[9] ⁹ Wilson GF, Russell CA. Real-time assessment of mental workload using psychophysiological measures and artificial neural networks. Hum Factors. 2003;45(4):635-44.

[10] ¹⁰ Richter M, Friedrich A, Gendolla GHE. Task difficulty effects on cardiac activity. Psychophysiology. 2008;45(5):869-75.

[11] ¹¹ Backs RW, Seljos KA. Metabolic and cardiorespiratory measures of mental effort: The effects of level of difficulty in a working memory task. Int J Psychophysiol. 1994;16(1):57-68

[12] ¹² Veltman JA, Gaillard AWK. Physiological workload reactions to increasing levels of task difficulty. Ergonomics. 1998;41(5):656-69.

[13] ¹³ McGarrigle R, Munro KJ, Dawes P, Stewart AJ, Moore DR, Barry JG et al. Listening effort and fatigue: what exactly are we measuring? A British Society of Audiology Cognition in Hearing Special Interest Group 'white paper'. Int J Audiol. 2014;53(7):433-40.

[14] ¹⁴ Mendes KDD, Silveira RCCP, Galvão CM. Revisão integrativa: método de pesquisa para a incorporação de evidências na saúde e na enfermagem. Texto & Contexto Enferm. 2008;17(4):758-64.

[15] ¹⁵ Whittemore R, Knafl K. The integrative review: updated methodology. J Adv Nurs. 2005;52(5):546-53.

[16] ¹⁶ Miles K, Mc Mahon C, Boisvert I, Ibrahim R, de Lissa P, Graham P et al. Objective assessment of listening effort: coregistration of pupillometry and EEG. Trends Hear. 2017;21:1-13.

[17] ¹⁷ McGarrigle R, Dawes P, Stewart AJ, Kuchinsky SE, Munro KJ. Measuring listening-related effort and fatigue in school aged children using pupillometry. J Exp Child Psychol. 2017;161:95-112.

[18] ¹⁸ Wagner AE, Toffanin P, Baskent D. The timing and effort of lexical access in natural and degraded speech. Front. Psychol. 2016;7(398):1-14.

[19] ¹⁹ Bianchi F, Santurette S, Wendt D, Dau T. Pitch discrimination in musicians and non-musicians: effects of harmonic resolvability and processing effort. J. Assoc. Res. Otolaryngol. 2016;17(1):69-79.

[20] ²⁰ Wendt D, Dau T, Hjortkjær J. Impact of background noise and sentence complexity on processing demands during sentence comprehension front. Psychol. 2016;7(345):1-12.

[21] ²¹ Francis AL, MacPherson MK, Chandrasekaran B, Alvar AM. Autonomic nervous system responses during perception of masked speech may re?ect constructs other than subjective listening effort. Front. Psychol. 2016;7(263):1-15.

[22] ²² Mackersie CL, Calderon-Moultrie N. Autonomic nervous system reactivity during speech repetition tasks: heart rate variability and skin conductance. Ear Hear. 2016;37(1):118S-25S.

[23] ²³ McMahon, Boisvert I, de Lissa P, Granger L, Ibrahim R, Lo CY et al. Monitoring alpha oscillations and pupil dilation across a performance-intensity function. Front. Psychol. 2016;7(745):1-12.

[24] ²⁴ Wagner A, Pals C, Blecourt CM, Sarampalis A, Baskent D. Does signal degradation affect top-down processing of speech? In: Van Dijk P, Baskent D, Gaudrain E, Kleine E, Wagner A, Lanting C (orgs). Physiology, psychoacoustics and cognition in normal and impaired hearing. Switzerland: Springer Open, 2016. p. 297-306.

[25] ²⁵ Damian A, Corona-Strauss FI, Hannemann R, Strauss DJ. Towards the assessment of listening effort in real life situations: mobile EEG recordings in a multimodal driving situation. IEEE. 2015;2015(1):8123-6.

[26] ²⁶ Bertoli S, Bodmer D. Effects of age and task difficulty on ERP responses to novel sounds present during a speech perception in noise test. Clin Neurophysiol. 2016;127(1):360-8.

[27] ²⁷ Koelewijna T, Kluivera H, Shinn-Cunninghamb BG, Zekvelda AA, Krame SE. The pupil response reveals increased listening effort when it is difficult to focus attention. Hear Res. 2015;323:81-90.

[28] ²⁸ Zekveld AA, Heslenfeld DJ, Joshrude IS, Versfeld NJ, Kramer SE. The eye as a window to the listening brain neural correlates of pupil size as a measure of cognitive listening load. Neuroimage. 2014;101:76-86.

[29] ²⁹ Bernarding C, Strauss DJ, Hannemann R, Corona-Strauss FI. Quantification of listening effort correlates in the oscillatory EEQ activity: a feasibility study. IEEE. 2012;2012(1):4615-8.

[30] ³⁰ Bernarding C, Strauss DJ, Latzel M, Hannemann R, Chalupper J, Corona-Strauss FI. Simulations of hearing loss and hearing aid: effects on electrophysiological correlates of listening effort. IEEE. 2011;2011(1):2319-22.

[31] ³¹ Zekveld AA, Kramer SE, Festen JM. Pupil response as an indication of effortful listening: the influence of sentence intelligibility. Ear Hear. 2010;31(4):480-90.

[32] ³² Bernarding C, Corona-Strauss FI, Latzel M, Strauss DJ. Auditory streaming and listening effort: an event related potential study. IEEE. 2010;2010(1):6817-20.