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Improving Sensitivity of the Digits-in-Noise Test using
Antiphasic Stimuli
Karina C. De Sousa,1 De Wet Swanepoel,1,2 David R. Moore3,4, Hermanus
Carel Myburgh5 and Cas Smits6
1 Department of Speech-Language Pathology and Audiology, University
of Pretoria, Pretoria, Gauteng, South Africa.
2 Ear Sciences Centre, School of Surgery, University of Western
Australia, Nedlands, Australia.
3 Communication Sciences Research Center, Cincinnati Childrens’
Hospital Medical Center and University of Cincinnati, Ohio, USA.
4 Manchester Centre for Audiology and Deafness, University of
Manchester, United Kingdom.
5 Department of Electrical, Electronic and Computer Engineering,
University of Pretoria, Pretoria, Gauteng, South Africa.
6 Amsterdam UMC, Vrije Universiteit Amsterdam, Otolaryngology-Head
and Neck Surgery, Ear and Hearing, Amsterdam Public Health
research institute, De Boelelaan 1117, Amsterdam.
Financial Disclosures/ Conflict of Interest:
This research was funded by the National Institute of Deafness and Communication
Disorders of the National Institutes of Health under award number 5R21DC016241-
02. The 2nd author’s relationship with the hearX Group and hearZA includes equity,
consulting, and potential royalties. The third author’s relationship with the hearX
Group and hearZA includes equity, consulting and potential royalties.
Address correspondence to Amsterdam UMC, Vrije Universiteit Amsterdam,
Otolaryngology-Head and Neck Surgery, Ear and Hearing, Amsterdam Public Health
research institute, De Boelelaan 1117, Amsterdam. E-mail: C.Smits@vumc.nl
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ABSTRACT
Objective: The digits-in-noise test (DIN) has become increasingly popular as a
consumer-based method to screen for hearing loss. Current versions of all DINs either
test ears monaurally or present identical stimuli binaurally (i.e., diotic noise and
speech, NoSo). Unfortunately, presentation of identical stimuli to each ear inhibits
detection of unilateral sensorineural hearing loss (SNHL), and neither diotic nor
monaural presentation sensitively detects conductive hearing loss (CHL). Following
an earlier finding of enhanced sensitivity in normally hearing listeners, this study tested
the hypothesis that interaural antiphasic digit presentation (NoSπ) would improve
sensitivity to hearing loss caused by unilateral or asymmetric SNHL, symmetric SNHL,
or CHL.
Design: This cross-sectional study, recruited adults (18-84 years) with various levels
of hearing, based on a four-frequency pure tone average (PTA) at 0.5, 1, 2 and 4kHz.
The study sample was comprised of listeners with normal hearing (n=41; PTA ≤ 25 dB
HL in both ears), symmetric SNHL (n=57; PTA > 25 dB HL), unilateral or asymmetric
SNHL (n=24; PTA > 25 dB HL in the poorer ear) and CHL (n=23; PTA > 25 dB HL and
PTA air-bone gap ≥ 20 dB HL in the poorer ear). Antiphasic and diotic speech
reception thresholds (SRTs) were compared using a repeated-measures design.
Results: Antiphasic DIN was significantly more sensitive to all three forms of hearing
loss than the diotic DIN. SRT test-retest reliability was high for all tests (ICC r > 0.89).
Area under the receiver operating characteristics (ROC) curve for detection of hearing
loss (> 25 dB HL) was higher for antiphasic DIN (0.94) than for diotic DIN (0.77)
presentation. After correcting for age, PTA of listeners with normal hearing or
symmetric SNHL was more strongly correlated with antiphasic (rpartial[96]=0.69) than
diotic (rpartial=0.54) SRTs. Slope of fitted regression lines predicting SRT from PTA
was significantly steeper for antiphasic than diotic DIN. For listeners with normal
hearing or CHL, antiphasic SRTs were more strongly correlated with PTA
(rpartial[62]=0.92) than diotic SRTs (rpartial[62]=0.64). Slope of regression line with PTA
was also significantly steeper for antiphasic than diotic DIN. Severity of asymmetric
hearing loss (poorer ear PTA) was unrelated to SRT. No effect of self-reported English
competence on either antiphasic or diotic DIN among the mixed first-language
participants was observed
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Conclusions: Antiphasic digit presentation markedly improved the sensitivity of the
DIN test to detect SNHL, either symmetric or asymmetric, while keeping test duration
to a minimum by testing binaurally. In addition, the antiphasic DIN was able to detect
CHL, a shortcoming of previous monaural or binaurally diotic DIN versions. The
antiphasic DIN is thus a powerful tool for population-based screening. This enhanced
functionality combined with smartphone delivery could make the antiphasic DIN
suitable as a primary screen that is accessible to a large global audience.
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INTRODUCTION
Hearing loss presents a significant global health burden as the 4th leading contributor
to years lived with disability (Vos et al. 2016). Mounting evidence demonstrates
significant associations between hearing loss, depression (Fellinger et al. 2012),
unemployment (Ruben 2015), risk for hospitalization (Genther et al. 2013; Reed et al.
2018) and cognitive decline and dementia (Lin et al. 2011; Livingston et al. 2017).
Early detection is an essential first step to ameliorate the functional impairment of
hearing loss, yet a high proportion of cases remains undetected and untreated (Ki-
Moon 2016; Mackenzie and Smith 2009). Contributing to the disparity is lack of routine
adult hearing screening programs and rehabilitation options that are either unavailable
or prohibitively expensive (Chou et al. 2011; Wilson et al. 2017).
Poor awareness of hearing loss and existing models of clinic-based adult screening
among the lay public also contribute to hearing healthcare inaccessibility (Lin et al.
2016). In efforts to increase and decentralize access to detection of hearing loss,
screening methods such as the digits-in-noise test (DIN), as an internet or landline
phone-based hearing screen have been employed (Smits et al. 2004; Jansen et al.
2010; Watson et al. 2012; Zokoll et al. 2012). The DIN is a speech-in-noise test that
uses digit triplets (e.g. 5-9-2), typically presented in steady speech-shaped noise, to
measure the speech reception threshold (SRT), expressed in dB signal-to-noise ratio
(dB SNR), where a listener can recognize 50% of the digit triplets correctly. Compared
to pure tone audiometry or speech recognition in quiet, speech recognition in noise
has the advantage of being more characteristic of a person’s hearing ability in real-life
situations (Grant et al. 2013). Furthermore, DIN assessment of sensorineural hearing
loss (SNHL) correlates highly with pure tone audiometry and eliminates the need for
a soundproof booth, calibrated equipment and a test administrator (Smits et al. 2004;
Jansen et al. 2010; Potgieter et al. 2016, 2018; Koole et al. 2016).
The DIN was first developed as a national landline telephone test in the Netherlands
(Smits et al. 2004) and later also implemented as an internet-based test (Smits et al.
2006). Highly correlated with the audiometric pure tone average (r=0.77) it
demonstrated sensitivity and specificity of more than 90% to detect sensorineural
hearing loss (Smits et al. 2004). Four months after its release, the DIN saw
considerable uptake with more than 65,000 tests taken (Smits et al. 2005),
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demonstrating its role and potential as a large-scale hearing screening tool available
to the public. Using simple digits, the test does not require a high degree of linguistic
competence (Kaandorp et al. 2016). Various language versions of the DIN have been
developed, including British-English (Hall, 2006), American-English (Watson et al.
2012); Polish (Ozimek et al. 2009), French (Jansen et al. 2010) and German (Zokoll
et al. 2011).
Despite the success of the DIN in several countries, the need of landline telephones
to conduct testing can be problematic, especially in low-and-middle income countries
like South Africa where landline penetration is poor (STATSSA 2013). On the other
hand, global access to smartphones by adults is estimated to be 80% by the year
2020, providing a modern-day alternative (The Economist 2015). Whereas mobile
phone penetration is much higher, the cost to complete the test via a mobile phone
call could be more expensive. An alternative is to offer the DIN as a downloadable
smartphone application, allowing access to high fidelity broadband signals as opposed
to bandwidth signals used in standard telephone networks (Potgieter et al. 2016), and
removing the need for cellular connectivity once uploaded. While applicable
worldwide, using a mobile platform could potentially address the mostly nonexistent
access to hearing screening in low-and-middle income countries. In sub-Saharan
Africa, for instance, there is only one audiologist for every million people (Mulwafu et
al. 2017). As a result, the South African English DIN was developed and released as
the national hearing screening application in 2016, downloadable on iOS and Android
smartphones, called hearZATM (Potgieter et al. 2016; De Sousa et al. 2018). This
binaural test version allows for testing under 3 minutes, with high sensitivity (> 80%)
to detect SNHL (Potgieter et al. 2016; Potgieter et al. 2018).
There has been a growing interest in increasing the efficiency and sensitivity of
existing DINs using various test modifications. Using a fixed-SNR procedure, Smits
(2017) showed that the number of digit triplets in a DIN could be reduced to as few as
8 trials, without compromising sensitivity and specificity but sacrificing accurate
estimation of the SRT. Furthermore, with the early appearance and high prevalence
of high frequency hearing loss, use of low-pass filtered masking noise to improve
sensitivity of the DIN to high frequency hearing loss has been investigated, showing
either higher (Vlaming et al. 2014) or similar (Vercammen et al. 2018) area under the
receiver operating characteristics (ROC) curve compared to DINs with standard
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speech-shaped noise. Therefore, when using homogenized digits to ensure high test-
retest reliability, these modifications could make the DIN test more applicable to
persons with noise-induced or age-related hearing loss (Vlaming et al. 2014).
Current versions of all DINs either sequentially test each ear (monaurally) or present
the test stimuli binaurally and identically to each ear (homophasic or diotic). This
binaural DIN setup allows for rapid testing in approximately 3 minutes, whereas
sequential testing of each ear doubles test time and may thus reduce uptake and
completion. Using diotic presentation may, however, preclude detection of unilateral
or asymmetric sensorineural hearing loss (SNHL). These listeners may pass the diotic
DIN test because performance is largely based on the functionally better ear (Potgieter
et al. 2018). Furthermore, both monaural and diotic testing is insensitive to the
attenuation caused by conductive hearing loss (CHL) because most DINs are
presented at suprathreshold intensities. To improve the sensitivity of the DIN,
especially for listeners with unilateral, asymmetrical SNHL and CHL, this study
evaluated the use of a DIN test paradigm using digits that are phase inverted
(antiphasic) between the ears, while leaving the masking noise interaurally in-phase.
Such a configuration of stimuli (NoSπ) was shown to improve DIN SRTs in normal
hearing listeners (Smits et al. 2016).
Sensitivity differences between diotic and antiphasic auditory stimulus presentations
are commonly known as the binaural masking level difference (Hirsh 1948). Before
the widespread use of the auditory brainstem response, binaural masking level
difference was employed to distinguish between different types of hearing loss (Olsen
et al. 1976; Wilson et al. 2003). Binaural masking level difference was reported to be
poorer for listeners with various types and configurations of hearing loss compared to
normal hearing controls. Wilson and colleagues (1985) investigated speech masking
level difference for people with unilateral SNHL. In the diotic condition (NoSo), only
slight SNR variations were observed across a range of interaural level differences.
However, in the antiphasic condition (NoSπ), SNRs became worse with increasing
interaural level differences.
Smits and colleagues (2016) examined SRTs in different listening conditions for the
Dutch and American English DIN among normal hearing listeners. Results indicated
that the threshold advantage over monotic presentation provided by diotic (NoSo)
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presentation was small (≅ 1 dB). However, the use of antiphasic digits (NoSπ) provided
a further ≅ 5 dB advantage. Listeners with unilateral SNHL or CHL are not expected
to have full access to the antiphasic advantage due to subtle timing irregularities
caused by peripheral hearing loss, either sensorineural (Jerger et al. 1984; Thornton
et al. 2012; Wilson et al. 1985) or conductive (Hartley and Moore 2003; Jerger et al.
1984). In cases of symmetric hearing loss, the antiphasic advantage is expected to
decrease as the degree of hearing loss increases because of increasing threshold and
timing cue deterioration (Wilson et al. 1994). These findings support the idea that
antiphasic digit presentation could sensitize the DIN for a wider range of hearing loss
types while using a single binaural test. This would improve the function of current
consumer-based DINs.
The objective of this study was, therefore, to determine whether antiphasic digit
presentation improves detection of hearing loss relative to the diotic presentation.
MATERIALS AND METHODS
Study design and participants
A cross-sectional, repeated-measure study of the DIN SRT comparing diotic and
antiphasic presentation within and between listeners of varying types and degrees of
hearing loss was conducted. Listeners were recruited from a student population, a
University clinic, and hospital and private practices in the Gauteng province of South
Africa. Adults (18-84 years; Table 1) with various levels of hearing were recruited,
based on a four frequency (0.5,1,2 and 4kHz) pure tone average (PTA). The study
sample included normal hearing (n=41; pure tone average (PTA) ≤ 25 dB HL in both
ears), symmetric SNHL (n=57; PTA > 25 dB HL) and unilateral or asymmetric SNHL
(n=24; PTA > 25 dB HL in the poorer ear). The better ear PTA of listeners with
asymmetric SNHL did not exceed 45 dB HL. A sample of listeners with CHL (n=23;
PTA > 25 dB HL and PTA air-bone gap ≥ 20 dB HL in the poorer ear) was also
recruited, including 3 listeners with symmetric and 20 with unilateral or asymmetric
hearing loss. Bone conduction pure tone average thresholds (0.5,1,2,4 kHz) for the
poorer ear did not exceed 25 dB HL, except for one listener with CHL with poorer ear
bone conduction PTA of 28 dB HL. Asymmetric hearing loss was defined as an
interaural difference >10 dB (PTA). Hearing sensitivity categories were based on
poorer ear PTA and categorized as excellent (0-15 dB HL), minimal (16-25 dB HL),
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mild (26-40 dB HL), moderate (41-55 dB HL) and severe-to-profound (56-120 dB HL).
For analyses, the ‘excellent’ and ‘minimal’ categories were combined into a single
‘normal’ category. Listeners had various levels of English-speaking competence. Non-
native English speakers self-reported their level of competence on a non-standardized
scale from 1-10, a higher score indicating better competence (Potgieter et al. 2018).
The Health Sciences Research Ethics Committee, University of Pretoria approved the
study protocol (number 58/2017). All eligible participants were informed on the study
aims and procedures and provided consent before participation.
Procedures and Equipment
The smartphone application for the South African English DIN was adapted for
antiphasic stimulus presentation. Original homogenized diotic digits were phase
reversed for antiphasic presentation. The phase inversion was completed in MatLab
by multiplying each sample in one channel of the digit triplet sound file by -1. The DIN
application was designed in Android Studio version 2.3.0 and written in Java version
1.8.0, consistent with the original hearZA App. The application stored a list of 120
different digit-triplets, randomly selected for presentation at the beginning of each test
(Potgieter et al. 2016). Randomized triplet selection was done with replacement,
meaning that the same triplet could occur more than once in one test. Triplets were
presented with 500 ms silent intervals at the beginning and end of each digit-triplet.
Successive digits were separated by 200 ms of silence with 100 ms of jitter (Potgieter
et al. 2016). The test used a fixed noise level and variable speech level when triplets
with negative SNRs were presented. To prevent clipping of the signal, the speech level
was fixed, and the noise level varied once the SNR became positive (Potgieter et al.
2016). The speech-weighted masking noise was delivered interaurally in-phase, and
the digits were either in-phase (diotic; NoSo) or were phase inverted between the two
ears (antiphasic; NoSπ). To prevent possible learning of the masking noise (Lyzenga
and Smits 2011), noise ‘freshness’ was ensured for each trial by creating a long noise
file and selecting successive fragments from a random offset within the first 5 seconds.
Both diotic and antiphasic versions of the DIN consisted of 23 digit-triplets. The SNR
varied in fixed step sizes (4 dB SNR for the first three steps, thereafter continuing in 2
dB steps) starting at 0 dB SNR using a one-up one-down staircase procedure, tracking
the SNR at which 50% of the digit triplets were correctly identified (Smits et al. 2004;
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Potgieter et al. 2016). For the first three steps, SNR became progressively more
negative by 4 dB per step for correct responses but increased by 2 dB per step for
incorrect responses. A digit-triplet was only considered correct when all digits were
entered correctly. The SRT was calculated by averaging the last 19 SNRs, in line with
the currently used hearZA test.
After completion of pure tone audiometry, participants completed five DIN tests, each
lasting about 3 minutes, on a Samsung Trend Neo smartphone coupled with
manufacturer supplied (wired) earbuds in a quiet, office-like room. The first training
test used antiphasic presentation. The remaining four DIN tests alternated between
antiphasic and diotic DIN, with a test and retest for each participant. The test order
was therefore: (1) antiphasic training list, (2) antiphasic test, (3) diotic test, (4)
antiphasic re-test and (5) diotic re-test.
Statistical Analysis
Statistical analysis was done using the Statistical Package for the Social Sciences
(IBM SPSS v25.0). A sample size of 122 listeners (24 with normal hearing PTA ≤ 25
dB HL, 24 with asymmetric hearing loss and 74 with either symmetric normal hearing
PTA ≤ 25 dB HL or symmetric sensorineural hearing loss with PTA ≥ 26 dB HL) would
provide a medium effect size (Cohen’s f = 0.25), with 80% statistical power at two-
tailed significance level of 0.05, to test both hypotheses. The sample of 23 listeners
with CHL was subsequently added.
The effect of test condition (i.e., diotic or antiphasic) and hearing loss category (i.e.,
type and symmetry of hearing loss) on the SRT was assessed using repeated-
measures analysis of variance. Post hoc comparisons used Bonferroni adjustment for
multiple comparisons. In cases where sphericity was violated, Greenhouse-Geisser
corrections were applied. Analysis of covariance was used to determine effects of age
and English-speaking competence on the diotic and antiphasic SRT. General linear
regression was used to test whether the slope of the relation between PTA and SRT
differed between antiphasic and diotic testing. The effect of test repetition on
antiphasic SRT was investigated using a paired sample t-test. Intraclass correlation
coefficients (ICC) were calculated and were based on a mean rating of the number of
observations (i.e. test and retest; k=2) of both diotic and antiphasic test conditions,
absolute agreement, and a two-way mixed-effects model. In addition, measurement
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error between test-retest for diotic and antiphasic presentation was calculated by
determining quadratic mean (√2) of within-subject standard deviations for the test-
retest measures. All subsequent analyses were conducted by averaging the test and
retest SRT values for the diotic and antiphasic DIN. Associations between poorer ear
PTA and SRT were examined using Pearson’s partial correlation. Receiver operating
characteristics (ROC) curves were calculated to determine the sensitivity and
specificity of the DIN tests for different cutoff values, to detect mild hearing loss and
worse (PTA > 25 dB HL) and moderate hearing loss and worse (PTA > 40 dB HL).
SRT cut-off values corresponding to reasonably high sensitivity and specificity were
chosen, while demonstrating the trade-off between sensitivity and specificity (i.e.
higher sensitivity with consequent lower specificity).
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RESULTS
Listeners with normal hearing had lower SRTs than those with hearing loss using both
diotic and antiphasic testing (Fig. 1). However, antiphasic testing was significantly
more sensitive to all three forms of hearing loss than diotic testing (Table 1).
Across all hearing categories, after controlling for age, poorer ear PTA was
significantly correlated to both diotic and antiphasic SRT (p < 0.001). The correlation
was, however, stronger for antiphasic (rpartial[145]=0.82] than diotic SRT
(rpartial[145]=0.44). For listeners with either normal hearing or symmetric SNHL, poorer
ear PTA was significantly (p < 0.001) correlated with both antiphasic (rpartial[96]=0.69)
and diotic (rpartial[96]=0.54) SRTs (Fig. 2). However, the slope of the fitted regression
was significantly steeper for antiphasic SRTs (t(1)=7.79.14, p < 0.001). Antiphasic
SRTs of listeners with normal hearing or CHL, were more strongly correlated to poorer
ear PTA (rpartial[62]=0.92) than diotic SRTs (rpartial[62]=0.54). The slope of the fitted
regression was also significantly steeper for antiphasic compared to diotic SRTs
(t[1]=11.84, p < 0.0001), indicative of greater sensitivity of the antiphasic DIN. The
severity of unilateral or asymmetric SNHL (poorer ear PTA) was unrelated to SRT. For
the diotic DIN, there was substantial overlap between the SRTs of normal hearing
listeners and those in each of the three hearing loss groups (Fig. 2A), even for PTAs
in the moderate or greater hearing loss ranges (Table 2). The SRT overlap was less
substantial for the antiphasic DIN, with listeners with mild poorer ear hearing loss
corresponding in SRTs to the normal hearing group (Fig. 2B).
ROC analysis, including poorer ears of all participants, (Fig. 3) showed higher areas
under the curve for antiphasic DIN compared to diotic DIN to detect PTA >25 dB HL
(0.95; 95% CI, 0.91 to 0.98 vs 0.78; 95% CI, 0.69 to 0.86) and >40 dB HL (0.96; 95%
CI, 0.93 to 0.99 vs 0.80; 95% CI, 0.73 to 0.87). Antiphasic DIN was, therefore, more
sensitive and specific to hearing loss (of either type and symmetry) compared to diotic
DIN. SRT cut-offs in Table 3 demonstrate the trade-off between sensitivity and
specificity to detect PTA > 25 dB HL and > 40 dB HL.
Antiphasic DIN test repetition produced a significant mean SRT improvement (0.9 dB
SNR; 95% CI 0.61 to 1.4) across hearing categories following the presentation of the
initial antiphasic training list (t[144]=5.1, p < 0.001; Fig. 1). However, between the
subsequent test and retest, the mean SRT difference was not significant (p = 0.86).
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Similarly, diotic test and retest showed no significant SRT difference (p = 0.6). SRT
test-retest reliability was high for listeners with normal hearing or SNHL for both diotic
DIN (ICC=0.89; 95% CI 0.85 to 0.93) and antiphasic DIN (ICC=0.94; 95% CI, 0.91 to
0.96). Listeners with CHL had high test-retest reliability for antiphasic DIN, with ICC of
0.88 (95% CI, 0.72 to 0.95; p < 0.001), but had poorer ICC of 0.61 for homophasic DIN
(95% CI, 0.09 to 0.83; p < 0.05). Diotic DIN had lower measurement error (1.1 dB;
95% CI 0.9 to 1.2) than the antiphasic DIN (1.4 dB; 95% CI 1.2 to 1.5) for the whole
sample, but the variance between listeners was much higher for the antiphasic DIN
than for the diotic DIN (Table 2).
The effect of competence in the English language on SRT was assessed by dividing
listeners into high competence (>7; n=73) and lower competence (≤7; n=72) groups.
Controlling for poorer ear PTA and age, no significant SRT difference (p = 0.16) was
found between the two groups for either the diotic DIN (F[1,141]=2.47, partial η2=0.02)
or the antiphasic DIN (F[1,141]=1.98, partial η2 = 0.02).
Table 1. Analysis of variance statistics for the effect of test presentation and category
of hearing loss.
df
F
Sig
Partial Eta
Squared
Test Type
(Diotic vs Antiphasic)
1, 141
497.06
< .001
0.78
Hearing Category
3,141
31.88
< .001
0.41
Test Type*Hearing
Category
3, 141
57.81
< .001
0.55
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Table 2. Diotic and antiphasic DIN SRT for listeners with normal hearing, symmetric SNHL, unilateral or
asymmetric SNHL and CHL according to PTA hearing loss categories.
Excellent
(0-15 dB)
Minimal
(16-25 dB)
Mild
(26-40 dB)
Moderate
(41-55 dB)
Severe-
profound
(56-120 dB)
NH &
SHL
n
26
15
23
24
10
Age Range
(Years)
19-67
23-84
39-84
51-84
67-79
Diotic
DIN
Mean
SRT(SD)
-11.1 (0.8)
-9.7 (1.1)
-10 (1.1)
-8.7 (0.9)
-6.4 (1.5)
SE
0.16
0.28
0.22
0.19
0.49
Antiphasic
DIN
Mean
SRT(SD)
-18·4 (1.4)
-16.7 (1.6)
-15.7 (1.8)
-12.4 (2.1)
-8.2 (2.7)
SE
0.28
0.41
0.37
0.43
0.85
UHL
n
0
0
0
4
20
Age Range
(Years)
-
-
-
25-63
18-72
Diotic
DIN
Mean
SRT(SD)
-
-
-
-10.8 (0.3)
-9.4 (1.2)
SE
-
-
-
0.15
0.27
Antiphasic
DIN
Mean
SRT(SD)
-
-
-
-12.5 (1.9)
-11.3 (2.2)
SE
-
-
-
0.93
0.49
CHL
n
0
0
4
4
15
Age Range
(Years)
18-44
19-62
20-68
Diotic
DIN
Mean
SRT(SD)
-
-
-10.7 (0.7)
-9.8 (2.3)
-9.3 (1)
SE
-
-
0.35
1.12
0.27
Antiphasic
DIN
Mean
SRT(SD)
-
-
-13.7 (0.9)
-11.7 (2.4)
-10.1 (1.8)
SE
-
-
0.43
1.19
0.46
DIN: digits-in-noise, NH; Normal Hearing, SHL; Symmetric SNHL, UHL; unilateral sensorineural hearing loss, CHL;
conductive hearing loss
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14
DISCUSSION
Antiphasic presentation improved the test characteristics of the smartphone DIN test
with higher sensitivity and specificity to detect hearing loss of various degrees, types
and symmetries than the diotic DIN. With monaural testing, it is possible to segregate
a ‘better’ ear from a ‘poorer’ ear. Traditionally, emphasis has been placed on the
function of the ‘better’ ear to assess activity and participation, but there is now
considerable evidence that asymmetric or unilateral hearing loss can reduce these
aspects of hearing health almost, or as much as symmetric binaural HL (Firszt et al.
2015; Rothpletz et al. 2012; Vannson et al. 2015). It is thus important to assess the
function of both ears, working together. Binaural tests, as used here, are more
dependent on the relative function of both ears (Supplementary Figure) but, because
of interaural summation and unmasking effects that interaction is complex (Hall et al.,
1995, 1998). A screening test should be rapid, is not intended to be diagnostic, and
persons who fail the test must be referred for diagnostic testing (Wilson and Jungner,
1968). The antiphasic DIN is a rapid test compared to sequential monaural testing and
aims to detect all hearing losses that require further diagnostic assessment.
Mechanisms of antiphasic advantage
Listeners with normal hearing in both ears were at a significant advantage for
understanding speech-in-noise compared to listeners with either type or symmetry of
hearing loss. This advantage is due to several mechanisms, but the primary one is
binaural integration. In spatial hearing, when sound from a lateral source arrives at the
nearer ear earlier than the far ear, interaural phase differences are processed as
spatial cues. Brainstem neurons detect interaural timing differences as small as 10 s
(Brughera et al. 2013), equal to about 2° of space (Middlebrooks and Green 1991). In
the antiphasic DIN, the 180° interaural phase difference of the digits simulates an
interaural timing difference, separating virtually the target speech from the noise. We
introduced a phase inversion in the speech signals between the ears, leaving the noise
in-phase (NoSπ) since the SRT improvement is larger compared to the NπSo condition
(Olsen et al. 1976). Listeners in our study with ‘normal’ hearing had 6-8 dB better
antiphasic than diotic SRT, in line with the study of Smits et al. (2016). Peripheral
hearing loss disrupts interaural timing differences by desynchronizing neural activity
from the affected ear(s), reducing the antiphasic advantage (Jerger et al. 1984; Welsh
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et al. 2004; Vannson et al. 2017). Predicted poorer diotic SRTs due to loss of outer
hair cell function and associated cochlear compression were also observed for
listeners with symmetric SNHL. Antiphasic SRTs, however, demonstrated greater
threshold differences in listeners with symmetric SNHL between the various categories
of hearing sensitivity compared to diotic SRT.
Unilateral hearing loss
Diotic presentation in unilateral SNHL does not result in strongly elevated SRTs
compared to listeners with bilateral normal hearing, because performance mainly
reflects the better ear. Furthermore, the 1 dB advantage provided by binaural
summation (Smits et al. 2016), was ineffective to detect unilateral SNHL. Listeners in
this study, with moderate unilateral SNHL achieved diotic SRTs comparable to
listeners with normal hearing. Similarly, diotic SRTs of those with severe-to-profound
unilateral or asymmetric SNHL compared to those with only mild symmetric SNHL.
Since listeners with unilateral SNHL could only adequately hear the digits presented
to the better ear, binaural interaction was either minimal or entirely absent. Antiphasic
SRTs were, as expected, significantly poorer and better reflected the degree of
hearing loss in the poorer ear than did diotic SRTs.
Listeners with strongly asymmetric hearing loss could increase the overall
presentation level of the DIN test by self-selecting a higher listening level. Some of
these listeners may then have enough residual hearing in the poorer ear to achieve a
degree of binaural advantage in antiphasic conditions when the signal intensity is
brought to threshold in that ear. However, the degree to which overall level adjustment
compensates for asymmetric hearing loss is also restricted to the tolerance of masking
noise in the better ear (Jerger et al. 1984). Three listeners with primarily high-
frequency unilateral SNHL had antiphasic SRTs within the normal range. Since
interaural timing differences are low frequency (< 1500 Hz) dependent (Middlebrooks
and Green 1991), it is expected that the favourable antiphasic SRTs obtained in these
three listeners was due to involvement of their residual low-frequency hearing.
Conductive hearing loss
The antiphasic test paradigm was very successful in detecting listeners with CHL. A
person with symmetric CHL could overcome loudness attenuation of the standard
diotic signals by increasing the overall presentation level, thereby achieving near-
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normal standard SRTs, as seen in listeners with mild and moderate CHL. Diotic SRTs
were slightly poorer across consecutive hearing sensitivity categories (mild, moderate
and severe-to-profound), but in most cases (20/23) were still within the normal hearing
range. Earlier studies demonstrated that antiphasic processing is disrupted by acute
CHL that both attenuates and delays sound passing through the ear (Hartley and
Moore 2003). Chronic CHL commencing in infancy can impair antiphasic listening
even after CHL has resolved (Moore et al. 1991; Pillsbury et al. 1991) and produces a
number of neurological changes affecting binaural integration (Polley et al. 2013). Due
to the disruption in interaural timing difference caused by CHL, antiphasic SRTs in our
study deviated considerably from listeners with normal hearing, in contrast to diotic
SRTs.
Training and reliability
Listeners with normal hearing and SNHL had a small training effect between the
antiphasic training list and test condition. There were no significant SRT differences
between the diotic DIN and antiphasic DIN test and retest measurements. Similar
findings were reported by Smits et al. (2013), suggesting that SRT improvement from
the training list to the first test condition is due to a procedural learning effect in naïve
listeners. Overall, the antiphasic DIN test-retest reliability was high and better detected
CHL as opposed to diotic SRTs. Overall, across the entire sample in this study,
antiphasic DIN test characteristics for detecting mild and moderate hearing loss was
high. The area under the receiver operating characteristics curve for antiphasic test
accuracy for hearing losses of >25 dB HL and >40 dB HL was significantly higher (0.94
and 0.96) than for diotic testing (0.78 and 0.80).
Clinical implications
The high sensitivity of a 3-minute antiphasic DIN to detect hearing loss of various
types, symmetries and degrees holds significant potential for population-based
screening. CHL, in the form of otitis media, is typically more prevalent among
underserved, remote and poor populations than other forms of hearing loss (Hunter et
al. 2007; Cameron et al. 2014) but is not easily detected with currently used DIN tests.
Since the DIN can be used in children as young as 4 years of age (Koopmans et al.
2018), the antiphasic DIN test may be a means of early identification in those
populations, once age-specific normative SRT scores are established. School-aged
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screening programs where the DIN has already been successfully implemented
(Denys et al. 2018), could similarly benefit from an antiphasic variant to improve
sensitivity and reduce test duration from a monaural to a binaural test. Of course, the
completion of a single antiphasic DIN test would not be able to differentiate between
either CHL or SNHL, or as with monaural testing, between unilateral or bilateral
hearing loss. However, following up on initial screening with other DIN variants (e.g.
monaural, filtered or modulated noise) for those who fail the antiphasic test could
potentially allow for categorization into bilateral, unilateral or CHL.
A smartphone platform of test delivery has proved a successful method of screening,
allowing for directed referrals from cloud-based data management platforms (De
Sousa et al. 2018), thereby optimizing resource allocation. Furthermore, it has been
shown that the test can be done reliably across various smartphone devices (either
iOS or Android operated) and transducers (Potgieter et al., 2016; De Sousa et al.
2018). Analysis of the hearZA tests taken approximately a year and a half after its
release, showed high test uptake (> 30 000 tests), especially among an important
target population of users younger than 40 years (De Sousa et al. 2018). The
development of the antiphasic DIN test in other language variants, however, is
recommended to make it accessible to a large global audience.
In conclusion, antiphasic SRTs correlated significantly better to poorer ear PTA than
diotic SRTs. As a result, antiphasic presentation markedly improved sensitivity to
detect SNHL and CHL, either symmetric or asymmetric, making it a powerful tool for
population-based screening.
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ACKNOWLEDGEMENTS
The authors thank all the participants of this study, Steve Biko Academic Hospital and
all participating private practices for their assistance with data collection. The authors
thank Li Lin for assistance with data analysis.
This research was funded by the National Institute of Deafness and Communication
Disorders of the National Institutes of Health under award number 5R21DC016241-
02. Additional funding support was obtained from the National Research Foundation
(Grant PR_CSRP190208414782). The 2nd author’s relationship with the hearX Group
and hearZA includes equity, consulting, and potential royalties. The last author’s
relationship with the hearX Group and hearZA includes equity, consulting and potential
royalties. David Moore is supported by Cincinnati Children’s Research Foundation and
by the NIHR Manchester Biomedical Research Centre.
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19
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FIGURES
Figure 1. Antiphasic and diotic SRT according to hearing category.
Figure 2. Correlations of the diotic DIN and antiphasic DIN to poorer ear PTA.
Figure 3. ROC curves presenting test characteristics of the antiphasic DIN and diotic
for detecting poorer ear PTA > 25 dB HL (left) and > 40 dB HL (right).
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