The results of this study show that CI recipients can have improved speech recognition in noise with processing options available clinically. ADRO processing demonstrated results similar to STD processing (i.e. no additional processing). This finding agrees with James et al (2002)
, who found no difference between these processing options in noise for adult CI recipients. Dawson et al (2004)
, however, did find a difference between ADRO and standard processing in noise with pediatric CI recipients. The difference between these studies may be due to the participants tested, as the Dawson study used pediatric CI recipients and the James study used adult CI recipients. ADRO performance also remained relatively stable when the noise level was increased. This stability across noise levels can most likely be explained by the maximum gain rule of ADRO processing, which does not allow the gain to exceed a specified maximum amount. At the moderate noise level used in this study, the amplification of background noise had already met the maximum amount of allowable gain and therefore, no additional amplification was provided when the noise level was increased.
This study found that BEAM processing resulted in significantly better performance than STD and ADRO processing at both noise levels. The ability of BEAM to improve speech recognition in noise for CI recipients has been demonstrated in previous research. Wouters and Vanden Berghe (2001)
and Spriet et al (2007)
found larger improvements in SNRs than the current study. However, these models used different noise stimuli (speech-weighted noise and multi-talker babble), which were presented from one to three noise sources. The current study used R-SPACE™ (live restaurant) noise presented from a diffuse field. The R-SPACE™ noise has been previously found to result in a poorer RTS than other noise. Valente and colleagues (2006)
tested bilateral hearing aid users in the R-SPACE™ and found that the RTS was1.3 dB poorer for R-SPACE™ noise than for HINT noise, which is filtered to match the average long-term spectrum of HINT sentences. Therefore, speech recognition tasks may be more difficult when the R-SPACE™ noise is used compared to other continuous noise types.
The difference in the R-SPACE™ configuration may also explain the difference between the current findings and previous research. The R-SPACE™ configuration presents noise from all eight loudspeakers. Therefore, the front speaker presents both speech and noise. BEAM utilizes directionality to divide speech from spatially separated noise. When the speech and noise are presented together from the front speaker, BEAM relies on the adaptive noise cancellation stage to reduce the noise. BEAM may be more effective at improving speech recognition in noise when the noise source is spatially separated from the speech signal. Since typical real-world listening situations often include combined speech and noise, previous studies may have overestimated the absolute performance of BEAM, and current results may better predict the performance of BEAM processing in real-world situations similar to that replicated by the R-SPACE™.
BEAM processing showed a significant decrease in performance with the increase in noise level. This reduction in performance is probably due to the second stage of BEAM, which utilizes adaptive noise cancellation. This may affect the clarity of the speech reference by filtering out portions of the speech signal along with the noise.
ASC processing resulted in the best performance at the loud noise level and was almost as good as BEAM at the moderate noise level. This result agrees with the findings of Wolfe et al (2009)
, where ASC improved speech recognition in the presence of loud noise levels. ASC processing also maintained performance across noise levels having almost equivalent performance at 60 and 70 dB SPL. The benefit of ASC processing at a louder noise level was not necessarily expected in the R-SPACE™, as ASC processing limits background noise by increasing the AGC kneepoint. This results in reduction in amplification for distant, softer sounds, and increased amplification of closer, louder sounds. One would postulate that in the diffuse noise environment of the R-SPACE™, where the noise sources and speech are the same distance, that ASC processing would not significantly benefit speech recognition. The noise sources were not equidistant from the listener in the Wolfe et al (2009)
study. The rear noise sources were further from the listener than the front noise sources, and the speech signal was closer to the listener than all noise sources. It is possible in the current study that the regular directional microphone increased the sensitivity of sounds arriving from the front, and the ASC processing maximized suppression of background noise. These two features working in conjunction may be responsible for the performance in a diffuse noise field. Regardless of the mechanisms at work, the findings suggest that ASC processing is a good option to limit amplification of background noise at moderate and loud levels while maintaining speech intelligibility.
It is also important to note the possible effect of infinite compression on speech recognition in noise. The Nucleus Freedom processor, at default settings, codes inputs from 25 to 65 dB SPL into the electrical dynamic range. The threshold (25 dB SPL) can be adjusted in the programming software, but the upper limit (65 dB SPL) is fixed (Wolfe et al, 2009
). Therefore, any signal greater than 65 dB SPL would be exposed to high levels of compression.
The RTSs obtained in this study resulted in infinite compression being activated for the majority of participants across processing conditions and noise levels. Five participants were not subject to infinite compression in the 60 dB SPL noise condition, as they obtained RTSs below +5 dB across all processing conditions. Seven participants had infinite compression in some conditions and not in others, as they obtained RTSs above and below +5 dB across processing conditions. The remaining 18 participants were subject to infinite compression across all processing conditions in both noise levels. In addition, ASC changes the magnitude of infinite compression, as ASC aims to keep the noise floor at least 15 dB below the AGC kneepoint. Limiting the background noise to below the point where speech is compressed may be the reason ASC performed best at the loud input level.
The three bilateral participants demonstrated a bilateral benefit with almost all processing options at both noise levels. This supports the findings of previous bilateral CI studies that showed improved speech recognition in noise with binaural hearing. Several studies attribute the majority of bilateral benefit to the head-shadow effect (Gantz et al, 2002
; Tyler et al, 2003
; van Hoesel and Tyler, 2003
; Litovsky et al, 2006
; Buss et al, 2008
; Basura et al, 2009
). In the current study, the noise source is diffuse. The exact SNR at each ear varies as the R-SPACE™ noise changes in real-time. The R-SPACE™ noise is uncorrelated, so the exact level of noise coming out of each loudspeaker may be higher or lower than other loudspeakers at any moment in time. The overall SNR at each ear should be similar when averaged over time. It is possible that a rapid-changing head shadow effect may contribute to the observed bilateral improvement.
The current results with the three bilateral participants showed a mean bilateral improvement as high as 9 dB compared to unilateral performance. Previous studies estimated the head-shadow effect to improve the SNR between 4 and 7 dB (van Hoesel and Tyler, 2003
; Litovsky, 2006
; Basura et al, 2009
). The greater bilateral benefit observed in this study may be attributed to the central phenomena of binaural squelch and redundancy. The noise presented from each speaker is not identical, allowing the brain to use differences in the timing and spectrum of the input signal to separate the speech and noise (Tyler et al, 2002
; Tyler et al, 2003
; Ching et al, 2007
; Brown & Balkany, 2007
). Also, the speech presented from the front loudspeaker is perceived by both ears providing redundant information to the brain. This redundancy should allow the brain to develop a better representation of the message (Dillon, 2001
; Ching et al, 2007
The variation in results between the current study and previous ones could also be ascribed to characteristics of the individual participants. These three participants were experienced listeners with their bilateral CIs (mean bilateral experience of 2.7 years). Some studies have measured bilateral benefit shortly after the second CI (Gantz et al, 2002
; Tyler et al, 2002
; Litovsky et al, 2006
). Recent research has indicated that the effect of binaural squelch increases over time (Buss et al. 2008
; Basura et al, 2009
; Eapen et al, 2009
; Litovsky et al, 2009
). Eapen et al (2009)
found that the squelch effect significantly increased after the first year of bilateral experience. All three of the participants in this study had over one year of bilateral experience, which may have resulted in increased benefit from binaural squelch.
The bilateral participants demonstrated similar speech understanding in quiet with each ear alone. This equivalent performance between right and left ears may allow better integration of the binaural signal in noise. It is unclear how differences between the ears may impact bilateral performance. Finally, the difference in noise types and arrays may also play a role in the variation. The R-SPACE™ noise may better demonstrate the brain’s ability to analyze the differences and similarities between inputs from the two ears to improve the internal representation of speech and noise. However, the small sample size of the current study makes it difficult to draw conclusions or comparisons to other studies.
In addition to the difference in performance between unilateral and bilateral stimulation of these participants, the effect of the noise level is a fascinating finding. These participants’ unilateral performance was similar to the mean unilateral performance of the group, with a poorer performance at the louder noise level. However, this was not true when they were stimulated bilaterally. Their bilateral RTS was better when the noise got louder. This was true for all the bilateral participants with three of the processing options (ADRO, ASC, and BEAM). The bilateral improvement found at the higher noise level suggests that bilateral benefit may be greater as the listening situation becomes more challenging. It is feasible that many traditional clinical measures may not provide adequate evaluation for bilateral stimulation. It has been suggested that bilateral benefit measured in studies may underestimate the benefit received by bilateral CI recipients. It is often the case that the subjective reports of bilateral benefit exceed the measured benefit (Litovsky et al, 2006
; Laske et al, 2009
). The large bilateral benefit seen in this study may better estimate CI recipients’ everyday performance. The assessment of bilateral benefit, however, is difficult and may vary between individuals and tasks. Although the bilateral trend seen in this study is interesting, results should be interpreted with caution due to the small number of bilateral participants.
Although different processing options can improve the speech recognition in noise for CI recipients, they still perform notably poorer than normal-hearing individuals. In this study, the best speech recognition for the unilateral participants was found with BEAM processing in 60 dB SPL noise, which resulted in a mean RTS of 8.3 dB. This is 11 dB poorer than that reported by Nilsson et al (1992)
for normal-hearing individuals using HINT sentences in spectrally-matched noise. For bilateral participants, the best RTS of 0.4 dB was found with BEAM processing in 70 dB SPL noise. The performance of the bilateral participants is on average closer to that of normal-hearing individuals, but is still poorer. Valente et al (2006)
evaluated twenty-five bilateral hearing aid users with mild to moderately-severe sensorineural hearing loss using HINT sentences in the R-SPACE™. Average performance of the hearing aid users showed an RTS of 2.0 dB and −0.3 dB with an omnidirectional and directional microphone, respectively. The unilateral and bilateral CI participants in the current study performed poorer than bilateral hearing aid users. However, the average bilateral CI performance was only 1.1 dB poorer than that of bilateral hearing aid users. ASC and BEAM processing improve the ability of CI users to understand speech in background noise, but performance with these strategies is still poorer than bilateral hearing aid users and far from that of normal-hearing individuals.
The results of this study suggest the type of processing and the noise level interact to produce different degrees of speech recognition within the same individual. This has important clinical relevance for programming of different processing options and counseling CI recipients on the use of different processing strategies. This finding supports CI recipients’ subjective reports of preferences for different processing options in different listening environments. Typically, patients are given one program to use in noisy listening environments. However, this study supports providing the patient with two separate noise programs, BEAM for moderate levels of background noise and ASC for loud levels.