Cognition noise and hearing aid signal processing
The first studies that systematically investigated individual cognitive differences and the benefit of compression speed were those of Gatehouse, Naylor and Elberling (2003, 2006). Later studies have corroborated the early findings (Lunner & Sundewall-Thorén, 2007; Rudner et al., 2011).
Testing individual benefit of compression release settings in background noise
Early studies indicated that cognitive factors correlate with aided speech measures, depending on the speed and complexity of the signal-processing, and the nature of the speech and background signals used to test performance (Gatehouse, Naylor and Elberling 2003, 2006; Humes 2003, 2007; Lunner 2003).
This study aimed to replicate the patterns of candidature in the domain of cognitive capacity found by Gatehouse et al. (2006). Working memory performance was assessed in the same way as in Gatehouse et al. (2006). Speech recognition in noise was assessed by the Dantale 2 speech corpus. Test conditions were fast- and slow-acting compression, in either unmodulated or modulated backgrounds. Tests were performed after 9 weeks of acclimatisation to the compression setting.
Consistent with Gatehouse et al. (2006), this study indicated that subjects showing a low score in a cognitive (working memory) test performed better in the speech recognition test with slow time constants than with fast time constants. They also performed better in unmodulated noise than in modulated noise. Meanwhile, subjects with high scores in the cognitive test showed the opposite pattern.
Furthermore, cognitive test scores were significantly correlated with the differential advantage of fast-acting versus slow-acting compression in conditions of modulated noise. The pure tone average threshold explained 30% of the variance in aided speech recognition in noise under relatively simple listening conditions, while cognitive test scores explained about 40% of the variance under more complex, fluctuating listening conditions, where the pure-tone average explained less than 5% of the variance. This suggests that speech recognition under steady-state noise conditions may underestimate the role of cognition in real-life listening.
Two more studies with a similar test design were published after the aforementioned initial studies: Cox & Xu (2010), with a somewhat shorter test time (4 weeks) for each compression setting, and Rudner et al. (2011), with a longer test time (9 weeks). Cox and Xu (2010) showed a different pattern of results, while Rudner et al. (2011) corroborated the earlier findings.
Thus the pattern of results seems to be the following: The effect of cognition on speech understanding in modulated noise with fast-acting compression may only pertain after a period of four to nine weeks of familiarisation. Prior to this, persons with lower cognitive capacity may benefit more from slow-acting compression.
The finding that persons with high working memory capacity performed better in modulated noise with fast-acting compression agrees with findings of previous studies involving a familiarisation period of at least nine weeks.
Cox RM, Xu J (2010). Short and long compression release times: speech understanding, real
world preferences, and association with cognitive ability. Journal of the American Academy of Audiology, 21, p. 121-138.
Humes LE (2003). Modeling and predicting hearing aid outcome. Trends in Amplification,7, p. 41-75.
Humes LE. (2007). The contributions of audibility and cognitive factors to the benefit
provided by amplified speech to older adults. Journal of the American Academy of Audiology 18(7), p. 590-603.
Lunner T (2003). Cognitive Function in Relation to Hearing Aid Use. International journal of audiology. Supplement, 42(1), 1S49-1S58.
Gatehouse S, Naylor G, Elberling C (2003). Benefits from hearing-aids in relation to the interaction between the user and the environment. International journal of audiology. Supplement 1, 42(1), 1S77-1S85.
Gatehouse S, Naylor G, Elberling C (2006). Linear and nonlinear hearing-aid fittings -1. Patterns of benefit. International journal of audiology, 46(3), p.130-152.
Gatehouse S, Naylor G, Elberling C (2006). Linear and nonlinear hearing aid fittings -2. Patterns of candidature. International journal of audiology, 46(3), p.153-171.
Lunner T, Thorén ES (2007). Interactions between Cognition, Compression, and Listening Conditions: Effects on Speech-in-Noise Performance in a Two-Channel Hearing Aid. Journal of the American Academy of Audiology, 18(7), p. 604-617.