Effects of Directionality, Compression, and Working Memory on Speech Recognition
Authors: Rallapalli Varsha , Ellis Gregory , Souza Pamela 
Published: November 2020

Objectives:
Previous research has shown that the association between hearing aid–processed speech recognition and individual working memory ability becomes stronger in more challenging conditions (e.g., higher background noise levels) and with stronger hearing aid processing (e.g., fast-acting wide dynamic range compression, WDRC). To date, studies have assumed omnidirectional microphone settings and collocated speech and noise conditions to study such relationships. Such conditions fail to recognize that most hearing aids are fit with directional processing that may improve the signal to noise ratio (SNR) and speech recognition in spatially separated speech and noise conditions. Here, we considered the possibility that directional processing may reduce the signal distortion arising from fast-acting WDRC and in turn influence the relationship between working memory ability and speech recognition with WDRC processing. The combined effects of hearing aid processing (WDRC and directionality) and SNR were quantified using a signal modification metric (cepstral correlation), which measures temporal envelope changes in the processed signal with respect to a linearly amplified reference. It was hypothesized that there will be a weaker association between working memory ability and speech recognition for hearing aid processing conditions that result in overall less signal modification (i.e., fewer changes to the processed envelope).

Design:
Twenty-three individuals with bilateral, mild to moderately severe sensorineural hearing loss participated in the study. Participants were fit with a commercially available hearing aid, and signal processing was varied in two dimensions: (1) Directionality (omnidirectional [OMNI] versus fixed-directional [DIR]), and (2) WDRC speed (fast-acting [FAST] versus slow-acting [SLOW]). Sentence recognition in spatially separated multi-talker babble was measured across a range of SNRs: 0 dB, 5 dB, 10 dB, and quiet. Cumulative signal modification was measured with individualized hearing aid settings, for all experimental conditions. A linear mixed-effects model was used to determine the relationship between speech recognition, working memory ability, and cumulative signal modification.

Results:
Signal modification results showed a complex relationship between directionality and WDRC speed, which varied by SNR. At 0 and 5 dB SNRs, signal modification was lower for SLOW than FAST regardless of directionality. However, at 10 dB SNR and in the DIR listening condition, there was no signal modification difference between FAST and SLOW. Consistent with previous studies, the association of speech recognition in noise with working memory ability depended on the level of signal modification. Contrary to the hypothesis above, however, there was a significant association of speech recognition with working memory only at lower levels of signal modification, and speech recognition increased at a faster rate for individuals with better working memory as signal modification decreased with DIR and SLOW.

Conclusions:
This research suggests that working memory ability remains a significant predictor of speech recognition when WDRC and directionality are applied. Our findings revealed that directional processing can reduce the detrimental effect of fast-acting WDRC on speech cues at higher SNRs, which affects speech recognition ability. Contrary to some previous research, this study showed that individuals with better working memory ability benefitted more from a decrease in signal modification than individuals with poorer working memory ability.