Lexical tone information provides redundant cues for the recognition of Mandarin sentences in listeners with normal hearing in quiet conditions. The contribution of lexical tones to Mandarin sentence recognition in listeners with hearing aids (HAs) is unclear. This study aimed to remove lexical tone information and examine the effects on Mandarin sentence intelligibility in HA users. The second objective was to investigate the contribution of cognitive abilities (i.e., general cognitive ability, working memory, and attention) on Mandarin sentence perception when the presentation of lexical tone information was mismatched.
A text-to-speech synthesis engine was used to manipulate Mandarin sentences into three test conditions: (1) a Normal Tone test condition, where no alterations were made to lexical tones within sentences; (2) a Flat Tone test condition, where lexical tones were all changed into tone 1 (i.e., the flat tone); and (3) a Random Tone test condition, where each word in test sentences was randomly assigned one of four Mandarin lexical tones. The manipulated sentence signals were presented to 32 listeners with HAs in both quiet and noisy environments at an 8 dB signal to noise ratio.
Speech intelligibility was reduced significantly (by approximately 40 percentage points) in the presence of mismatched lexical tone information in both quiet and noise. The difficulty in correctly identifying sentences with mismatched lexical tones among adults with hearing loss was significantly greater than that of adults with normal hearing. Cognitive function was not significantly related to a decline in speech recognition scores.
Contextual and other phonemic cues (i.e., consonants and vowels) are inadequate for HA users to perceive sentences with mismatched lexical tone contours in quiet or noise. Also, HA users with better cognitive function could not compensate for the loss of lexical tone information. These results highlight the importance of accurately representing lexical tone information for Mandarin speakers using HAs.