The purpose of this study was to investigate the influence of the type of speech material on the benefit obtained from temporal fine structure (TFS) information in speech for young normal-hearing (YNH) and older hearing-impaired (OHI) participants.
The design was based on the work of Hopkins et al. (2008). They measured the speech reception thresholds for a target talker in a background talker as a function of the frequency range over which TFS information was available. The signal was split into 32 channels, each with a bandwidth equal to the equivalent rectangular bandwidth of the “normal” auditory filter at the same center frequency. Above a cutoff (CO) channel, channels were vocoded and contained only temporal envelope information. Channels up to and including CO were not processed. Hopkins et al. found that, as CO was increased, speech reception thresholds decreased more for normal-hearing participants than for participants with cochlear hearing loss, suggesting that the latter were less able to use TFS information. We used the same design, but compared results when the target speech materials were open-set sentences, as used by Hopkins et al., and when they were more predictable sentences with a closed word set (Danish Dantale 2).
With the open-set material, YNH listeners benefited more from TFS information than OHI listeners, replicating Hopkins et al. (2008). For the YNH participants, the benefit of adding TFS was greater for the open-set material than for the closed-set material, while no difference in TFS benefit across speech materials was found for the OHI participants.
The choice of speech material is important when assessing the benefit of TFS. Several factors may facilitate recognition in the absence of TFS cues, including small set size, predictable temporal structure of the target speech, and contextual effects. We speculate that TFS information is useful for reducing informational masking, by providing cues for the perceptual segregation of the target and background. When the target speech is highly predictable, informational masking may be minimal, rendering TFS cues unnecessary.