The advantage of knowing the talker.

Many audiologists have observed a situation where a patient seems to understand something spoken by his/her spouse but not the same information spoken by a stranger. We know from both anecdotal observations and research (Scarinci et al., 2008) that spouses or family members make accommodations for the hearing-impaired conversation partner. In long-term relationships in which one partner has hearing impairment, the talker may raise her voice, speak more clearly, or choose vocabulary or syntax that are familiar to the listener. We also know that such strategies have been shown to improve communication within the couple (Preminger, 2008). In other words, audiologists may be observing the benefits of communication strategies rather than voice familiarity per se. However, there is also evidence that familiarity with a talker’s voice can improve speech recognition under some circumstances. Research in this area has focused on deliberately-trained recognition of previously unknown speakers, usually by young listeners with normal hearing. In general, those data indicate that speech recognition is improved when stimuli are produced by a previously-heard talker compared to a novel talker (Palmeri et al., 1993; Nygaard, Sommers and Pisoni, 1992; Nygaard and Pisoni, 1998; Yonan and Sommers, 2000; Sheffert et al., 2002; Newman and Evers, 2007). To understand why this happens, consider the wide variability in speech production. Listeners are able to identify speech sounds despite intra-talker differences in fundamental frequency, speech rate, frequency location of vocal tract resonances, and overriding voice qualities such as tremor or hoarseness. Originally, this information was viewed as a type of noise that listeners had to discard in order to abstract the identifiable components of speech for comparison to stored lexical representations (Pisoni, 1981). More recent work has argued that information about the talker is stored and may be used in combination with lexical information (Palmieri et al., 1993; Nygaard et al., 1994, 1995; Goldinger, 1996; Remez et al, 2007). This second view is consistent with the idea that a familiar voice might aid in speech recognition. If learning to understand a particular talker is anything like language learning in general, we would expect talker familiarity to be a gradient phenomenon that improves with increased exposure. However, because of practical constraints, most published studies evaluated deliberate short-term exposure (hours to days), usually with strictly controlled test material. In those cases, the listener did not know the talker or interact with them in person, but simply heard their recorded voice. In such studies it has been found that mere exposure to a voice does not necessarily confer an advantage across varying task conditions. As one example of this, exposure to sentences by a target talker improved recognition of single words by that talker, but exposure to single words did not improve recognition of sentences (Nygaard and Pisoni, 1998; Yonan and Sommers, 2000). A plausible explanation for such findings is the idea that familiarity might be aided by exposure to a richer variety of utterances which cover a range of topics, prosody and context, as would occur in a long-term relationship. Indeed, one study which failed to show an advantage of implicit familiarity exploited a short-term relationship (professor/student) which existed under constrained communication conditions (classroom teaching) (Newman and Evers, 2007). An additional group of studies focused on deliberate auditory training of listeners with hearing loss, often via a computer-based program that the listener completed on an individual basis over weeks or months. Such programs may be completed either with or without hearing aids; may employ syllables, words or sentences; and typically increase in difficulty (such as increasing level of background noise) throughout the course of the training. Like the short-term training studies with normal-hearing listeners, these studies also show that learning does not generalize to improved recognition of novel speech materials that are dissimilar to those used in the training programs (see Boothroyd, 2010 and Sweetow and Palmer, 2005 for recent reviews). For example, in one study training words for a single talker produced improvement in words by novel talkers, but not on connected speech using the same words (Burk & Humes, 2008). Thus, a real-life paradigm of interest to clinicians is the benefit of long-term, implicit learning over many years of interaction within a relationship. The case of long-term relationships in which one individual has a hearing loss presents an interesting scenario. First, we focus here on older talker-listener pairs. There has been less attention to the advantage conferred by a familiar (or trained) voice for older listeners, although the available data hint that older listeners may respond differently. Several studies have identified age deficits for speaker identification (Yonan and Sommers, 2000; Helfer and Freyman, 2008; Rossi-Katz and Arehart, 2009) or auditory priming tasks (Schacter et al., 1994; Huang et al., 2010). However, Yonan and Sommers (2000) found that older listeners trained over a two-day period derived as much or more benefit from talker familiarity than younger listeners. Taken together, these data suggest that older listeners may process and/or store voice information differently than younger listeners, but that does not preclude using voice information to improve perception. A final point with regard to age is that all of the training studies conducted thus far have used test materials created within the laboratory. Although talker age was usually not specified, we can assume that the materials were produced by “ideal” (and probably younger) talkers. In the real-life situation we consider, the talker’s voice may also be subject to age-related changes, such as vocal tremor or hoarseness (Kendall, 2007) which could make the voice more immediately identifiable, but perhaps alsoless intelligible. Second, we consider a situation in which the listener has hearing loss. There is some evidence that the advantage of talker familiarity may be more beneficial under degraded listening conditions. McLennan and Luce (2005) have argued that familiarity effects are more likely to be observed when processing of the input signal is slowed, as might be the case when the signal is impoverished. Available data for normal-hearing younger listeners presented with low-pass filtered speech (Church and Schacter, 1994) or speech in noise (Nygaard and Pisoni, 1998; Yonan and Sommers, 2000) support this idea. Previous work on talker familiarity, including those studies focused on older listeners (Yonan & Sommers, 2000; Huang et al., 2010), tested adults with normal- to near-normal auditory thresholds. Thus, available data may not reflect the effects of familiarity that occur in the older hearing-impaired population. A further consideration is the fact that most older, hearing-impaired listeners show considerable difficulty understanding speech in noise. This is most apparent in situations where there is both energetic and informational masking (as when the background consists of other speech). Under such circumstances, Newman and Evers (2007) proposed that familiarity with the target voice could aid auditory stream segregation. It is not known what specific aspects of the familiar voice might contribute to this, but we know that listeners can use a wide variety of acoustic cues to familiarity, including fundamental frequency (Church and Schacter, 1994) and formant trajectories (Sheffert et al., 2002). Familiarity aside, we know that ability to track such frequency variations aids source segregation (Binns and Culling, 2007; Miller et al., 2010). Finally, the ability to follow frequency variations is impaired in some older listeners (Souza et al., 2011). Taken together, these data suggest that even if familiarity enhances those cues for younger listeners, that enhancement may not be the same in older listeners. Because previous work used broad-band noise maskers rather than speech (Yonan and Sommers, 2000), this issue has not been investigated. The current study addressed the issue of long-term talker familiarity for older individuals with hearing impairment in a variety of listening situations. In each case, the talker-listener pair for the familiar condition represented a close, long-term relationship (spouse or close friend). In Experiment 1, we measured the benefit of a familiar voice when the difficulty level was manipulated by controlling the level of a speech-shaped background noise. In Experiment 2, we measured the benefit of a familiar voice when the background noise was other speech (introducing aspects of informational masking).