The impact of noise and hearing loss on the processing of simultaneous sentences.

In crowded listening environments, selective attention enables information to be extracted from a talker of interest. However, in many cases it is desirable to retrieve information from a talker who is outside the immediate focus of attention (e.g., when two people talk at once). While some early studies showed that listeners with normal hearing perform poorly when asked to recall messages from unattended talkers (e.g. Cherry 1953), subsequent studies indicated that listeners are able to process unattended speech to some extent (e.g. Moray 1959; Conway et al. 2001; Rivenez et al. 2006) and can perform remarkably well at following two talkers when instructed to do so in advance (Best et al. 2006; Gallun et al. 2007; Ihlefeld and Shinn-Cunningham 2008). A recent survey of listeners with hearing loss (Gatehouse and Noble 2004) revealed that the self-perception of communication handicap is strong in listening situations calling for divided or rapidly shifting attention, such as following two talkers at once, catching the beginning of what a new speaker says, or engaging in conversation with a group of people. Given these subjective reports, we hypothesized that hearing loss impairs one’s ability to deal effectively with simultaneous messages and that this difficulty would be revealed in objective dual-task listening experiments. While several studies have examined divided listening in listeners with hearing-impairment (e.g. Strouse et al. 2000; Mackersie et al. 2001; Humes et al. 2006; Singh et al. 2008), the majority have used older listeners, making it difficult to factor out the differential contributions of age and hearing loss to the results. For this reason, we recruited a group of young listeners with sensorineural hearing loss and compared their performance to a similar group of young listeners with normal hearing. We chose a task in which listeners were required to respond to two simultaneously presented messages (see Best et al. 2006; Ihlefeld and Shinn-Cunningham 2008). In order to focus on the task of processing two simultaneous messages, we made an effort to eliminate other factors that might interfere with the ability of hearing-impaired listeners to hear out the messages. First, the overall presentation level was adjusted on an individual basis to reduce audibility as a factor in the hearing-impaired group. Second, the two messages were presented dichotically (one to each ear) to avoid peripheral interference between the stimuli that would be likely to impair speech intelligibility in the hearing-impaired group (Duquesnoy 1983; Festen and Plomp 1990; Summers and Leek 1998). Note that this design is similar to the classic dichotic digits paradigm introduced by Broadbent (1954) and used or adapted by others since to study attention (e.g. Treisman 1971), hemispheric dominance effects (e.g. Kimura 1961; Bryden 1963; reviewed in Hugdahl 2003), and auditory processing disorders (e.g. Fifer et al. 1983; Jerger and Martin 2006). Finally, the difficulty of speech reception was varied parametrically by adding noise to the messages at a range of signal-to-noise ratios. This allowed us to assess dual-task performance over various levels of listening difficulty within a group, but more importantly enabled us to compare the two groups at SNRs that roughly equate their speech reception performance.