Time Course and Cost of Misdirecting Auditory Spatial Attention in Younger and Older Adults Gurjit Singh,1,2,3 M. Kathleen Pichora-Fuller,3,4 and Bruce A. Schneider3,4 Objectives: The effects of directing, switching, and misdirecting auditory spatial attention in a complex listening situation were investigated in 8 younger and 8 older listeners with normal-hearing sensitivity below 4 kHz.
difficulty communicating in challenging listening situations. These difficulties understanding speech in noise may arise from age-related deficits in peripheral, central auditory, and/or cognitive processing. Reduced working-memory capacity, age-related slowing in the speed of information processing, and inhibitory or other attentional deficits are some of the most likely cognitive sources of the difficulties that older listeners experience in understanding language spoken in challenging situations (for a review see Cohen 1987; Pichora-Fuller & Singh 2006; Schneider et al. 2010). Considerable research has been conducted investigating the contributions of working memory and speed of processing to the speech-understanding difficulties of older adults, but the role of attentional factors has received less scrutiny. Attention could be especially important in situations such as communicating with friends at a noisy restaurant when a listener may need to monitor multiple simultaneous voices. Recently, the ability to attend to a target in a multi-talker situation (e.g., Kidd et al. 2005; see also Spence & Driver 1994; Mondor & Zatorre 1995) has been studied using the well-established attentional cueing paradigm (Posner et al. 1980). In this paradigm, an observer is typically asked to detect a target signal at one of two (or more) locations, with a pretrial cue indicating the most and/or least likely locations at which the signal will be presented. One common variant of the cueing paradigm is to provide observers with pretrial cues indicating the probability of a target appearing at a particular location. For example, in a probabilistic spatial cueing study, a “valid” or “likely” trial occurs when a target is presented from a location with a high probability of occurrence and an “invalid” or “unlikely” trial occurs when a target is presented from a location with a low probability of occurrence. In a previous study using the cueing paradigm, we found that younger and older adults with normal audiometric thresholds below 4 kHz were better able to identify targets presented from likely than unlikely locations, but that both groups experienced a similar benefit from knowing where to listen (Singh et al. 2008). Failure to observe a significant age-related difference in benefit from cueing could be evidence that there are no age-related differences in switching attention from one location to another. Alternatively, the failure to observe age-related differences may have occurred because the listening task was too easy to detect differences that might have been observed in more challenging situations where listeners must rapidly switch their attentional focus from one source to another. One reason the task may have been too easy was that there was a relatively long time delay between the cue and the target and this delay may have accommodated slower switching by older adults. Another possible explanation is that the listeners in our previous study were only required to allocate attention to the location where the cue was heard and did not have to use the cue to redirect attention to another location. For example, in multi-talker situations, it could be that when a listener switches attention from one source to another, younger listeners may be more able than older listeners to “recover” if attention is
Design: In two companion experiments, a target sentence was presented from one spatial location and two competing sentences were presented simultaneously, one from each of two different locations. Pretrial, listeners were informed of the call-sign cue that identified which of the three sentences was the target and of the probability of the target sentence being presented from each of the three possible locations. Four different probability conditions varied in the likelihood of the target being presented at the left, center, and right locations. In Experiment 1, four timing conditions were tested: the original (unedited) sentences (which contained about 300 msec of filler speech between the call-sign cue and the onset of the target words), or modified (edited) sentences with silent pauses of 0, 150, or 300 msec replacing the filler speech. In Experiment 2, when the cued sentence was presented from an unlikely (side) listening location, for half of the trials the listener’s task was to report target words from the cued sentence (cue condition); for the remaining trials, the listener’s task was to report target words from the sentence presented from the opposite, unlikely (side) listening location (anticue condition). Results: In Experiment 1, for targets presented from the likely (center) location, word identification was better for the unedited than for modified sentences. For targets presented from unlikely (side) locations, word identification was better when there was more time between the call-sign cue and target words. All listeners benefited similarly from the availability of more compared with less time and the presentation of continuous compared with interrupted speech. In Experiment 2, the key finding was that age-related performance deficits were observed in conditions requiring anticue but not cue responses. Conclusions: The findings from Experiment 1 suggest that for both age groups, stream continuity mediates the process of allocating and maintaining auditory spatial attention when the target originates at an expected location, but that time is needed for the reallocation of auditory spatial attention when the target originates at an unexpected location. The findings from Experiment 2 suggest that when attention is momentarily misdirected, difficulties disengaging attention may help explain why older adults with good hearing report difficulty communicating in multi-talker listening situations. (Ear and Hearing 2013;34;711–721)
INTRODUCTION Hearing impairment is one of the most common chronic conditions affecting listeners who are more than 60 years of age, with nearly 1 of 2 older adults experiencing some form of hearing loss (Cruickshanks et al. 1998). Furthermore, even in the absence of elevated audiometric thresholds, older listeners often report 1 Phonak Canada Ltd, Mississauga, Ontario, Canada; 2Department of Speech-Language Pathology, University of Toronto, Toronto, Ontario, Canada; 3Toronto Rehabilitation Institute, Toronto, Ontario, Canada; and 4Department of Psychology, University of Toronto Mississauga, Mississauga, Ontario, Canada.
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momentarily misdirected to the wrong source. Two experiments were conducted to explore these two possible reasons for the absence of an age-related difference in our original study.
Role of Time in the Ability of Younger and Older Listeners to Switch Attention Although there is substantial literature exploring the time course of visual spatial attention (for a review see Egeth & Yantis 1997), very few studies have explored the time course of auditory spatial attention. On the one hand, the gap in our understanding regarding the time course of allocating auditory spatial attention is not surprising because the conditions under which spatial cueing yields robust effects on listening performance were only recently delineated (e.g., Arbogast & Kidd 2000; Kidd et al. 2005). On the other hand, this gap is surprising given the long-standing observation that successful listening in noisy environments requires the rapid deployment of auditory attention to whichever sound source captures the listener’s interest (Cherry 1953). For example, when communicating over dinner with colleagues at a noisy restaurant, listeners must frequently switch their focus of attention from one talker to the next, and listening performance may depend on time for several reasons: time may be required to switch attention (Shinn-Cunningham & Best 2008); additional time could provide an opportunity to further process information (Barrouillet et al. 2004); or time may facilitate the formation of auditory objects (Best et al. 2008). Because slowing of information processing and problems dividing attention are commonly observed age-related declines in cognition, it is expected that older adults should be less able to function in situations in which rapid switching of attention is necessary.
Experiment 1 Effect of Time Available for Listeners to Switch Attention The primary goal of Experiment 1 was to investigate the influence on word identification of age and the time available to
switch auditory attention in a multi-talker environment in which there is uncertainty about the location of the target talker. This was investigated in healthy younger and older listeners with normal audiometric thresholds up to 4 kHz. A probabilistic cueing paradigm was used in which the time available to allocate attention from one source to another was systematically manipulated.
Participants and Methods Participants • Eight younger (mean = 22.4 years; SD = 4.4 years) and 8 older adults (mean = 70.9 years; SD = 2.5 years) participated in the study. All participants were native English speakers, in good overall health, and had clinically normal puretone air conduction thresholds (≤25 dB HL) from 0.25 to 3 kHz bilaterally (see Fig. 1). All the participants had participated in prior experiments concerning communication and aging. Two of the 16 participants (1 younger and 1 older adult) had participated in our prior study (Singh et al. 2008). Participants provided informed consent and were paid $10 per hr. Stimuli • The stimuli consisted of sentences from the Coordinate Response Measure (CRM) corpus spoken by the 4 male talkers (Bolia et al. 2000). The sentences have the format: “Ready (call-sign) go to (color) (number) now,” with all possible combinations of eight callsigns (Arrow, Baron, Charlie, Eagle, Hopper, Laker, Ringo, Tiger), four colors (red, white, blue, green), and eight numbers (1, 2, 3, 4, 5, 6, 7, 8). The call-sign provided the cue to enable listeners to identify the target sentence on a given trial. The color and number were the target words that were scored. In addition to the original CRM sentences, listeners were also presented edited CRM sentences. For the original sentences, the words “go to” intervened between the call-sign cue word and the target words. For the edited sentences, the words “go to” were replaced by a silent pause of 300, 150, or 0 msec. The maximum pause duration to be tested was chosen to be 300 msec because the duration of the words “go to” ranged between 225 and 300 msec in a random sample of 50 sentences. Conditions with no pause or an intermediate pause duration of 150 msec were also
Fig. 1. Mean left (×) and right (○) ear audiometric thresholds (± standard error of the mean) for older (black/dashed lines) and younger (gray/solid lines) participants from Experiment 1 (left panel) and Experiment 2 (right panel).
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tested to determine whether performance would be reduced by decreasing the time between the call-sign cue and target words. Each of the 1024 sentences was edited manually, with the editing points determined by visual examination of the time waveform. To minimize click artefacts, editing points occurred at zero crossing in the soundfiles of the sentence materials. Equipment • All testing was conducted in a 3.3 m × 3.3 m single-walled sound-attenuating booth. The stimuli were controlled and presented via custom software. The stimuli were routed from a Dell computer to a Tucker-Davis Technologies System III to a Harmon/Kardon (model HK3380) amplifier (Alachua, FL). Sentences were converted to analog (RP2.1) at a sampling rate of 24.414 kHz by a 24-bit D/A converter, attenuated (PA5), conditioned (SA1), and presented over three loudspeakers (model 1761–9630; Grason-Stadler Inc., Eden Prairie, MN) located at approximately the same height as the listener’s head when seated. Visual cueing was displayed on a 17-in touchscreen monitor on a table at a height of 0.46 m in front of the listener. The response choices were also displayed visually, as was feedback regarding whether or not each response was correct. Calibration was performed using a B&K sound level meter (type 2260) with a 0.5-in condenser microphone (type 4189) using the A-weighting equivalent to measure concatenated CRM sentences. Procedures • On each trial, three sentences were presented simultaneously, with each sentence being presented from one of three possible locations: in front of, to the left, or to the right of the listener. Participants were instructed to face directly ahead. Two types of cues, call-sign cues and location certainty cues, were used on each trial in the study and they were both visually presented 1 second before each trial. The call-sign cue indicated the name (e.g., Baron) near the beginning of the sentence, which enabled the listener to identify the target sentence. On each trial, the call-sign cue could change. The call-sign cue was presented visually before the presentation of the auditory stimuli. One of four different location certainty cues (“0-100-0,” “10-80-10,” “20-60-20,” “33-33-33”) indicated the probability of the cued sentence being presented from the left, center, or right loudspeaker. For example, 10-80-10 indicated that the cued sentence would be presented from the left loudspeaker in
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10% of the trials, from the center loudspeaker in 80% of the trials, and from the right loudspeaker in 10% of the trials. In contrast to the conditions where the location was certain (0-100-0) or random (33-33-33), for the 10-80-10 and 20-60-20 conditions, the location of the target in a given trial could be either likely or unlikely. A likely trial occurred when the cued sentence was presented from the spatial location with the highest probability of occurrence (i.e., the loudspeaker located at 0°) and an unlikely trial occurred when the cued sentence was presented from a location with the lower probability of occurrence (i.e., the loudspeaker located to the left or right of the listener; see left half of Fig. 2). The location certainty cue remained constant for a block of 30 trials. The location of the cued sentence was randomly selected from the left, center, and right locations, with the constraint that the certainty cue was accurate across the block of 30 trials. All sentences were presented at 60 dB A. For all trials, both the correct color and number were required for a response to be scored as correct. Feedback (correct or incorrect) was provided after every trial, and summary scores were presented at the end of each block. At the start of each visit, participants completed practice trials for a minimum of 30 min. Design • Word-identification accuracy was measured using a 4 (target location certainty: 0-100-0, 10-80-10, 20-60-20, and 33-3333) × 4 (sentence: unedited and edited with 300,150, or 0 msec pauses) within-subjects design. For each participant, data were collected on 4 separate days, with each visit lasting about 2 hr. At each visit, participants were assigned to one of the four sentence conditions. For each of the sentence conditions, data were collected in four blocks of 30 trials. A different target certainty condition was assigned randomly without replacement for each of the four blocks. For each visit, participants completed two runs of the four blocks of 30 trials. Thus, at the end of each visit, a participant completed 60 trials for each of the four location certainty conditions for a given sentence condition. The order of the four sentence conditions for half of the participants was as follows: (1) 0 msec, (2) 150 msec, (3) 300 msec, and (4) unedited sentences; the order was reversed for the other half of the participants.
Fig. 2. Task sequence for the 0.8 and 0.6 location certainty conditions for both the unconditional and conditional set of instructions when the cued sentence was presented at either the likely or unlikely listening location. In this example, the pretrial call-sign cue is “Charlie.”
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Fig. 3. Mean percent correct word-identification scores (± standard error of the mean) for younger (left panel) and older (right panel) adults depicted across the four location certainty conditions. The data presented are from the present study (black bars), and the studies by Singh et al. (2008) (white bars), and Kidd et al. (2005) (gray bars).
Results The results from the baseline condition in the present study will be compared with the results from previous studies, and the general results will also be presented followed by the specific findings most pertinent to the main goal of the experiment, which was to determine whether there was an age-related effect on performance of the time available for listeners to switch auditory spatial attention. Baseline Condition and General Results • Figure 3 depicts the performance of listeners for targets presented with real spatial separation in the present and in two previous studies (Kidd et al. 2005; Singh et al. 2008). Overall, the results of the present study are in excellent agreement with previous findings regarding this baseline condition. In general, examination of the overall results revealed two patterns of interest (see Fig. 4). The most robust finding was that performance decreased as uncertainty about the target location increased. When the data were collapsed across the four timing
conditions and two age groups, performance was 80% correct when listeners were certain about the location of the target and 51% correct under conditions where the location of the target was random. The second pattern was that differences in performance were observed between the unedited and edited sentence conditions; word identification was 67% correct for the unedited sentences and ranged from 61 to 63% correct for the 0, 150, and 300 msec edited sentences. To confirm these descriptions a 2 × 4 × 4 repeated-measures analysis of variance (RMANOVA) was conducted with age (younger and older) as a between-subjects factor, and target location certainty (1.0, 0.8, 0.6, and 0.33) and sentence condition (unedited and edited sentences with 0, 150, and 300 msec pauses) as within-subjects factors. There was a significant main effect of target location certainty (F[3,42] = 99.20; p < 0.001). A post hoc Student–Newman–Keuls (SNK) test revealed significant differences among each of the four target location certainty specifications (p < 0.01). A main effect of sentence condition (F[3,42] = 4.86; p < 0.01) was also observed. Post
Fig. 4. Mean percent correct word-identification scores (± standard error of the mean) for the four location certainty conditions for the younger (left panels) and older (right panels) adult groups. White, light gray, dark gray, and black bars represent the 0, 150, 300 msec and original (unedited) sentence conditions, respectively.
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hoc SNK testing indicated that performance was better in the unedited original sentence condition compared with the 0 and 150 msec edited sentence conditions, and that performance was better when in the 300 msec compared with the 0 msec edited sentence condition (p < 0.05). No other significant effects were observed (p > 0.05). Note that there was neither a significant main effect of age nor any significant interaction with age. Listening Expectations: Effect of Pause Duration • To determine whether there was an age-related effect on performance of the time available time to switch attention, the relevant subset of conditions are the intermediate (0.8 and 0.6) location certainty conditions where both likely and unlikely trials occurred. Specifically, it was assumed that listeners would expect the target to be presented at the likely center location such that it would be necessary to switch attention if the target was presented from an unlikely side location, whereas in the 1.0 condition the target was always presented at the center location and there would be no need to switch attention, and in the 0.33 condition there was no likely condition. Because a similar pattern of performance was observed for the 0.8 and 0.6 location certainty conditions, the results across these two conditions were collapsed (see Fig. 5). A 2 (likely versus unlikely) × 4 (unedited and edited sentences with 0, 150, and 300 msec pauses) × 2 (younger versus older) RMANOVA was conducted where spatial listening expectations and sentence condition were within-subject variables and age was a between-subjects variable. This analysis revealed a significant main effect of spatial listening expectations, where performance was better when targets were presented from likely rather than unlikely listening locations (F[1,14] = 77.46; p < 0.001). There was also a significant main effect of sentence condition (F[3,42] = 4.49; p < 0.01). Post hoc SNK testing indicated that performance was significantly better for the unedited sentences than the sentences with either 0 or 150 msec gaps (p < 0.05; all other differences were not statistically different, p > 0.05). Finally, a significant two-way interaction of spatial listening expectations and sentence condition (F[3,42] = 4.10; p
0.05), the younger group significantly outperformed the older group when the instructions were conditional (p < 0.05). Third, word-identification performance was significantly better in the conditions
Fig. 7. Mean percent correct word-identification scores (± standard error of the mean) for younger (open circles) and older (filled circles) adults for the four certainty conditions. Solid and dashed lines indicate performance for the unconditional and conditional instructions, respectively.
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in which location certainty was greater compared with those with less location certainty (F[3,42] = 61.62; p < 0.001). Moreover, a significant Location Certainty × Instruction interaction was observed (F[3,42] = 6.17; p < 0.01), such that the benefit of increased location certainty was greater when listeners followed the conditional instruction compared with when they followed the unconditional instruction. Specifically, whereas performance was similar for the unconditional instruction when target location certainty was 0.8, 0.6, or 0.33, for the conditional instruction, performance was significantly poorer at 0.6 and 0.33 than at 0.8 (p < 0.05). No other significant effects were observed (p > 0.05). Spatial Listening Expectations • To consider the role of spatial listening expectations, the following analysis focuses on the intermediate target location certainty specifications (0.8 and 0.6). As in Experiment 1, because a similar pattern of performance was observed for each location certainty condition (0.8 and 0.6), for ease of presentation, the results across these conditions were collapsed. The descriptions of the results were confirmed statistically by a RMANOVA where age (younger versus older) was a between-subjects variable and instruction (conditional versus unconditional) and location expectation (likely versus unlikely) were within-subject variables. As shown in Figure 7, the younger group outperformed the older group (F[1,14] = 5.36; p < 0.05), performance was better when targets were presented from the likely compared with the unlikely listening locations (F[1, 12] = 28.92; p < 0.001), and performance in the unconditional instruction condition was better than the conditional instruction condition (F[1,14] = 21.38; p < 0.001). In addition to these main effects, a significant Location Expectation × Instruction interaction was observed (F[1,14] = 13.45; p < 0.01). Whereas at the likely listening location performance was similar whether participants were provided the unconditional or conditional set of instructions, at the unlikely listening location, performance was poorer when the instruction was conditional than when it was unconditional (p < 0.05). Furthermore, it was found that age-related differences in word identification also depended on location expectancy (F[1,14] = 14.61; p < 0.01). Although both age groups performed similarly when targets were presented from the likely listening location, younger listeners outperformed older listeners when targets were presented from the unlikely listening location (p < 0.05). Finally, a significant three-way interaction was observed between age, instruction, and location expectancy (F[1,14] = 5.56; p < 0.05]. Post hoc SNK testing revealed that when targets were presented at the unlikely listening location, age-related differences in word identification also depended on the instruction. Although both groups performed similarly with the unconditional instruction, younger adults outperformed older adults with the conditional instruction (p < 0.05; see Fig. 8). No other significant effects were observed (p > 0.05). Practice Effects • In light of the potentially challenging task demands of the study, considerable care was taken to ensure that participants were provided with adequate training. To this end, each participant completed a minimum of 30 min of practice and 3.5 hr of testing for each set of instructions. Nevertheless, to conclude that age-related differences in learning did not account for the Age × Instruction × Location Expectancy interaction, we examined how word-identification performance
Fig. 8. Mean percent correct key word-identification scores (± standard error of the mean) for younger and older adults on trials for targets presented from the likely (white bars) and unlikely (black bars) listening locations when given the unconditional or conditional instructions.
varied across the seven testing sets for each instruction condition. Figure 9 shows that the performance for both age groups was relatively constant over time for both types of instruction. To confirm this description, a RMANOVA was conducted using the data from the spatial listening expectation condition of interest (i.e., the unlikely data) where testing set (1 to 7) and instruction (conditional versus unconditional) were within-subject variables and age (younger versus older) was a between-subjects variable. Neither significant main effects nor the interaction of age with testing set was observed (p > 0.05).
Discussion The primary goal of Experiment 2 was to investigate whether there were age-related costs of misdirecting auditory spatial attention in a complex listening environment of the sort
Fig. 9. Mean percent correct word-recognition scores (± standard error of the mean) for younger (unfilled circles) and older (filled circles) adults for the unconditional (solid lines) and conditional (dashed lines) instructions for targets presented from the unlikely listening location. Data are presented for the seven testing sets.
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that many older adults describe as challenging for understanding speech in everyday life. We found that when the cued sentence was presented at an unlikely listening location and the instruction was conditional, younger adults outperformed older adults. The results suggest that older compared with younger adults could experience substantially greater costs of misdirecting attention as might be required in many everyday listening situations. The results from the present experiment are consistent with the accounts of Madden et al. (1994) and Greenwood and Parasuraman (1999) who suggested that older adults have more difficulty than younger adults disengaging attentional resources from a cued location when switching attention to a target at another location (the disengaging attention hypothesis). More generally, this account is consistent with theories of aging, which suggest that there are failures of cognitive control on tasks of executive function (Kramer et al. 1994; Braver & Barch 2002). It is important to note that in the present study and in the study by Madden et al., age-related differences were not observed in conditions where listeners used location-based cues to switch spatial attention from a likely to an unlikely location. A general finding seems to be that processes that facilitate attention switching per se are relatively unaffected by aging, both in studies of auditory spatial attention (Experiment 1; Singh et al. 2008) and visual spatial attention (Hartley et al.1990, 1992; Folk & Hoyer 1992; Greenwood & Parasuraman 1994). Furthermore, the disproportionately poorer performance of older compared with younger adults in Experiment 2 was only observed in conditions that required repeated engaging and disengaging of attentional resources, a finding that until now has only been reported in studies of visual spatial attention (e.g., Greenwood & Parasuraman 1994, 1999; Madden et al. 1994). Alternative Explanations for the Age × Task × Location Interaction • To conclude that differences in the ability to disengage attention account for the age-related difference in word-identification performance observed in Experiment 2, it is necessary to consider how competing explanations might account for the overall pattern of data. Alternative explanations to be considered are that the results may have been influenced by age-related differences in hearing, divided attention, or working memory. If any one of these three explanations were to be responsible for the three-way interaction of age, location uncertainty, and instructional set, it would need to explain why age differences were observed only when the instructions required repeated engaging and disengaging of auditory spatial attention. With respect to hearing acuity, it should be noted that the audiometric thresholds of the older group were 14 dB HL worse than those of the younger group at 4 kHz. Nevertheless, the thresholds of the older group were within normal range at frequencies up to and including 3 kHz in all cases, and in all but two cases they were also in the normal range at 4 kHz. Thus, the stimulus level should have been above the 15 dB SL criteria used by Humes (2007). Furthermore, for this study, the targets were selected from small closed-set choices of highly frequent words (i.e., colors and numbers), which would also make the word-recognition task easier than it would be in an experiment in which open-set responses are required. Finally, in the 100% probability condition both age groups performed highly accurately and equivalently. Crucially, an audibility
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advantage could conceivably account for a main effect within the data, but we do not believe that audibility could account for the pattern of age-related differences we observed only under specific instructional conditions. Of more relevance for Experiment 2, listeners had to use acoustic cues to spatial location to perform the task. Hence we might expect age-differences in hearing to produce age-related differences in performance irrespective of type of instructions participants were given; however, this was not the case. It should be mentioned that in the high- (likely) and low-probability (unlikely) spatial location conditions, targets were always presented from the center or a side, respectively, and that word-identification performance can potentially be affected due to differences in sensitivity arising from “better ear” mechanisms (see Carhart 1965; Olsen & Carhart 1967; Zurek 1993). However, such mechanisms likely did not influence the overall pattern of results in the present study. Allen et al. (2009) found that for targets presented between 0° and 60° azimuth, speech intelligibility performance was not dependent on a listener’s spatial listening expectations. Because we tested locations less than ± 60° azimuth, it is assumed that the differences in word identification observed in the present study did not result from location-based sensitivity differences arising from better ear mechanisms. Another possibility is that listeners may have adopted a strategy of dividing attention to listen to all three sentence streams simultaneously rather than adopting a strategy of listening to different sentence streams in a sequential fashion. Here again, if older adults are poorer than younger adults at simultaneously following multiple streams of information (i.e., the divided-attention attention hypothesis; Jerger et al. 1994; Humes et al. 2006), we would expect to find age differences when either unconditional or conditional instructions were given. Yet another possible explanation is that the cue and anticue tasks may reflect age-related declines in working memory rather than processes involving disengaging attention. Indeed, it has been argued that there is an interface between working memory and attention insofar as complex working memory tasks reflect differences in how well individuals can control attention to activate and keep the information relevant to the current goal or task available and also dampen or inhibit irrelevant information under conditions where there is risk of interference or distraction (Barrett et al. 2004; Engle 2010). In conditions where participants were provided the more memory-demanding (i.e., conditional) instructions, we observed age-related differences in performance when targets were presented from the unlikely listening location. It is important to note that we failed to observe age-related differences when targets were presented from the likely listening location, despite participants having to maintain in working memory the conditional nature of the instruction. We are unable to definitively rule out working memory as a possible explanation; however, the overall pattern of results suggests that attentional control rather than working memory in general is key, given the differences in performance between the likely and unlikely listening location.
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GENERAL DISCUSSION Overall, the motivation for conducting this research was to better understand why older adults experience difficulty understanding speech in the presence of competing speech. We investigated factors that might contribute to a listener’s ability to focus auditory spatial attention. Assuming that listeners allocate attentional resources to the likely listening location before the start of the trial, the benefit of directing focused attentional resources would be greatest when the cued target words were presented from the likely location. In both experiments 1 and 2, performance was similar for younger and older listeners when targets were presented with 100% certainty from the likely listening location, suggesting that difficulties in directing focused attention do not account for the disproportionate everyday listening difficulties experienced by older adults. When the cued sentence is presented from the unlikely listening location and the unconditional instructions are provided, it is possible to gauge the influence of auditory spatial attention switching (i.e., switching attention from the likely to the unlikely listening location) on word-identification accuracy by determining how much performance is reduced by comparing performance to when the cued sentence is presented from the likely listening location. In both experiments 1 and 2 and the study by Singh et al. (2008), age-related differences were not observed regardless of whether targets were presented from likely or unlikely locations. This suggests that relatively simple switching of auditory attention is preserved in older adults. Furthermore, the results from Experiment 1 suggest that even when there is minimal time to switch attentional resources from a likely location in front of a listener to an unlikely side location, attention switching abilities are preserved in older listeners. Finally, when participants are given the conditional instructions and the cued sentence is presented from an unlikely listening location such that an anticue response is required, it is possible to gauge the effect of momentarily misdirecting attentional resources by determining whether performance is reduced in comparison with performance when a cue response is required. The key finding from Experiment 2 is that whereas both age groups performed similarly when performing a cue task, the performance of older but not younger listeners was reduced when performing an anticue task. These findings suggest that in listening conditions requiring multiple switches of attention, there are age-related deficits in word-identification performance if attention is momentarily misdirected. The demonstration of age-related deficits in the ability to perform the anticue task is important both in terms of highlighting similarities between the visual and auditory spatial processing abilities of younger and older adults and highlighting a possible explanation for why older adults with little or no clinically significant audiometric hearing loss may experience difficulty understanding target speech in the presence of competing speech. When the location of the target talker changes frequently and may be cued by another talker, the most reasonable explanation for the pattern of findings seems to be that older adults have difficulty disengaging attention in conditions requiring multiple switches of attention. It would be of benefit for future research to consider how training might improve performance (Basak et al. 2008a, b).
ACKNOWLEDGMENTS The authors thank Robert Quelch for his assistance with data collection, Huiwen Goy for her assistance in editing the stimuli, James Qi for programming and technical support, and the participants for their valuable contributions. This research was made possible by support from the Canadian Institutes of Health Research (MOP-15359) and the Natural Sciences and Engineering Research Council of Canada (RGPIN 138472-05). Portions of this article were presented at the annual meeting of the American Auditory Society in Scottsdale, Arizona, in May 2010, the Cognitive Aging Conference in Atlanta, Georgia in April, 2010, and at the 159th meeting of the Acoustical Society of America in Baltimore, Maryland, in April 2010. The authors declare no conflict of interest. Address for correspondence: Gurjit Singh, Department of Speech-Language Pathology, Rehabilitation Sciences Building, Faculty of Medicine, University of Toronto, 160-500 University Avenue, Toronto, Ontario, Canada, M5G 1V7. E-mail: [email protected] Received September 1, 2012; accepted April 19, 2013.
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difficulty communicating in challenging listening situations. These difficulties understanding speech in noise may arise from age-related deficits in peripheral, central auditory, and/or cognitive processing. Reduced working-memory capacity, age-related slowing in the speed of information processing, and inhibitory or other attentional deficits are some of the most likely cognitive sources of the difficulties that older listeners experience in understanding language spoken in challenging situations (for a review see Cohen 1987; Pichora-Fuller & Singh 2006; Schneider et al. 2010). Considerable research has been conducted investigating the contributions of working memory and speed of processing to the speech-understanding difficulties of older adults, but the role of attentional factors has received less scrutiny. Attention could be especially important in situations such as communicating with friends at a noisy restaurant when a listener may need to monitor multiple simultaneous voices. Recently, the ability to attend to a target in a multi-talker situation (e.g., Kidd et al. 2005; see also Spence & Driver 1994; Mondor & Zatorre 1995) has been studied using the well-established attentional cueing paradigm (Posner et al. 1980). In this paradigm, an observer is typically asked to detect a target signal at one of two (or more) locations, with a pretrial cue indicating the most and/or least likely locations at which the signal will be presented. One common variant of the cueing paradigm is to provide observers with pretrial cues indicating the probability of a target appearing at a particular location. For example, in a probabilistic spatial cueing study, a “valid” or “likely” trial occurs when a target is presented from a location with a high probability of occurrence and an “invalid” or “unlikely” trial occurs when a target is presented from a location with a low probability of occurrence. In a previous study using the cueing paradigm, we found that younger and older adults with normal audiometric thresholds below 4 kHz were better able to identify targets presented from likely than unlikely locations, but that both groups experienced a similar benefit from knowing where to listen (Singh et al. 2008). Failure to observe a significant age-related difference in benefit from cueing could be evidence that there are no age-related differences in switching attention from one location to another. Alternatively, the failure to observe age-related differences may have occurred because the listening task was too easy to detect differences that might have been observed in more challenging situations where listeners must rapidly switch their attentional focus from one source to another. One reason the task may have been too easy was that there was a relatively long time delay between the cue and the target and this delay may have accommodated slower switching by older adults. Another possible explanation is that the listeners in our previous study were only required to allocate attention to the location where the cue was heard and did not have to use the cue to redirect attention to another location. For example, in multi-talker situations, it could be that when a listener switches attention from one source to another, younger listeners may be more able than older listeners to “recover” if attention is
Design: In two companion experiments, a target sentence was presented from one spatial location and two competing sentences were presented simultaneously, one from each of two different locations. Pretrial, listeners were informed of the call-sign cue that identified which of the three sentences was the target and of the probability of the target sentence being presented from each of the three possible locations. Four different probability conditions varied in the likelihood of the target being presented at the left, center, and right locations. In Experiment 1, four timing conditions were tested: the original (unedited) sentences (which contained about 300 msec of filler speech between the call-sign cue and the onset of the target words), or modified (edited) sentences with silent pauses of 0, 150, or 300 msec replacing the filler speech. In Experiment 2, when the cued sentence was presented from an unlikely (side) listening location, for half of the trials the listener’s task was to report target words from the cued sentence (cue condition); for the remaining trials, the listener’s task was to report target words from the sentence presented from the opposite, unlikely (side) listening location (anticue condition). Results: In Experiment 1, for targets presented from the likely (center) location, word identification was better for the unedited than for modified sentences. For targets presented from unlikely (side) locations, word identification was better when there was more time between the call-sign cue and target words. All listeners benefited similarly from the availability of more compared with less time and the presentation of continuous compared with interrupted speech. In Experiment 2, the key finding was that age-related performance deficits were observed in conditions requiring anticue but not cue responses. Conclusions: The findings from Experiment 1 suggest that for both age groups, stream continuity mediates the process of allocating and maintaining auditory spatial attention when the target originates at an expected location, but that time is needed for the reallocation of auditory spatial attention when the target originates at an unexpected location. The findings from Experiment 2 suggest that when attention is momentarily misdirected, difficulties disengaging attention may help explain why older adults with good hearing report difficulty communicating in multi-talker listening situations. (Ear and Hearing 2013;34;711–721)
INTRODUCTION Hearing impairment is one of the most common chronic conditions affecting listeners who are more than 60 years of age, with nearly 1 of 2 older adults experiencing some form of hearing loss (Cruickshanks et al. 1998). Furthermore, even in the absence of elevated audiometric thresholds, older listeners often report 1 Phonak Canada Ltd, Mississauga, Ontario, Canada; 2Department of Speech-Language Pathology, University of Toronto, Toronto, Ontario, Canada; 3Toronto Rehabilitation Institute, Toronto, Ontario, Canada; and 4Department of Psychology, University of Toronto Mississauga, Mississauga, Ontario, Canada.
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momentarily misdirected to the wrong source. Two experiments were conducted to explore these two possible reasons for the absence of an age-related difference in our original study.
Role of Time in the Ability of Younger and Older Listeners to Switch Attention Although there is substantial literature exploring the time course of visual spatial attention (for a review see Egeth & Yantis 1997), very few studies have explored the time course of auditory spatial attention. On the one hand, the gap in our understanding regarding the time course of allocating auditory spatial attention is not surprising because the conditions under which spatial cueing yields robust effects on listening performance were only recently delineated (e.g., Arbogast & Kidd 2000; Kidd et al. 2005). On the other hand, this gap is surprising given the long-standing observation that successful listening in noisy environments requires the rapid deployment of auditory attention to whichever sound source captures the listener’s interest (Cherry 1953). For example, when communicating over dinner with colleagues at a noisy restaurant, listeners must frequently switch their focus of attention from one talker to the next, and listening performance may depend on time for several reasons: time may be required to switch attention (Shinn-Cunningham & Best 2008); additional time could provide an opportunity to further process information (Barrouillet et al. 2004); or time may facilitate the formation of auditory objects (Best et al. 2008). Because slowing of information processing and problems dividing attention are commonly observed age-related declines in cognition, it is expected that older adults should be less able to function in situations in which rapid switching of attention is necessary.
Experiment 1 Effect of Time Available for Listeners to Switch Attention The primary goal of Experiment 1 was to investigate the influence on word identification of age and the time available to
switch auditory attention in a multi-talker environment in which there is uncertainty about the location of the target talker. This was investigated in healthy younger and older listeners with normal audiometric thresholds up to 4 kHz. A probabilistic cueing paradigm was used in which the time available to allocate attention from one source to another was systematically manipulated.
Participants and Methods Participants • Eight younger (mean = 22.4 years; SD = 4.4 years) and 8 older adults (mean = 70.9 years; SD = 2.5 years) participated in the study. All participants were native English speakers, in good overall health, and had clinically normal puretone air conduction thresholds (≤25 dB HL) from 0.25 to 3 kHz bilaterally (see Fig. 1). All the participants had participated in prior experiments concerning communication and aging. Two of the 16 participants (1 younger and 1 older adult) had participated in our prior study (Singh et al. 2008). Participants provided informed consent and were paid $10 per hr. Stimuli • The stimuli consisted of sentences from the Coordinate Response Measure (CRM) corpus spoken by the 4 male talkers (Bolia et al. 2000). The sentences have the format: “Ready (call-sign) go to (color) (number) now,” with all possible combinations of eight callsigns (Arrow, Baron, Charlie, Eagle, Hopper, Laker, Ringo, Tiger), four colors (red, white, blue, green), and eight numbers (1, 2, 3, 4, 5, 6, 7, 8). The call-sign provided the cue to enable listeners to identify the target sentence on a given trial. The color and number were the target words that were scored. In addition to the original CRM sentences, listeners were also presented edited CRM sentences. For the original sentences, the words “go to” intervened between the call-sign cue word and the target words. For the edited sentences, the words “go to” were replaced by a silent pause of 300, 150, or 0 msec. The maximum pause duration to be tested was chosen to be 300 msec because the duration of the words “go to” ranged between 225 and 300 msec in a random sample of 50 sentences. Conditions with no pause or an intermediate pause duration of 150 msec were also
Fig. 1. Mean left (×) and right (○) ear audiometric thresholds (± standard error of the mean) for older (black/dashed lines) and younger (gray/solid lines) participants from Experiment 1 (left panel) and Experiment 2 (right panel).
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tested to determine whether performance would be reduced by decreasing the time between the call-sign cue and target words. Each of the 1024 sentences was edited manually, with the editing points determined by visual examination of the time waveform. To minimize click artefacts, editing points occurred at zero crossing in the soundfiles of the sentence materials. Equipment • All testing was conducted in a 3.3 m × 3.3 m single-walled sound-attenuating booth. The stimuli were controlled and presented via custom software. The stimuli were routed from a Dell computer to a Tucker-Davis Technologies System III to a Harmon/Kardon (model HK3380) amplifier (Alachua, FL). Sentences were converted to analog (RP2.1) at a sampling rate of 24.414 kHz by a 24-bit D/A converter, attenuated (PA5), conditioned (SA1), and presented over three loudspeakers (model 1761–9630; Grason-Stadler Inc., Eden Prairie, MN) located at approximately the same height as the listener’s head when seated. Visual cueing was displayed on a 17-in touchscreen monitor on a table at a height of 0.46 m in front of the listener. The response choices were also displayed visually, as was feedback regarding whether or not each response was correct. Calibration was performed using a B&K sound level meter (type 2260) with a 0.5-in condenser microphone (type 4189) using the A-weighting equivalent to measure concatenated CRM sentences. Procedures • On each trial, three sentences were presented simultaneously, with each sentence being presented from one of three possible locations: in front of, to the left, or to the right of the listener. Participants were instructed to face directly ahead. Two types of cues, call-sign cues and location certainty cues, were used on each trial in the study and they were both visually presented 1 second before each trial. The call-sign cue indicated the name (e.g., Baron) near the beginning of the sentence, which enabled the listener to identify the target sentence. On each trial, the call-sign cue could change. The call-sign cue was presented visually before the presentation of the auditory stimuli. One of four different location certainty cues (“0-100-0,” “10-80-10,” “20-60-20,” “33-33-33”) indicated the probability of the cued sentence being presented from the left, center, or right loudspeaker. For example, 10-80-10 indicated that the cued sentence would be presented from the left loudspeaker in
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10% of the trials, from the center loudspeaker in 80% of the trials, and from the right loudspeaker in 10% of the trials. In contrast to the conditions where the location was certain (0-100-0) or random (33-33-33), for the 10-80-10 and 20-60-20 conditions, the location of the target in a given trial could be either likely or unlikely. A likely trial occurred when the cued sentence was presented from the spatial location with the highest probability of occurrence (i.e., the loudspeaker located at 0°) and an unlikely trial occurred when the cued sentence was presented from a location with the lower probability of occurrence (i.e., the loudspeaker located to the left or right of the listener; see left half of Fig. 2). The location certainty cue remained constant for a block of 30 trials. The location of the cued sentence was randomly selected from the left, center, and right locations, with the constraint that the certainty cue was accurate across the block of 30 trials. All sentences were presented at 60 dB A. For all trials, both the correct color and number were required for a response to be scored as correct. Feedback (correct or incorrect) was provided after every trial, and summary scores were presented at the end of each block. At the start of each visit, participants completed practice trials for a minimum of 30 min. Design • Word-identification accuracy was measured using a 4 (target location certainty: 0-100-0, 10-80-10, 20-60-20, and 33-3333) × 4 (sentence: unedited and edited with 300,150, or 0 msec pauses) within-subjects design. For each participant, data were collected on 4 separate days, with each visit lasting about 2 hr. At each visit, participants were assigned to one of the four sentence conditions. For each of the sentence conditions, data were collected in four blocks of 30 trials. A different target certainty condition was assigned randomly without replacement for each of the four blocks. For each visit, participants completed two runs of the four blocks of 30 trials. Thus, at the end of each visit, a participant completed 60 trials for each of the four location certainty conditions for a given sentence condition. The order of the four sentence conditions for half of the participants was as follows: (1) 0 msec, (2) 150 msec, (3) 300 msec, and (4) unedited sentences; the order was reversed for the other half of the participants.
Fig. 2. Task sequence for the 0.8 and 0.6 location certainty conditions for both the unconditional and conditional set of instructions when the cued sentence was presented at either the likely or unlikely listening location. In this example, the pretrial call-sign cue is “Charlie.”
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Fig. 3. Mean percent correct word-identification scores (± standard error of the mean) for younger (left panel) and older (right panel) adults depicted across the four location certainty conditions. The data presented are from the present study (black bars), and the studies by Singh et al. (2008) (white bars), and Kidd et al. (2005) (gray bars).
Results The results from the baseline condition in the present study will be compared with the results from previous studies, and the general results will also be presented followed by the specific findings most pertinent to the main goal of the experiment, which was to determine whether there was an age-related effect on performance of the time available for listeners to switch auditory spatial attention. Baseline Condition and General Results • Figure 3 depicts the performance of listeners for targets presented with real spatial separation in the present and in two previous studies (Kidd et al. 2005; Singh et al. 2008). Overall, the results of the present study are in excellent agreement with previous findings regarding this baseline condition. In general, examination of the overall results revealed two patterns of interest (see Fig. 4). The most robust finding was that performance decreased as uncertainty about the target location increased. When the data were collapsed across the four timing
conditions and two age groups, performance was 80% correct when listeners were certain about the location of the target and 51% correct under conditions where the location of the target was random. The second pattern was that differences in performance were observed between the unedited and edited sentence conditions; word identification was 67% correct for the unedited sentences and ranged from 61 to 63% correct for the 0, 150, and 300 msec edited sentences. To confirm these descriptions a 2 × 4 × 4 repeated-measures analysis of variance (RMANOVA) was conducted with age (younger and older) as a between-subjects factor, and target location certainty (1.0, 0.8, 0.6, and 0.33) and sentence condition (unedited and edited sentences with 0, 150, and 300 msec pauses) as within-subjects factors. There was a significant main effect of target location certainty (F[3,42] = 99.20; p < 0.001). A post hoc Student–Newman–Keuls (SNK) test revealed significant differences among each of the four target location certainty specifications (p < 0.01). A main effect of sentence condition (F[3,42] = 4.86; p < 0.01) was also observed. Post
Fig. 4. Mean percent correct word-identification scores (± standard error of the mean) for the four location certainty conditions for the younger (left panels) and older (right panels) adult groups. White, light gray, dark gray, and black bars represent the 0, 150, 300 msec and original (unedited) sentence conditions, respectively.
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hoc SNK testing indicated that performance was better in the unedited original sentence condition compared with the 0 and 150 msec edited sentence conditions, and that performance was better when in the 300 msec compared with the 0 msec edited sentence condition (p < 0.05). No other significant effects were observed (p > 0.05). Note that there was neither a significant main effect of age nor any significant interaction with age. Listening Expectations: Effect of Pause Duration • To determine whether there was an age-related effect on performance of the time available time to switch attention, the relevant subset of conditions are the intermediate (0.8 and 0.6) location certainty conditions where both likely and unlikely trials occurred. Specifically, it was assumed that listeners would expect the target to be presented at the likely center location such that it would be necessary to switch attention if the target was presented from an unlikely side location, whereas in the 1.0 condition the target was always presented at the center location and there would be no need to switch attention, and in the 0.33 condition there was no likely condition. Because a similar pattern of performance was observed for the 0.8 and 0.6 location certainty conditions, the results across these two conditions were collapsed (see Fig. 5). A 2 (likely versus unlikely) × 4 (unedited and edited sentences with 0, 150, and 300 msec pauses) × 2 (younger versus older) RMANOVA was conducted where spatial listening expectations and sentence condition were within-subject variables and age was a between-subjects variable. This analysis revealed a significant main effect of spatial listening expectations, where performance was better when targets were presented from likely rather than unlikely listening locations (F[1,14] = 77.46; p < 0.001). There was also a significant main effect of sentence condition (F[3,42] = 4.49; p < 0.01). Post hoc SNK testing indicated that performance was significantly better for the unedited sentences than the sentences with either 0 or 150 msec gaps (p < 0.05; all other differences were not statistically different, p > 0.05). Finally, a significant two-way interaction of spatial listening expectations and sentence condition (F[3,42] = 4.10; p
0.05), the younger group significantly outperformed the older group when the instructions were conditional (p < 0.05). Third, word-identification performance was significantly better in the conditions
Fig. 7. Mean percent correct word-identification scores (± standard error of the mean) for younger (open circles) and older (filled circles) adults for the four certainty conditions. Solid and dashed lines indicate performance for the unconditional and conditional instructions, respectively.
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in which location certainty was greater compared with those with less location certainty (F[3,42] = 61.62; p < 0.001). Moreover, a significant Location Certainty × Instruction interaction was observed (F[3,42] = 6.17; p < 0.01), such that the benefit of increased location certainty was greater when listeners followed the conditional instruction compared with when they followed the unconditional instruction. Specifically, whereas performance was similar for the unconditional instruction when target location certainty was 0.8, 0.6, or 0.33, for the conditional instruction, performance was significantly poorer at 0.6 and 0.33 than at 0.8 (p < 0.05). No other significant effects were observed (p > 0.05). Spatial Listening Expectations • To consider the role of spatial listening expectations, the following analysis focuses on the intermediate target location certainty specifications (0.8 and 0.6). As in Experiment 1, because a similar pattern of performance was observed for each location certainty condition (0.8 and 0.6), for ease of presentation, the results across these conditions were collapsed. The descriptions of the results were confirmed statistically by a RMANOVA where age (younger versus older) was a between-subjects variable and instruction (conditional versus unconditional) and location expectation (likely versus unlikely) were within-subject variables. As shown in Figure 7, the younger group outperformed the older group (F[1,14] = 5.36; p < 0.05), performance was better when targets were presented from the likely compared with the unlikely listening locations (F[1, 12] = 28.92; p < 0.001), and performance in the unconditional instruction condition was better than the conditional instruction condition (F[1,14] = 21.38; p < 0.001). In addition to these main effects, a significant Location Expectation × Instruction interaction was observed (F[1,14] = 13.45; p < 0.01). Whereas at the likely listening location performance was similar whether participants were provided the unconditional or conditional set of instructions, at the unlikely listening location, performance was poorer when the instruction was conditional than when it was unconditional (p < 0.05). Furthermore, it was found that age-related differences in word identification also depended on location expectancy (F[1,14] = 14.61; p < 0.01). Although both age groups performed similarly when targets were presented from the likely listening location, younger listeners outperformed older listeners when targets were presented from the unlikely listening location (p < 0.05). Finally, a significant three-way interaction was observed between age, instruction, and location expectancy (F[1,14] = 5.56; p < 0.05]. Post hoc SNK testing revealed that when targets were presented at the unlikely listening location, age-related differences in word identification also depended on the instruction. Although both groups performed similarly with the unconditional instruction, younger adults outperformed older adults with the conditional instruction (p < 0.05; see Fig. 8). No other significant effects were observed (p > 0.05). Practice Effects • In light of the potentially challenging task demands of the study, considerable care was taken to ensure that participants were provided with adequate training. To this end, each participant completed a minimum of 30 min of practice and 3.5 hr of testing for each set of instructions. Nevertheless, to conclude that age-related differences in learning did not account for the Age × Instruction × Location Expectancy interaction, we examined how word-identification performance
Fig. 8. Mean percent correct key word-identification scores (± standard error of the mean) for younger and older adults on trials for targets presented from the likely (white bars) and unlikely (black bars) listening locations when given the unconditional or conditional instructions.
varied across the seven testing sets for each instruction condition. Figure 9 shows that the performance for both age groups was relatively constant over time for both types of instruction. To confirm this description, a RMANOVA was conducted using the data from the spatial listening expectation condition of interest (i.e., the unlikely data) where testing set (1 to 7) and instruction (conditional versus unconditional) were within-subject variables and age (younger versus older) was a between-subjects variable. Neither significant main effects nor the interaction of age with testing set was observed (p > 0.05).
Discussion The primary goal of Experiment 2 was to investigate whether there were age-related costs of misdirecting auditory spatial attention in a complex listening environment of the sort
Fig. 9. Mean percent correct word-recognition scores (± standard error of the mean) for younger (unfilled circles) and older (filled circles) adults for the unconditional (solid lines) and conditional (dashed lines) instructions for targets presented from the unlikely listening location. Data are presented for the seven testing sets.
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that many older adults describe as challenging for understanding speech in everyday life. We found that when the cued sentence was presented at an unlikely listening location and the instruction was conditional, younger adults outperformed older adults. The results suggest that older compared with younger adults could experience substantially greater costs of misdirecting attention as might be required in many everyday listening situations. The results from the present experiment are consistent with the accounts of Madden et al. (1994) and Greenwood and Parasuraman (1999) who suggested that older adults have more difficulty than younger adults disengaging attentional resources from a cued location when switching attention to a target at another location (the disengaging attention hypothesis). More generally, this account is consistent with theories of aging, which suggest that there are failures of cognitive control on tasks of executive function (Kramer et al. 1994; Braver & Barch 2002). It is important to note that in the present study and in the study by Madden et al., age-related differences were not observed in conditions where listeners used location-based cues to switch spatial attention from a likely to an unlikely location. A general finding seems to be that processes that facilitate attention switching per se are relatively unaffected by aging, both in studies of auditory spatial attention (Experiment 1; Singh et al. 2008) and visual spatial attention (Hartley et al.1990, 1992; Folk & Hoyer 1992; Greenwood & Parasuraman 1994). Furthermore, the disproportionately poorer performance of older compared with younger adults in Experiment 2 was only observed in conditions that required repeated engaging and disengaging of attentional resources, a finding that until now has only been reported in studies of visual spatial attention (e.g., Greenwood & Parasuraman 1994, 1999; Madden et al. 1994). Alternative Explanations for the Age × Task × Location Interaction • To conclude that differences in the ability to disengage attention account for the age-related difference in word-identification performance observed in Experiment 2, it is necessary to consider how competing explanations might account for the overall pattern of data. Alternative explanations to be considered are that the results may have been influenced by age-related differences in hearing, divided attention, or working memory. If any one of these three explanations were to be responsible for the three-way interaction of age, location uncertainty, and instructional set, it would need to explain why age differences were observed only when the instructions required repeated engaging and disengaging of auditory spatial attention. With respect to hearing acuity, it should be noted that the audiometric thresholds of the older group were 14 dB HL worse than those of the younger group at 4 kHz. Nevertheless, the thresholds of the older group were within normal range at frequencies up to and including 3 kHz in all cases, and in all but two cases they were also in the normal range at 4 kHz. Thus, the stimulus level should have been above the 15 dB SL criteria used by Humes (2007). Furthermore, for this study, the targets were selected from small closed-set choices of highly frequent words (i.e., colors and numbers), which would also make the word-recognition task easier than it would be in an experiment in which open-set responses are required. Finally, in the 100% probability condition both age groups performed highly accurately and equivalently. Crucially, an audibility
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advantage could conceivably account for a main effect within the data, but we do not believe that audibility could account for the pattern of age-related differences we observed only under specific instructional conditions. Of more relevance for Experiment 2, listeners had to use acoustic cues to spatial location to perform the task. Hence we might expect age-differences in hearing to produce age-related differences in performance irrespective of type of instructions participants were given; however, this was not the case. It should be mentioned that in the high- (likely) and low-probability (unlikely) spatial location conditions, targets were always presented from the center or a side, respectively, and that word-identification performance can potentially be affected due to differences in sensitivity arising from “better ear” mechanisms (see Carhart 1965; Olsen & Carhart 1967; Zurek 1993). However, such mechanisms likely did not influence the overall pattern of results in the present study. Allen et al. (2009) found that for targets presented between 0° and 60° azimuth, speech intelligibility performance was not dependent on a listener’s spatial listening expectations. Because we tested locations less than ± 60° azimuth, it is assumed that the differences in word identification observed in the present study did not result from location-based sensitivity differences arising from better ear mechanisms. Another possibility is that listeners may have adopted a strategy of dividing attention to listen to all three sentence streams simultaneously rather than adopting a strategy of listening to different sentence streams in a sequential fashion. Here again, if older adults are poorer than younger adults at simultaneously following multiple streams of information (i.e., the divided-attention attention hypothesis; Jerger et al. 1994; Humes et al. 2006), we would expect to find age differences when either unconditional or conditional instructions were given. Yet another possible explanation is that the cue and anticue tasks may reflect age-related declines in working memory rather than processes involving disengaging attention. Indeed, it has been argued that there is an interface between working memory and attention insofar as complex working memory tasks reflect differences in how well individuals can control attention to activate and keep the information relevant to the current goal or task available and also dampen or inhibit irrelevant information under conditions where there is risk of interference or distraction (Barrett et al. 2004; Engle 2010). In conditions where participants were provided the more memory-demanding (i.e., conditional) instructions, we observed age-related differences in performance when targets were presented from the unlikely listening location. It is important to note that we failed to observe age-related differences when targets were presented from the likely listening location, despite participants having to maintain in working memory the conditional nature of the instruction. We are unable to definitively rule out working memory as a possible explanation; however, the overall pattern of results suggests that attentional control rather than working memory in general is key, given the differences in performance between the likely and unlikely listening location.
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GENERAL DISCUSSION Overall, the motivation for conducting this research was to better understand why older adults experience difficulty understanding speech in the presence of competing speech. We investigated factors that might contribute to a listener’s ability to focus auditory spatial attention. Assuming that listeners allocate attentional resources to the likely listening location before the start of the trial, the benefit of directing focused attentional resources would be greatest when the cued target words were presented from the likely location. In both experiments 1 and 2, performance was similar for younger and older listeners when targets were presented with 100% certainty from the likely listening location, suggesting that difficulties in directing focused attention do not account for the disproportionate everyday listening difficulties experienced by older adults. When the cued sentence is presented from the unlikely listening location and the unconditional instructions are provided, it is possible to gauge the influence of auditory spatial attention switching (i.e., switching attention from the likely to the unlikely listening location) on word-identification accuracy by determining how much performance is reduced by comparing performance to when the cued sentence is presented from the likely listening location. In both experiments 1 and 2 and the study by Singh et al. (2008), age-related differences were not observed regardless of whether targets were presented from likely or unlikely locations. This suggests that relatively simple switching of auditory attention is preserved in older adults. Furthermore, the results from Experiment 1 suggest that even when there is minimal time to switch attentional resources from a likely location in front of a listener to an unlikely side location, attention switching abilities are preserved in older listeners. Finally, when participants are given the conditional instructions and the cued sentence is presented from an unlikely listening location such that an anticue response is required, it is possible to gauge the effect of momentarily misdirecting attentional resources by determining whether performance is reduced in comparison with performance when a cue response is required. The key finding from Experiment 2 is that whereas both age groups performed similarly when performing a cue task, the performance of older but not younger listeners was reduced when performing an anticue task. These findings suggest that in listening conditions requiring multiple switches of attention, there are age-related deficits in word-identification performance if attention is momentarily misdirected. The demonstration of age-related deficits in the ability to perform the anticue task is important both in terms of highlighting similarities between the visual and auditory spatial processing abilities of younger and older adults and highlighting a possible explanation for why older adults with little or no clinically significant audiometric hearing loss may experience difficulty understanding target speech in the presence of competing speech. When the location of the target talker changes frequently and may be cued by another talker, the most reasonable explanation for the pattern of findings seems to be that older adults have difficulty disengaging attention in conditions requiring multiple switches of attention. It would be of benefit for future research to consider how training might improve performance (Basak et al. 2008a, b).
ACKNOWLEDGMENTS The authors thank Robert Quelch for his assistance with data collection, Huiwen Goy for her assistance in editing the stimuli, James Qi for programming and technical support, and the participants for their valuable contributions. This research was made possible by support from the Canadian Institutes of Health Research (MOP-15359) and the Natural Sciences and Engineering Research Council of Canada (RGPIN 138472-05). Portions of this article were presented at the annual meeting of the American Auditory Society in Scottsdale, Arizona, in May 2010, the Cognitive Aging Conference in Atlanta, Georgia in April, 2010, and at the 159th meeting of the Acoustical Society of America in Baltimore, Maryland, in April 2010. The authors declare no conflict of interest. Address for correspondence: Gurjit Singh, Department of Speech-Language Pathology, Rehabilitation Sciences Building, Faculty of Medicine, University of Toronto, 160-500 University Avenue, Toronto, Ontario, Canada, M5G 1V7. E-mail: [email protected] Received September 1, 2012; accepted April 19, 2013.
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