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|Year : 2005 | Volume
| Issue : 27 | Page : 11--26
Strength of noise effects on memory as a function of noise source and age
E Boman1, I Enmarker1, S Hygge2,
1 Kungl Tekniska Hogskolan - Royal Institute of Technology, Sweden
2 University of Gävle, Sweden
Centre for Built Environment, Laboratory of Applied Psychology, University of Gävle, SE- 801 76 Gävle
The objectives in this paper were to analyze noise effects on episodic and semantic memory performance in different age groups, and to see whether age interacted with noise in their effects on memory. Data were taken from three separate previous experiments, that were performed with the same design, procedure and dependent measures with participants from four age groups (13-14, 18-20, 35-45 and 55-65 years). Participants were randomly assigned to one of three conditions: (a) meaningful irrelevant speech, (b) road traffic noise, and (c) quiet. The results showed effects of both noise sources on a majority of the dependent measures, both when taken alone and aggregated according to the nature of the material to be memorized. However, the noise effects for episodic memory tasks were stronger than for semantic memory tasks. Further, in the reading comprehension task, cued recall and recognition were more impaired by meaningful irrelevant speech than by road traffic noise. Contrary to predictions, there was no interaction between noise and age group, indicating that the obtained noise effects were not related to the capacity to perform the task. The results from the three experiments taken together throw more light on the relative effects of road traffic noise and meaningful irrelevant speech on memory performance in different age groups.
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Boman E, Enmarker I, Hygge S. Strength of noise effects on memory as a function of noise source and age.Noise Health 2005;7:11-26
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Boman E, Enmarker I, Hygge S. Strength of noise effects on memory as a function of noise source and age. Noise Health [serial online] 2005 [cited 2021 Mar 5 ];7:11-26
Available from: https://www.noiseandhealth.org/text.asp?2005/7/27/11/31636
Field studies of children show a consistent negative association between chronic noise exposure and different cognitive outcomes. The most consistent finding is impaired reading, which is also the most researched outcome. Other negative cognitive outcomes, although not as reliably documented, include language mastery, motivation, long-term memory and attention (cf. Evans and Hygge, 2000; Evans and Lepore, 1993). For adults much less is reported on noise and cognitive impairments.
Non-experimental field studies, and in particular cross-sectional field studies, suffer methodological shortcomings in not being able to rule out self-selection into the different noise dose groups, and a confounding between noise being present both in the testing situation and the prolonged learning situation before the testing, as alternative explanations for any noise effect. However, when this is experimentally controlled by manipulation of the noise exposure and testing for the effects in silence (Hygge 1997, 2003), essentially the same conclusion as in the field studies on children emerge, that is, reading and language based memory is particularly vulnerable to noise exposure in children.
Although reliable noise effects on reading and memory in children have been demonstrated, there are as yet no comprehensive accounts of how cognitive and memory processes are differently affected by different noise sources. The sparse evidence available (Hygge, 1997, 2003) indicated that performance on cued recall of text reading in children aged 12-14 was more impaired by aircraft noise than by road traffic noise, which in turn was more detrimental than train noise. Further, the noise effect was less evident on recognition than on cued recall of the text. In all, these are the sparse empirical findings on how different noise sources affect memory in children, but as yet there are no comparative studies on adults.
In general terms it has been guessed that children compared to adults, or cognitively less able children compared to able children, are more vulnerable to noise (cf. Evans and Lepore, 1993; Hygge, 1997, 2003), but there is not much of an empirical support for such a statement. First, as regards children's vulnerability to noise compared to adults, there are no comparative studies. Pertaining to the second part of the statement, Hygge (1997, 2003) did not find more noise impairment in a group of pupils with less capacity to perform the memory tasks (aged 1214 years) compared to more able pupils.
Recent studies from our own laboratory, however, have addressed the issues of noise source and age from a more theoretically driven point of view. In three separate experimental studies (Boman, 2004; Enmarker, 2004; Hygge, Boman and Enmarker, 2003) details were worked out about noise effects on performance in episodic and semantic memory tasks (Tulving, 1993, 2001), and whether there was a difference between road-traffic noise and meaningful irrelevant speech as noise sources. These studies had the same design, procedure and dependent measures, and included younger pupils aged 13 14 years, older pupils aged 18-20 years, as well as teachers aged 35-45 and 55-65 years. The original studies reported on noise effects across the various individual dependent measures within each study, but did not extend the analysis to comparing age groups, nor to combining the individual dependent measures into aggregated content related groups of measures.
The main objective with the present paper was to make those extensions by analyzing the three previous experiments as one experiment. By taking advantage of increased sample sizes and power, age comparisons were possible for a large set of identical memory tests. Also, testing hypotheses about interactions between noise conditions and age groups became possible. Further, forming sub-sets of inter-related dependent measures within the episodic and semantic memory according to the nature of the material to be memorized or retrieved (e.g. memory of the text, memory for faces and names, memory for sentences, general word comprehension and word fluency), made it possible to also analyze noise effects on aggregated memory measures. This aggregation by the nature of the material was hoped to further specify the details of the noise effects on episodic and semantic memory tasks.
The rationales for the choice of the dependent measures, noise sources and age groups are worked out in the separate papers, but will for the sake of clarity be briefly summarized here. Doing that will also be of help in clarifying the expected outcomes of our analyses, bearing in mind that our expectations of outcomes have two different roots. One root stems from the knowledge of Tulving's (1993, 2001) memory systems and how memory performance changes with age (Nilsson, 2003), which are not explicit about noise effects. The other root stems from the separate results in our three earlier experiments, and what would be expected if they were analysed as one unit.
All the three noise experiments by Boman (2004), Enmarker (2004), and Hygge, et al., (2003), employed the same test battery which was derived from Tulving's memory theory (Tulving, 1993, 2001). This battery included 18 dependent measures (see [Table 1], [Table2] meant to cover a spectrum of attention, episodic and semantic memory. Some of the tests were adapted from a large (N > 3000) prospective Swedish study on memory, health and aging (the Betula project, see Nilsson et al., 1997 for a description), and other tests were taken over from our own work (Hygge, 1997, 2003). For memory tasks with a distinct time differentiation between encoding and retrieval, the testing was performed in silence to unconfound noise effects at encoding and retrieval.
Road traffic noise is the most prevalent source of noise disturbance in Western societies (Berglund and Lindvall, 1995), and chatter from other students is often rated as the most disturbing noise in schools (Boman and Enmarker, 2004; Lundquist, Holmberg and Landstrom, 2000). Road traffic noise has been found to impair performance on cued recall of text reading in Hygge (1997, 2003), and irrelevant speech has reliably been found to impair short-term memory serial recall, and sometimes also reading comprehension and proof-reading (Banbury and Berry, 1998; Ellermeier and Zimmer, 1997;
Jones, 1990; Jones, Miles, and Page, 1990; Martin, Wogalter, and Forlano, 1988; Oswald, Tremblay and Jones, 2000; Tremblay, Nicholls, Alford, and Jones, 2000). Road traffic noise and meaningful irrelevant speech were therefore chosen as noise sources and were matched against each other on mean energy level (dBA L eq -level), background level, time profile, and number and duration of noise peaks. These two noise sources were compared to a quiet control condition. The impairing effect of the noise exposure vs. the quiet condition was reliable for some of the episodic and semantic memory tasks in the separate analyses of the three experiments. Although there were reliable noise effects, there was not a reliable difference in effect between the road traffic noise and the irrelevant speech noise, and therefore, the road traffic noise and irrelevant speech were not expected to have different effects on the memory measures when combining the three studies.
In retrieval from episodic memory a distinction has been made between recall and recognition on the assumption that they reflect different retrieval demands, that is, recall is assumed to involve more effortful processing than recognition (Craik and McDowd, 1987). In our three experiments analysed separately there were significant noise impairments in performance on cued recall of the text, and to some extent also on recognition of the text. Thus, it seems that the presence of good retrieval cues reduced the noise effects. For the other episodic memory tasks included in the studies, there were no significant noise effects obtained, however, the overall pattern was that performance was lower in the noise conditions than in the silent condition. In the present paper a main effect of noise was expected for the reading comprehension task, with a stronger effect on cued recall than on recognition of the text. The same pattern of outcome was also expected for the other included episodic memory tests, with a higher degree of impairment for free and cued recall than for recognition.
For the semantic memory tasks the obtained noise effects showed a mixed pattern in the separate studies, with an impairment on word fluency tests for teachers and for older pupils, aged 18-20 years, while the performance on word comprehension was impaired for the youngest group. On the basis of increased sample sizes and reduced error terms, performance on semantic memory tasks were also expected to become significant when combining the three studies, in spite of being non-significant in some of the studies taken separately.
Since semantic memory is supposed to include more automatic access compared to retrieval from episodic memory (cf. Nyberg, Backman, Erngrund, Olofsson and Nilsson, 1996; Roediger and McDermott, 1993), and divided attention at encoding reduces performance considerable but has a smaller effect on retrieval (cf. Andersson, Craik, and Naveh-Benjamin, 1998), noise effects on semantic memory tasks were expected to be weaker compared to the episodic memory tasks. Thus, it was predicted that both noise sources would impair all episodic and semantic memory tasks, however, the noise effects on semantic memory tasks were expected not to be of the same strength as on the episodic memory tasks.
We also included a test of attention in the analyses to have some indication of attention in relation to noise exposure, and to check on its possible role as a mediator of noise effects.
The rationale for choosing the age groups for the participants in the three studies comes from studies indicating that the capacity to perform episodic and semantic memory tasks have their peaks at different periods in life (Craik, 2000, Nilsson, 2003; Nilsson et al., 1997; Salthouse, 1998). Performance on episodic memory tasks is assumed to be at its peak around the age of 1820 years and is thus expected to decline both for groups older and younger than 18-20 years. Semantic memory, on the other hand, is assumed to have its peak performance at an age of up to 55-60 years, however, education has been reported to be more important than age for this memory system. (Nilsson, 2003; Nilsson et al., 1997; Nyberg et al., 2003). Thus, the predictions for the main effects of age are that we expect a peak performance in episodic memory tasks for the group aged 18-20 years, and the worst performance for the youngest group. For semantic memory tasks the age group 55-65 years was expected to perform best and the youngest group worst. For attention no main effects of age were expected.
Interactions between noise and age were expected on episodic memory tasks to the effect that the age group 13-14 years would be most impaired by noise and the group 18-20 years the least. For performance on semantic memory tasks noise was expected to interact with age, to the effect that the older age groups would be less impaired by noise than the group aged 18-20 years, and the youngest group more impaired. Further, empirical findings from the Betula project (Nilsson, et al., 1997; Nyberg, et al., 2003) and Craik and McDowd (1987) indicate that recall is more age sensitive (declining faster with ages from 55-65 years and above) than recognition. Together with the hypothesis that episodic memory performance is at its peak around the age of 18-20 years, it was predicted that performance of cued recall of the text would be more impaired by noise than recognition on the text both in the groups younger and older than 18-20 years.
Neither the children aged 13-14 years nor the older groups showed any consistent noise effects on any of the measures of attention in the three separate studies. Therefore, no explicit predictions were made on the noise effects on attention or any interaction with age.
Participants and Basic Design
Participants were randomly assigned to one of three noise exposure groups: (a) meaningful irrelevant speech, (b) road traffic noise, and (c) silence. There were ninety-six participants in each of the three experiments, aged 18-20 years in the first experiment (Hygge et al., 2004), 1314 years in the second experiment (Boman, 2004), 35-45 (n=48) and 55-65 years (n=48) in the third experiment (Enmarker, 2004). The pupils were recruited from local elementary and upper secondary schools and paid to participate. The teachers were recruited from the same schools as the pupils. There were 16 males and 16 females in each of the three noise groups in each of the first two experiments. In the third experiment there were 16 participants with 9-10 females and 6-7 males in each of the three noise conditions for each of the two age-groups (35-45 and 55-65 years).
The experiments were run in a climate chamber (4 x 6 m) with controlled air temperature (21oC) and light level (900 lx). Two to four participants stayed in the experimental room at the same time, but worked on the tasks individually. They were seated in a row at a table and there was a computer screen in front of them. Altogether, the experimental session lasted for approximately two hours and the noise exposure for one hour. All sessions were run in the afternoon. See [Table 1] for order of tasks and time limits. The only difference between the three groups was the different noise conditions during first part of the experiment. At the outset of the experiments, the participants were informed that the study was about memory. They were told that they would be given separate instructions and time limits ahead of each task.
In the noise conditions digital recordings of meaningful irrelevant speech and road traffic noise were played back through loudspeakers in front of the room. The equivalent sound level (L eq ) in the noise conditions was set to 66 dBA 2 m in front of the loudspeakers. The sound level in the quiet control group was 38 dBA L eq .
The road traffic noise recording was made up of a background of continuous traffic noise (~62 dBA) with superimposed segments of trucks passing by. The peaks (fast) in the superimposed segments were at 78 dBA and occurred on the average once per minute and with different duration. The meaningful irrelevant speech recording consisted of background babble (~62 dBA) without any discernable meaning. Segments from a conversation between teenagers, only one person talking at a time, were superimposed on the babble background to match the dBA-against-time history of the truck noise. The dominant frequency range for the road traffic noise (100-300 Hz) was lower than that for the meaningful irrelevant speech (5001500 Hz).
The description the dependent measures that follows here is not as detailed as in the original three articles, which the reader should consult when in doubt.
Episodic memory. The participants read a text about a fictitious ancient culture for 15 min at the beginning of the experiment and were tested in writing in silence for cued recall and recognition of the text at the end of the experiment. See [Table 1], test 3, 15. The text was a revision of one of the texts about real cultures used by Hygge (1997, 2003) used in the classroom experiments. The questions and the scoring of the written replies were established in a set of classroom experiments (Hygge, 1997, 2003).
In the Betula project, Nilsson et al. (1997) developed a face and name recognition task for testing intentional and incidental learning and recognition for non-verbal material in episodic memory. This test was computerised and adapted to group presentation in our experiments. There were 16 colour-pictures of faces of 10-year-old children. Each picture was presented for 8 s on a computer screen, clearly visible to all participants. Together with the picture made-up, but common, first and family names were presented. The participants were instructed that they later would be tested for recognition of faces and family names, but nothing was mentioned about first names. Approximately 40 min later they were tested for recognition of faces and family names (see [Table 1], task 4, 13). In addition to that, and without having been so instructed at encoding, they were also given a recognition test for the first names. On the computer screen 24 faces and names were presented during testing. Twelve were target faces and names of the 16 presented initially, and 12 were distractor faces and names. Target and distractor faces appeared one by one for 15 s on the computer screen in a random order. The faces were presented twice, first without any names and then with four different first-family-name combinations. Intentional episodic memory was tested with paper-and-pencil recognition test of faces and family names, and incidental memory with recognition of first names.
In testing sentences with and without enactment (Nilsson et al., 1997) the participants were presented with two successive lists in imperative form (e.g., knock on the pan, roll the pineapple) with 16 sentences each. For one of the lists the encoding was done with enactment (making the movements but imagining the object) and for the other without enactment (Engelkamp, 1995). The lists were counterbalanced. Each sentence was presented on the computer-screen for 8 s. Around 15 min later there was a free recall test in silence and immediately after that a cued recall test with category names presented where the task was to recall nouns from the sentences (see [Table 1], task 11, 12). After another 12 min a phrase containing the verb from the earlier presentation was presented on the screen. The task was to fill in the missing noun in the phrase on the reply sheet (see [Table 1], task 14). These tests made it possible to study noise effects on episodic memory with and without involvement of a motoric component.
Semantic memory. In the word fluency test, a semantic memory task, three sets of words were generated, each set starting with a letter of its own. The sets were: words, five-letter words and professions (Nilsson et al., 1997). Each set was given one minute and the participants wrote down their replies.
In the word comprehension task participants were presented with a list of 30 target words on a paper. Next to each target five other words were presented, one of which being synonymous to the target word. For the group 13-14 years, four words were presented instead of five. This task was a test of the noise impact on the general knowledge in semantic memory.
Attention. In the search and memory task (SMT) (Smith and Miles, 1987) participants were presented with lines of random letters and a set of five target letters at the beginning of each line. The task was to memorize the given targets, search through the given line only once, and to mark all targets found. Each line contained 59 letters, 0-4 of which were targets. Lines were arranged in sets of six, with 11-14 targets distributed through each set. The task was scored both for accuracy (percentage of targets missed) and speed (number of letters completed). The SMT was performed at the beginning of the experiment and as the last task before the quiet period (see [Table 1]). By administrating the task twice, both initial and accumulated noise effects on attention can be assessed. In the SMT there is not much of a learning effect (Smith and Miles, 1987).
Analyses of variance (Software SPSS 11.5 for Windows) were performed with noise condition and age group as independent variables. Follow up tests were made by inspecting the confidence intervals in the tables of means for the significant interactions. (This short-hand procedure does not overload text with statistical detail, which in the present context, but not always, is an advantage). [Table 2] shows the means and SDs for the 18 dependent measures as a function of noise condition and age group, and [Table 3] summarizes the separate ANOVAs for these dependent measures. [Table 4] shows the within subject analysis of variance between cued recall and recognition of the text. [Table 5] shows the means and SDs for the aggregated dependent measures as a function of noise and age group, and [table 6] summarizes the separate ANOVAs for these measures.
Performance on six out of the twelve episodic memory tests showed an effect of noise, all except one an effect of age group, and only one memory test an interaction between noise and age (see [Table 3]). Judging from the size of the Fratios, which is not always justified, performance on cued recall and recognition of the text stand out as particularly sensitive to noise. Follow-up tests, by means of confidence intervals, indicate that for cued recall and recognition, the quiet condition had higher means than both the road traffic and speech conditions, which did not differ. The age effect for cued recall had a peak for the 18-20 years old, as predicted, while recognition against predictions was best for the oldest group, followed by the 35-45 and 18-20 years old group, which did not differ. The youngest group was significantly worst off both for cued recall and recognition, as predicted.
A direct comparison of the cued recall and recognition scores, as a within-subject variable (M), in an ANOVA, showed significant interactions M x Noise, M x Age, and M x Noise x Age (see [Table 4]). As predicted, this indicated more impairment from noise on cued recall than on recognition (see [Figure 1] and [Figure 2]). Contrary to predictions, the youngest group did not show more noise impairment than the other age groups. However, in line with expectation that recall is more age sensitive there was a tendency for the noise effect on cued recall to be larger for the two older groups than for the two younger groups (For cued recall Noise x Age group, F(6,276) = 1.85, p = 0.086). The significant three way interaction M x Noise x Age, and an inspection of the corresponding confidence interval limits, indicated that only in the oldest group the means for cued recall were lower both in the road traffic noise group than in the quiet group, and lower in the speech group than in the road traffic noise group. For recognition meaningful irrelevant speech was significantly more impairing than the quiet condition in the youngest group and the group aged 35-45 years, but road traffic was not.
For the other episodic memory tasks (se [Table 2] and [Table 3] that showed a noise effect on performance, the pattern was that the quiet condition yielded higher means than both the road traffic noise and speech conditions, which did not differ. The significant age effect as a rule indicated that the two oldest groups performed best, followed by the 18-20 years old group, which in turn performed better than the youngest group. The significant interaction between noise and age group showed that for the age group 3545 years, the performance on free recall without enactment showed the highest impairment in the noise conditions compared to the silent condition.
Thus taken separately, performance on a majority of the episodic memory tasks showed an effect of noise, all but one an age effect, and only one an interaction between noise and age group. The noise effect was as predicted, the predicted age effect with a peak for the group aged 18-20 years old came out only for the cued recall of the text, and none of the predicted interactions between noise and age on memory were supported.
Performance of three out of the four semantic memory tasks showed a predicted reliable effect of noise, and all four a reliable age effect, but there were no interactions between noise and age group on the semantic memory tasks. When there was a noise effect, the quiet group had higher scores than the road traffic and speech noise groups, which did not differ. Inspections of confidence intervals for the age effects indicated that the oldest group never was significantly better than the group aged 35-45 years, which was contrary to predictions. The youngest groups, as predicted, always performed worst on the semantic tasks.
The SMT task was performed in two blocks during the experiment, in the beginning and at the end of the first part of the experiment, but since there was no interactions Noise x Blocks, N x Type (Accuracy and Speed), and Noise x Blocks x Type (Fs Aggregated Measures - Nature of the Material Starting from the univariate analyses in [Table 3], aggregated dependent measures were formed. Dependent measures that reflected different aspects in episodic and semantic memory were aggregated in the following way: The mean of the cued recall and recognition measures from the text reading, the mean of all the episodic memory tasks that involved remembering sentences (test 7, 11, 12, 14), and the mean for all tests involving memory or faces and names (test 4, 13) was formed. For semantic memory, word fluency measures were aggregated (test 5) and word comprehension formed its own measure (test 6, divided by 3 to keep values on the same scale range as the other aggregated measures). In this way, more general hypotheses about the noise effects on aggregated memory measures, rather than the individual memory tasks can be evaluated. Means and SDs for these five groups of aggregated measures are shown in [Table 5]. [Table 6] summarizes the ANOVAs and follow-up tests, and [Figure 3] and [Figure 4] show means by noise and age group.
Judging from [Figure 3] the line for the memory for the text has the steepest slope of the five measures, which indicates that this measure is the one most sensitive to noise manipulations. This measure also has the steepest slope from road traffic noise to meaningful irrelevant speech, indicating that this measure alone is reliably more impacted by speech noise than by road traffic noise, which is also borne out from the inspection of corresponding confidence intervals, as noted in [Table 6]. Thus, when the two memory measures for reading the text are averaged into one measure, the ensuing decrease in variance was sufficient to make the difference between the speech noise and the road traffic noise reliable. This was contrary to expectations. Noise effects were also found on memory for sentences, word fluency and word comprehension. However, for these measures there were no differentiation between road traffic noise and meaningful irrelevant speech. Another finding was that performance on recognition for faces and names was not impaired by noise.
[Figure 4] shows, as predicted that the peak performance for memory of the text is reached in the 18-20 years group, which is also borne out in the follow-up test in [Table 6]. For the second episodic memory tasks, memory for sentences, the peak performance is in the region 18-45 years, and for the third episodic memory related measure, memory for faces, there is no marked peak but rather a slight increase with age. Thus, the expected superiority in episodic memory for the 18-20 year old group was borne out only for the memory of the text. The predicted lowest performance for the youngest group, however, received support from all the three aggregated episodic memory tests, but none showed the predicted Age x Noise interaction. For the two semantic memory tasks, word fluency and word comprehension, performance reached a plateau at 35-45 years, that is, a peak plateau reached earlier than we predicted. Contrary to expectations there were no interactions between noise and age group. The noise effect for the aggregated semantic memory tests were according to predictions, that is, the performance was better in quiet than in road traffic noise or irrelevant speech, which did not differ.
A Note on Gender
In the three basic experiments gender was kept under control to balance the design, but not chosen to test hypotheses about interactions between gender and noise. However, since gender data is available a few remarks also on this variable are appropriate. For the aggregated measures memory for sentences, word fluency and memory for faces and names showed a significant gender effect (all Fs(1,264) > 5.69, ps 2.69, p =.015), indicating that older men were more impaired by speech noise than by road traffic noise, while the reverse was true for older women. Thus, there was a general trend towards females performing better than males, but no indication of a general interaction between gender and noise.
The main objectives were to analyze noise effects on episodic and semantic memory in different age groups, and whether age differences in memory interacted with noise. Our results showed reliable noise effects for several memory tests, both when taken alone and aggregated according to the nature of the material to be memorized. It was assumed that noise would have a more pronounced effect on episodic than on semantic memory tasks. As evident form [Figure 3] and [Table 6], there was a general tendency for the noise effects on episodic memory tasks to be stronger than on semantic memory tasks, and the result thus was in line with prediction in this respect.
The results further show that within episodic memory, the noise effects on performance on cued recall of the text were stronger than on recognition of the text, which in turn was stronger than on memory for sentences. Another finding was that recognition for faces and names were not impaired by noise. These results imply that the performance on complex tasks is more vulnerable to noise compared to easier ones, and it also give some support to that the presence of retrieval cues may compensate for encoding deficiencies and thereby reduce or prevent noise impairments. The assumption about complexity of the task is also in line with Cohen, Evans, Stokols and Krantz's (1986) model, which implies that task performance deteriorates when number of inputs one must attend to increases. They meant that noise increases demands on the limited capacity of attention and information processing reducing the information processing resources available for the memory task. This reduced capacity should thus have more disruption effects in a complex task than in a simpler one.
Contrary to predictions, however, there was no interaction between noise and age group, and in particular, the younger pupils were not more negatively affected by the noise than the older groups. This was true also if the relative, rather than the absolute, noise impairments in the separate age groups were calculated. This finding is somewhat counter intuitive, and should be paid attention to in coming research.
With regard to the nature of the sound, it was not expected that meaningful irrelevant speech and road traffic noise would differ. That expectation was based on the results from the three experiments taken separately, but originally our expectation was that meaningful irrelevant speech noise would be more detrimental to memory than road traffic noise. The results from the three experiments taken together throw a little more light on the relative effects of road traffic noise and speech noise. When memory tests were aggregated according to the nature of the material to be memorized, there is a differentiation between the two noise sources on memory of the text [Table 6], to the disadvantage of the speech noise. This may be an indicator of interfering semantic parallel processes between the noise and the task at hand (see Knez and Hygge, 2002 for similar reasoning).
Although we were primarily concerned with the effects of noise exposure and age on performance on episodic and semantic memory, the main effects of age in our studies can be compared to no-noise studies on memory and age, in order to see whether the present findings add to a general pattern or not.
The Betula study (Nilsson et al., 1997) is an obvious such point of reference because some of the memory tasks employed by us were also employed in the Betula study. In a recent summary of the ongoing Betula study, Nilsson (2003) stated that there are clear age deficits in episodic memory, and that similar age deficits do not exist for semantic memory. The decline of episodic memory seems to begin as early as around 20 years of age. The figures presented by Nilsson (2003) summarizing the first wave of the Betula study, show a steady decline in episodic memory from 35-40 years of age (lowest age span reported) to 75-80 years of age (highest age span reported). This is congruent with our results from the episodic memory for sentences for the 35-45 and 55-65 years old groups (see [Figure 4]).
The Betula study did not include any episodic memory test similar to our memory of the text. As evident from [Table 4], [Table 5] and [Table 6] and [Figure 3] and [Figure 4], the cued recall part of the memory for the text was the single most age sensitive test. The assumed peak in episodic memory tasks for young adults around 18 years of age, as suggested by for example Nilsson (2003), is clearly borne out with this test, and the peak at this age is more marked for memory of the text than for the other episodic memory tasks (see [Figure 3]).
For semantic memory in the Betula study, Nilsson et al. (1997) reported the highest performance at the age of 55-60 years, which was significantly higher than for the older age groups and for the age group 35-40, but not for the group the age span 45-50 years of age. Our results for word fluency and word comprehension do not indicate higher means in our oldest group compared to the second oldest group, 35-45 years. This difference between our results and the Betula study may be attributable to our groups being teachers, who because of their better than average education are being less negatively impacted by age in semantic memory (Nilsson et al., 1997; Nilsson, 2003).
In conclusion, in this paper it has been shown that both noise sources, independent of age, affect episodic and semantic memory performance. However, meaningful irrelevant speech impaired reading comprehension to a higher degree than road traffic noise did. This finding reported here suggests the need for future research concerning the noise and the nature of the task.
We are grateful to Lars-Goran Nilsson and his co-workers in the Betula project for permission to take over and adapt memory tests to our setting. We also are grateful to Anders Kjellberg and Igor Knez for valuable comments on the paper. The research reported here was supported by the National Institute for Working Life and University of Gavle.
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