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APPLIED ASPECTS OF AUDITORY DISTRACTION
|Year : 2010
: 12 | Issue : 49 | Page
|The effects of road traffic and aircraft noise exposure on children's episodic memory: The RANCH Project
Mark Matheson1, Charlotte Clark1, Rocio Martin2, Elise van Kempen3, Mary Haines4, Isabel Lopez Barrio2, Staffan Hygge5, Stephen Stansfeld1
1 Centre for Psychiatry, Wolfson Institute of Preventive Medicine, Barts and the London School of Medicine and Dentistry, Queen Mary University of London, Old Anatomy Building, Charterhouse Square, London EC1M 6BQ, United Kingdom
2 Instituto de Acustica, CSIC (Consejo Superior De Investigaciones Científicas), Madrid, Spain
3 Centre for Environmental Health Research, National Institute for Public Health and Environment RIVM, Bilthoven, The Netherlands
4 The Sax Institute, Sydney, NSW, Australia
5 Laboratory of Applied Psychology, Centre for Built Environment, University of Gavle, Gavle, Sweden
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|Date of Web Publication||21-Sep-2010|
Previous studies have found that chronic exposure to aircraft noise has a negative effect on children's performance on tests of episodic memory. The present study extended the design of earlier studies in three ways: firstly, by examining the effects of two noise sources, aircraft and road traffic, secondly, by examining exposure-effect relationships, and thirdly, by carrying out parallel field studies in three European countries, allowing cross-country comparisons to be made. A total of 2844 children aged between 8 years 10 months and 12 years 10 months (mean age 10 years 6 months) completed classroom-based tests of cued recall, recognition memory and prospective memory. Questionnaires were also completed by the children and their parents in order to provide information about socioeconomic context. Multilevel modeling analysis revealed aircraft noise to be associated with an impairment of recognition memory in a linear exposure-effect relationship. The analysis also found road traffic noise to be associated with improved performance on cued recall in a linear exposure-effect relationship. No significant association was found between exposure to aircraft noise and cued recall or prospective memory. Likewise, no significant association was found between road traffic noise and recognition or prospective memory. Taken together, these findings indicate that exposure to aircraft noise and road traffic noise can impact on certain aspects of children's episodic memory.
Keywords: Aircraft, children, episodic memory, noise, road traffic
|How to cite this article:|
Matheson M, Clark C, Martin R, van Kempen E, Haines M, Barrio IL, Hygge S, Stansfeld S. The effects of road traffic and aircraft noise exposure on children's episodic memory: The RANCH Project. Noise Health 2010;12:244-54
|How to cite this URL:|
Matheson M, Clark C, Martin R, van Kempen E, Haines M, Barrio IL, Hygge S, Stansfeld S. The effects of road traffic and aircraft noise exposure on children's episodic memory: The RANCH Project. Noise Health [serial online] 2010 [cited 2020 May 25];12:244-54. Available from: http://www.noiseandhealth.org/text.asp?2010/12/49/244/70503
| Introduction|| |
The existing studies of the effects of environmental noise exposure on children's cognitive functioning have compared high and low noise exposed groups. While this type of design allows noise effects to be identified, it does not provide knowledge of sufficient precision to feed into policy initiatives. In order to formulate legislation, it is necessary to know the effects which different levels of noise will have in order to identify a threshold at which noise levels become unacceptable. Accordingly, the present epidemiological study examined exposure-effect relationships between a range of noise exposures and children's performance.
Previous studies of environmental noise effects in children have employed different designs and have not used the same cognitive tests, therefore not allowing direct comparison between the studies. The study reported here formed part of a larger field study, the RANCH Project (Road traffic and aircraft noise exposure and children's cognition and health), which comprised three parallel field studies carried out in three European countries: the United Kingdom, Spain and the Netherlands. The design and the outcomes were the same in each of the three countries. This allowed the relationship between noise exposure and the outcome measures to be examined both for each country individually and for the pooled data set. This is the largest study to have been undertaken in this area to date and is the first to make cross-country comparisons. An overview of the RANCH Project is presented by Stansfeld et al.  Clark et al.  elaborate on the effects of noise on reading comprehension in the RANCH Project, while in this article we provide a detailed account of that aspect of the project which dealt with episodic memory.
The present study focused on identifying noise levels at which learning may be adversely affected. The issue of noise levels at which learning is optimal is a distinct research question. While there are no Europe-wide guidelines for unoccupied classroom noise exposure, countries tend to adopt guidelines of 30 or 35 dB L Aeq 30 minutes indoors. , L Aeq is shorthand for "equivalent continuous noise level" and is an averaged measure of noise exposure over a specified time period. Likewise, ANSI recommends that the ambient unoccupied classroom noise level should not exceed 35 dB L Aeq MAX . 
Many studies of environmental noise effects in children have focused on aircraft noise.  Despite its ubiquitous nature in Western societies, road traffic noise has received comparatively little attention. The present study has addressed this issue by examining both aircraft and road traffic noise. Furthermore, this study has examined exposure-effect relationships between aircraft and road traffic noise and episodic memory, as assessed through tests of delayed cued recall and recognition, and a test of prospective memory.
Previous studies of noise and memory
The effects of chronic noise exposure on children's cognitive performance have been the focus of a number of studies. ,,,, In these studies, the most widely found effect on children's cognition has been in terms of reading comprehension, ,,, with memory having received rather less attention. Episodic memory can be assessed in terms of recognition memory and recall memory.  Recall memory in turn can be assessed by tests of free recall in which subjects are asked to remember as much as they can about, for example, a text or by tests of cued recall in which subjects are prompted to remember specific pieces of information. Another type of memory which has received relatively little attention in the research literature is prospective memory. This refers to the type of memory involved in remembering to take a certain course of action in the future such as posting a letter or keeping an appointment.
The strongest evidence for an effect of chronic noise exposure on episodic memory comes from a landmark study which took advantage of a naturally occurring experiment when Munich Airport moved to a different location.  At each of the old and new airports, a noise exposed and matched control non-noise exposed group were identified. The children were required to read a text, their memory for which was assessed by means of a free recall test the following day. The children's reading comprehension was separately assessed by means of a standardized German reading test. Prior to the airport relocating, the children in the noise exposed 68 dB L Aeq 24 hour group at the old airport performed significantly worse on the free recall test than those in the non noise exposed 59 dB L Aeq 24 hour group at the old airport. Two years after the airport had moved, this difference was no longer observed. At the new airport, while there were no significant differences in the memory test scores prior to the airport moving, two years later, the group exposed to the noise from the new airport (62 dB L Aeq 24 hour ) performed significantly worse on the memory test than the matched control quiet 55 dB L Aeq24 hour group. These findings suggest that there may be a causal relationship between chronic exposure to aircraft noise and deficits in episodic memory. They also suggest that such effects may be reversible. While other studies have also found effects of chronic noise exposure on episodic memory,  a number of studies have failed to find such effects. ,,
Typically, chronic noise exposure has been examined by epidemiological studies, and effects of acute noise exposure examined in experimental, laboratory studies. Studies of whether acute rather than chronic exposure to noise can have a negative effect on episodic memory have produced equivocal results. Hygge  found that simulated aircraft noise impaired memory at both 55 and 66 db L Aeq 15 minutes . He also found that aircraft and road traffic noise had a greater effect than either train noise or irrelevant speech. No effects of acute noise were found, however, in studies by Hambrick-Dixon  or Johansson. 
It is noteworthy that of those studies which have focused on recognition memory, most have failed to find effects of chronic noise exposure. ,, However, Haines et al.  and Lercher et al.,  both found effects of noise exposure on recognition memory.
An explanation for these rather disparate findings might lie in the nature of the stimuli which subjects are required to remember.  Studies which have found effects of noise on memory seem to have complex, semantic stimuli, in common. A study by Hygge  illustrates this point: subjects were given cued recall and recognition tests for both easy and difficult material under conditions of both quiet and noise. Effects were found only in the case of the difficult stimuli. Similarly, Meis et al.  found that both chronic and simulated aircraft noise had effects on long-term recall, not on word production, the latter being a task requiring participants to recall words which they had not been instructed to recall in advance. This finding is particularly significant in that while long-term recall is an explicit memory task, that is, a task in which the subject is explicitly instructed to remember the stimuli, word production is an implicit task in which the subject is not instructed in this way. It may be that explicit memory is more vulnerable to the effects of noise in that it is more affected by distraction or divided attention than is the case with implicit memory. However, a difficulty with this hypothesis is that several studies have failed to find effects of noise on, e.g., attention.  Moreover, Lercher et al.  found effects of relatively modest levels of road traffic and train noise greater than 60 dB L Aeq Ldn on both explicit and implicit memory.
Several mechanisms have been proposed to account for noise effects on memory. Many theories focus on noise interference with verbal communication as the mechanism for the effect. Noise may cause disruption of teaching activities.  Theories of irrelevant sound effects  propose that performance on serial recall tasks are disturbed by the presence of low-intensity background noise, and recent studies have found that both road traffic noise and meaningful irrelevant speech had a similar effect on task performance, with noise effects being strongest for memory of texts, followed by episodic and semantic tasks. , However, whilst noise is proposed to influence cognitive performance by impairing the quality with which information is rehearsed or stored in memory, a later paper found that noise did not impair performance via a change in resource allocation or strategy. 
Further investigation of mechanisms has focused on the role of classroom acoustics in noise effects: whilst some studies simply describe the acoustic characteristics of classrooms, some specifically assess speech intelligibility, and a few relate acoustic conditions to performance. ,,,,,, Shield and Dockrell  identified that the average external noise outside of schools in the UK was 57 dB L Aeq 5 minutes : 86% of schools were exposed to road traffic noise and external noise levels affected children's reports of how easy it was to hear their teacher. Studies also suggest that individual noise events per se, as well as chronic noise, may play an important role in performance effects. ,
As noted above, there has been little research focusing on prospective memory. Indeed, to date the only study to examine the effects of noise on prospective memory has looked at the effects of acute noise on task performance.  This study required children to prospectively remember to write their names on the last page of a test booklet. In this study, performance on this test was found to be poorer in children exposed to acute noise. A test of prospective memory was included as an exploratory aspect of the present study.
In the present study, episodic memory was assessed by means of a test of cued recall. This was selected over the alternative, a test of free recall, as it offered greater ease of scoring which was considered important given the large sample size. A test of recognition memory was also administered. Socioeconomic variables were adjusted for as these are potentially confounding factors for both noise exposure and cognitive ability.
| Hypotheses|| |
It is clear from the foregoing discussion that previous studies which have examined the effects on noise on children's memory have produced mixed results. Predictions as to the results of the current study were therefore based on a judgment as to whether the existing evidence is greater either for or against noise effects being associated with particular types of memory. As no previous studies were identical to the present study in terms of design and tests used, it was also necessary in making these judgments to give consideration to the similarities and differences between the present and previous studies. On this basis, the following predictions were made:
1a. It was judged that the balance of evidence from previous studies such as those conducted by Meis et al. and Hygge et al. , that both aircraft noise and road traffic noise would be associated with impairments in cued recall in an exposure-effect relationship after adjustment for socioeconomic factors.
1b. It was predicted that the effect on cued recall would be found both when the test was scored in terms of correctly recalled information and in terms of conceptual recall. This distinction is fully explained in the method.
2. It was judged that the balance of evidence from previous studies such as those conducted by Hambrick-Dixon, Hygge, and Haines et al. ,, that recognition memory would not be associated with noise exposure.
3. On the strength of the one previous study to examine the effect of noise exposure on prospective memory,  it was predicted that this outcome would be associated with impairments in an exposure-effect relationship after adjustment for socioeconomic factors.
| Method|| |
The children were selected to take part in this cross-sectional field study according to their noise exposure at school. The schools were near one of three airports: London Heathrow in the UK, Madrid Barajas in Spain and Amsterdam Schiphol in the Netherlands. The schools were selected from a four by four matrix of increasing aircraft and road traffic noise exposure, allowing the individual effects of each of these noise sources to be examined (see for further details  ).
All elementary schools near each of the three airports were first identified and then were matched according to socioeconomic position, number of pupils eligible for free school meals and main language spoken at home beginning with those schools exposed to the highest levels of aircraft noise. Following noise surveys, schools were then excluded if they had a dominant noise source other than aircraft or road traffic noise. In the UK and Spain, two classes were selected in each school as far as possible and in the Netherlands one class was selected in each school. No children were excluded from the selected classes.
In the UK, aircraft noise exposure was based on 16-hour outdoor L Aeq contours provided by the Civil Aviation Authority from July to September 1999. This is an averaged measure of outdoor noise exposure over a 16-hour period. Road traffic noise was estimated on the basis of the proximity of each school to motorways, main roads, secondary roads, and traffic flow data based on the UK Calculation of Road Traffic Noise method.  This is a standardized method of estimating road traffic noise used in the UK, which produces L A10 18 hour values which were converted to L Aeq 16 hour .  In Spain, aircraft noise estimates were also based on L Aeq 16 hour , using contour information from July to September 2000. In Spain, road traffic noise was estimated entirely on the basis of visits made to each of the schools in order to take noise measurements. In the Netherlands, by contrast, both aircraft and road traffic noise exposure was based on modeled data linked to school locations by Geographical Information Systems; the contours were from October 1999 to November 2000. In order to check the accuracy of these estimates, each school was visited and noise measurements were taken by trained acousticians. These measures indicated the estimates to be accurate. In each country, schools were excluded from the study if noise surveys indicated either the presence of a dominant noise other than aircraft or road traffic noise or which were insulated against noise. In addition to assessing chronic noise exposure, measurements of acute noise, both inside and outside the classroom, were taken during testing to check for any unexpected noise sources and to assess acute noise exposure. A measure of indoor and outdoor acute noise L 10 was calculated representing the noise level exceeded for 10% of the duration of the testing session. For all analyses, noise was treated as a continuous variable. In our sample, aircraft noise at school ranged from 32 to 77 dB L Aeq 16 hour and road traffic noise at school ranged from 31 to 71dB L Aeq 16 hour .
Recall and recognition
Episodic memory was assessed in terms of delayed cued recall, delayed recognition and prospective memory. Delayed recall and recognition were tested by the Children's Memory Scale  adapted for group administration. The Children's Memory Scale is an episodic memory task widely used in the USA and less widely used in the UK. The test assesses the ability to process, encode and recall meaningful verbal material that is presented in a narrative format. Extensive psychometric piloting confirmed that this adapted version of the Children's Memory Scale was both reliable and valid; pilot reports are available from the authors on request.
The test consisted of two stories, taken from the Children's Memory Scale, which had been recorded on to audio CDs in each of the three countries in order to standardize the presentation of the stories. For the same reason, the stories were then played to the children using standard equipment across the three sites. This equipment was calibrated prior to testing and volume levels controlled during testing. The stories were respectively 86 and 75 words in length. The children were instructed to listen carefully to the stories because they would have to remember them later. There was then a 30-minute delay with an interference task which was a self-report questionnaire which is described below. The children were then given a cued recall test which consisted of two parts, each part relating to one of the stories. For each story, the children were asked to write as much as they could remember in response to each of four questions. These questions were scored in two distinct ways. Firstly, they were scored for correctly recalled specific pieces of information. In order to be awarded credit, the children were required to have recalled some of the details of the stories. Scoring for correct information was carried out in a very strict manner with credit only being awarded when the children had provided responses which exactly matched those listed on a score sheet. These were not required to be verbatim but did need to express precisely the same idea as those on the score sheet.
Secondly, the children's answers were also scored for their conceptual recall of the main themes, rather than the details, of the stories. Scoring for conceptual recall was conducted in a more lenient manner such that, e.g., in cases in which the children seemed to have recalled the essential elements of the story, but had perhaps not been sufficiently explicit about the way in which they had reported their memories, credit was given. Again, the correct answers were listed on a score sheet. The maximum total scores for stories 1 and 2 were 36 and 30 points, respectively, for correctly recalled information. For conceptual recall, the maximum total scores for Stories 1 and 2 were 9 and 6 points, respectively. For the purposes of analysis, the scores from the two stories were summed.
Immediately after having finished the cued recall questions, the children were given a test of delayed recognition. This consisted of two parts, each relating to one of the two stories. For each story, the experimenter read out 15 factual recall questions to which they were required to respond yes or no by ticking boxes on a response sheet. Each correctly answered question was given a score of 1 and each wrong answer a score of 0. There was a maximum total score for each story of 15. Each subject's total scores for each story were summed to produce a maximum total score of 30.
The test of prospective memory was based on that used by Shield and Dockrell.  The test consisted of two trials. In each case, the children were instructed to write their initials next to two specific question numbers on a reading test which was administered as part of the same test battery in which the above described tests were administered. The children were given the instructions for the prospective memory test immediately prior to commencing the reading test. Children are often requested by their teachers to identify their work by writing their name or initials and as such this is a naturalistic measure of prospective memory.
The test was scored both for what, if anything, the children wrote - specifically did they write their initials or something else, such as their name and for where they wrote it - specifically did they write it in the place they were instructed to, or somewhere else. If a child responded exactly as instructed on both trials, they were given a score of 1. If they made any errors on either of the trials, whether in terms of what they wrote or where they wrote it, they were given a score of 0.
Exactly the same tests were used across the three countries. The tests were translated from English into both Dutch and Spanish and then were back-translated into English in order to minimize the chance of any mis-translation. The tests were all extensively piloted in each of the three countries.
Data relating to potential confounding factors were collected through a questionnaire completed by the children and through a questionnaire completed by their mother or female carer. The questionnaires included items relating to socioeconomic status, perceived health including psychological health, sleep quality, perceptions of noise and annoyance, noise interference with activities at home and at school, and environmental attitudes. As there were a large number of potential confounding factors, analyses of covariance were carried out and a confounding factor was only included in the main analysis if it was significantly related to road traffic and/or aircraft noise exposure [Table 1].
|Table 1: Sociodemographic characteristics of the RANCH sample: Overall and by country|
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Age was collected both from school records and from the parent questionnaire. Employment status was a measure of the highest employment status in the child's household. Crowding was a measure of the number of people per room in the child's household. This was based on national definitions to take account of cultural variation. In the UK and Spain, crowding was defined as more than 1.5 people per room. In the Netherlands, crowding was defined as equal to or greater than one person per room. Home ownership was an indicator of whether the child's home was rented or owned/mortgaged. Mother's education was measured using a ranked index of standard qualifications in each country. In order to allow comparisons to be made between the different measures used in each country, a relative inequality index  was calculated for mother's educational attainment. The measure of child's long-standing illness (LSI) was based on parental report. Main language was an indicator of whether the child spoke the predominant language in their country, i.e., English, Spanish or Dutch. Parental support for school work was based on a self-report scale in the children's questionnaire. The analysis was conducted on a complete-case basis so that missing values were not imputed for any of the confounding variables used in the main analyses.
The memory tests and children's questionnaire were part of a larger test battery which was group administered during a 2-hour testing session. Testing took place in the children's normal classroom, in the morning, for the most part in the second quarter of the year. The tests and questionnaire were administered in a fixed order under careful supervision from the field team. Written consent was obtained from both parents and children.
In the UK, ethical approval for the study was given by the East London and the City Local Research Ethics Committee, East Berkshire Local Research Ethics Committee, Hillingdon Local Research Ethics Committee and the Hounslow District Research Ethics Committee. In the Netherlands, ethical approval was given by the medical ethics committee of TNO, Leiden. In Spain, ethical approval was given by the CSIC Bioethical Commission, Madrid.
Multilevel modeling was used to analyze the data. This allows the hierarchical nature of the data to be taken into account, enabling data at both the school level, e.g., aircraft noise exposure and the individual level, e.g., mother's educational attainment to be fitted in the same model. For the conceptual recall and correct information recall analyses, which were carried out in the UK, the statistical package MLWin  was used to fit the models to the data. For the recognition memory and prospective memory data, which were analyzed in Spain, an alternative statistical package, (SAS version 9.1) was used.
Separate analyses were conducted for aircraft noise exposure and road traffic noise exposure. For each noise exposure type, two models were run. Model 1 included noise exposure, either aircraft or road traffic noise, while Model 2 was further adjusted for classroom glazing, i.e., the type of windows in the child's classroom, age, sex, country (the UK, Spain and the Netherlands), mother's educational attainment, socioeconomic status which was measured by employment status, crowding and home ownership, LSI, main language spoken at home and parental support for school work and the other noise exposure variable, e.g., for aircraft noise analyses, road traffic noise exposure was adjusted for. This final model was additionally adjusted for parental report of the child having dyslexia and acute noise during testing. We tested the models for statistical significance by comparing the goodness of fit of different models using a chi square test of deviance. Regression estimates and 95% confidence intervals were calculated from these models, which take the school and individual level variance within the data into account, to assess effect sizes. Tests of heterogeneity were used to determine whether the effect between noise and performance was the same in each country. All results presented in this article are from the Model 2 analyses, which were adjusted for the confounding factors.
The exposure-effect associations between noise and memory performance were examined using the effect sizes which determine the change in task performance associated with a 1-dB L Aeq 16 hour change in noise exposure and by subsequent analyses examining the shape of the relationship. The possibility of a curvilinear exposure-effect relationship between either aircraft or road traffic noise and each outcome was investigated using fractional polynomial models.  The best fitting model from a set of two degree fractional polynomials of the form β1aircraft noise p1 + β2noise p2 where p1 and p2 belong to the set -2, -1, -0.5, 0, 0.5, 1, 2, 3 was chosen. Then, the goodness of fit deviance of this model was compared with the goodness of fit of a straight line model to test for departure from a straight line relationship.
| Results|| |
Sociodemographic characteristics of the sample
[Table 1] illustrates the sociodemographic characteristics of the overall RANCH sample. There were a total of 2844 children who participated in the study UK (n = 1174), Spain (n = 908) and NL (n = 762) from 89 schools (UK = 29, Spain = 27, NL = 33). The overall child response rate was 89% and the response rate for the parent questionnaire was 80%. The participation rate for children and parents did not vary significantly across noise exposures. Completed parent questionnaires were available for 2276 (80%) of the children sampled. The average age was 10 years 6 months and 53% of the sample were female. The table indicates that there were differences between the countries in terms of employment status, home ownership, crowding and main language spoken at home that reflect sociodemographic differences between the countries.
Noise exposure and information recall
Chronic aircraft noise exposure was not significantly related to poorer information recall: model fit (χ2 = 2.04, df = 1, P0 = 0.15) [Table 2]. Chronic road traffic noise exposure was related to improved information recall: model fit (χ2 = 3.70, df = 1, P = 0.0489) [Table 2]. As road traffic noise increased by 5dB L Aeq 16 hour , performance on information recall increased by 0.19 marks (B = 0.038 Χ 5). Children also performed better on the task if their parents were homeowners, if they spoke the main language of the country and if they had high parental support for school work; performance was poorer for children whose mother had a low educational attainment. [Table 2] also shows the effect size of road traffic noise on information recall from the adjusted model for each country: there was no significant difference in effect size between the countries (test of heterogeneity P = 0.93). [Figure 1] shows the information recall scores adjusted for age, gender and country by 5dB L Aeq 16 hour bands of road traffic noise: the exposure-effect relationship between road traffic noise and information recall was linear (P = 0.67) for comparison of straight line fit with best fitting fractional polynomial curve.
|Figure 1: The exposure-effect relationship for road traffic noise exposure and information recall; adjusted information recall score and 95% confidence interval bars for 5 dBLAeq 16 hour bands of road traffic noise adjusted for age, gender and center|
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|Table 2: Effect sizea for aircraft and road traffic noise exposure and information recall and conceptual recall|
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Noise exposure and conceptual recall
Chronic aircraft noise exposure was not significantly related to poorer conceptual recall: model fit (χ2 = 1.23, df = 1, P = 0.27) [Table 2]. Chronic road traffic noise exposure was associated with improved conceptual recall: model fit (χ2 = 6.75, df = 1, P = 0.0066) [Table 2]. As road traffic noise increased by 5 dB L Aeq 16 hour , performance on conceptual recall increased by 0.065 marks (B = 0.013 Χ 5). Children also performed better on the conceptual recall task if they were male, their parents were homeowners, if they spoke the main language of the country and if they had parental support for schoolwork; performance was poorer for children whose mother had low educational attainment. [Table 2] also shows the effect size of road traffic noise on conceptual recall from the adjusted model for each country; there was no significant difference in effect size between the countries (test of heterogeneity P = 0.73). [Figure 2] shows the exposure-effect relationship between road traffic noise exposure and conceptual recall, which was linear (P = 0.99) for comparison of straight line fit with best fitting fractional polynomial curve.
|Figure 2: The exposure-effect relationship for road traffic noise exposure and conceptual recall; adjusted conceptual recall score and 95% confidence interval bars for 5 dBLAeq 16 hour bands of road traffic noise adjusted for age, gender and country|
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Noise exposure and recognition memory
Chronic aircraft noise exposure was significantly related to poorer recognition memory: model fit (χ2 = 5.75, df = 1, P = 0.0141) [Table 3]. As aircraft noise increased by 5 dB L Aeq 16 hour , performance on the recognition task decreased by -0.09 marks (B = -0.018 Χ 5). Children also scored lower on recognition memory if they had a mother with low educational attainment and scored higher if their parent/s were homeowners, if their parent/s were employed, if they spoke the main language of the country and if the child perceived a high level of parental support for schoolwork. [Table 3] also shows the effect size of aircraft noise on recognition memory for each country; there was no significant difference in the effect size between countries (test of heterogeneity P = 0.105). [Figure 3] shows the relationship between aircraft noise and recognition memory which was linear (P0 = 0.34) for comparison of straight line fit with best fitting fractional polynomial curve. Chronic road traffic noise exposure was not significantly related to recognition memory: model fit (χ2 = 0.240, df = 1, P = 0.624) [Table 3].
|Figure 3: The exposure-effect relationship for aircraft noise exposure and recognition memory; adjusted recognition score and 95% confidence interval bars for 5 dBLAeq 16 hour bands of aircraft noise adjusted for age, gender and country|
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|Table 3: Effect sizea for aircraft and road traffic noise exposure and recognition memory and prospective memory|
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Noise exposure and prospective memory
Chronic aircraft noise exposure and road traffic noise exposure were not significantly related to prospective memory: model fit for aircraft noise (χ2 = 2.35, df = 1, P = 0.125); model fit for road traffic noise (χ2 = 2.22, df = 1, P = 0.136) [Table 3].
| Discussion|| |
The pattern of results from this study examining the effects of aircraft and road traffic noise on children's episodic memory was mixed and unexpected. They can be summarized as follows. Firstly, after adjustment for confounding factors, the pooled analysis revealed a linear exposure-effect relationship between exposure to aircraft noise and impaired recognition memory in children. In the same analysis, after adjustment for confounding factors, exposure to road traffic noise was found to be associated in a linear exposure-effect relationship with enhanced performance on a test of cued recall, both when scored for correct information and for conceptual recall. Aircraft noise was not found to be associated with cued recall or prospective memory, while road traffic noise was not found to be associated with recognition memory or prospective memory. These effects could not be accounted for by socioeconomic factors or acute noise.
The linear exposure-effect association between aircraft noise and recognition memory was not predicted but is consistent with that found by Haines et al.  The fact that this result differs from a later Haines et al.'s study  is interesting in view of the fact that the later study used tests of cued recall and recognition memory, which were very similar to that of the present study. The explanation may be that the present study had a much larger sample size than that of Haines et al.'s study,  providing more power to detect small effects and allowing potential confounding factors to be dealt with in a more rigorous manner. Indeed in the present study more potential confounding factors were examined.
The absence of a significant association between aircraft noise and cued recall was not predicted. Although it is consistent with some studies, e.g. Haines et al., , it is not consistent with the findings of the Munich Airport study.  Although both the Munich and the present studies aimed to examine the effects of noise on episodic memory in children, the methodology of the two studies differed in several important respects. Whereas in the present study the retrieval phase of the episodic memory task was carried out in the children's classroom, and therefore potentially in noise, in the Munich study retrieval took place in conditions of controlled silence. Again, whereas in the present study the children's episodic memory was tested by means of a test of cued recall, in the Munich study the test was of free recall. Perhaps the most important difference between the two studies, however, is that whereas in the present study the children listened to recordings of stories, in the Munich study the children were required to read the stories themselves. This gives rise to the possibility that the test of memory is confounded by reading ability. This is of particular significance given the large body of evidence, including from the Munich study itself, of an association between noise exposure and reading.  It could therefore be the case that the poorer performance of noise-exposed children on the test of episodic memory in the Munich study may in fact reflect deficits not in memory but in reading. Given the lack of evidence for an effect of aircraft noise on cued recall from the present large-scale, rigorously controlled study, together with the evidence from a number of previous studies, the balance of evidence would seem to suggest that aircraft noise does not in fact impact adversely on cued recall.
The lack of a significant association between aircraft noise and prospective memory is not consistent with the predicted effect or with the results of the only previous study to examine this cognitive function in relation to noise exposure.  This is a difficult type of memory to assess either experimentally or in a field study. The very nature of prospective memory demands that it cannot be tested over many trials. The challenge with this type of memory is to remember to do something which is not done habitually, or as a matter of routine. The more often a subject is tested, the more they are "primed" to carry out this task. However, from the point of view of assessing the children's performance on prospective memory, the fact that there were only two trials means that this is a relatively weak test. Further examination of the results revealed that the children tended either to respond entirely accurately to the task both in terms of what and where they wrote, or entirely inaccurately. There was also a high correlation between performance on the trials; if the child completed the first trial correctly they also completed the second correctly (r = 0.89, P = 0.001). For this reason, performance was recorded as a binomial variable, either 1 if the child responded exactly as instructed or 0 if any mistake was made. Further research is required in order to investigate this area more thoroughly.
The finding that exposure to road traffic noise was associated in a linear exposure-effect relationship with enhanced performance on the test of cued recall, measured both in terms of correct information and conceptual recall, was not predicted. In fact it was predicted that road traffic noise would have the opposite effect, i.e., that it would be associated with an impairment of performance on this test. These findings compare with those of Dockrell and Shield,  who found that children's performance on verbal reading tests was enhanced in conditions of exposure to environmental and babble noise, compared with a baseline condition, where children worked quietly. However, the memory tasks in the current study were non-verbal, and Dockrell and Shield demonstrated enhanced performance with environmental noise and babble, only for verbal and not non-verbal tasks. The explanation for these findings is not clear. It is unlikely to be attributable to increased arousal, which might in any case have a detrimental effect on performance on a difficult task of this type. As no ceiling effects were found for performance of the task in our data, it is clear that this is a difficult type of task. One possibility is that it can be explained in terms of the context-dependent memory hypothesis. ,, In general terms, this claims that memory retrieval may be enhanced if the environment in which it takes place is similar to that of the encoding environment. Thus, noise may actually improve memory if a similar noise is present both at encoding and at retrieval. In the present study, children exposed to road traffic noise at encoding were also very likely to have been exposed to road traffic noise at retrieval. This noise might therefore have acted as an aid in the cued recall test. That this enhanced performance should be associated with road traffic and not aircraft noise may be due to the fact that road traffic noise is more constant, whereas aircraft noise is intermittent. For this reason, there were more likely to be differences in the noise experienced at encoding and retrieval for those children exposed to aircraft noise. Under these circumstances, any advantage due to context-dependent memory would be reduced. Evidence for the context-dependent memory hypothesis is mixed, with some studies finding support , and others not.  This area clearly requires further investigation. A further explanation is that these could be chance findings, associated with the complexity of estimating road traffic noise exposure; whilst acousticians estimated the levels, there is potential for traffic flow to have been underestimated and exposure misclassification may also have occurred as classrooms were at varying distances from the faηade of the school building.
The absence of an association between road traffic noise and recognition memory was as predicted. The fact that this differs from the finding for aircraft noise suggests that the impact of noise on recognition memory does depend upon the type of noise.
The absence of an association between road traffic noise and prospective memory was not predicted. As with the absence of an association between aircraft noise and prospective memory, this finding may be attributed to the difficulty of measuring this type of memory, and the precise nature of the test used in this study.
Clearly, children do not spend all of their time within the school environment, and consequently, whether they are exposed to noise at home, may be an important determinant of whether they exhibit cognitive impairments. Home aircraft noise exposure was not included in these analyses, as it was found to be very highly correlated with aircraft noise exposure at school.  Although the correlation between school and home road traffic noise was much weaker, there is evidence that exposure to nighttime road traffic noise can adversely affect children's sleep quality and is associated with daytime sleepiness.  Unfortunately, data pertaining to road traffic noise exposure at the children's homes were not available in the UK or Spain, so we could not examine this issue further.
The study reported in this article forms part of the largest study to date to examine is mis-spelt the effects of noise on children's cognitive functioning. This study has addressed limitations of earlier studies by examining exposure-effect relationships between noise and performance, and by carrying out parallel studies in three countries, thereby testing whether the findings generalize between countries. The large sample size together with the use of multilevel modeling has allowed the role of potential confounding factors, for the most part socioeconomic factors, to be dealt with in a rigorous way. The study has produced evidence for a negative association between aircraft noise and recognition memory as well as for a positive association between road traffic noise and cued recall. No association was found between aircraft noise and either cued recall or prospective memory, nor was any association found between road traffic noise and either recognition memory or prospective memory. Taken together, these findings indicate that exposure to aircraft noise and road traffic noise can impact on certain specific aspects of children's episodic memory. Our interpretation of the mixed and unexpected results highlights that more theoretical and empirical research is needed before we have a complete understanding of the impacts of noise on children's episodic memory.
| Acknowledgments|| |
The RANCH project was funded by the European Community QLRT-2000-00197. In the UK, co-funding was provided by the Department of Environment, Food and Rural Affairs. In the Netherlands, co-funding was provided by the Dutch Ministry of Public Health, Welfare and Sports, Dutch Ministry of Spatial Planning, Housing and Environment, and the Dutch Ministry of Transport, Public Works and Water Management.
Other members of the RANCH team were Eldar Aarsten, Tamuno Alfred, Rebecca Asker, Φsten Axelsson, Birgitta Berglund, Bernard Berry, Sarah Brentnall, Rachel Cameron, Hugh Davies, Paul Fischer, Anita Gidlφf Gunnarsson, Emina Hadzibajramovic, Jenny Head, Maria Holmes, Mats E Nilsson, Evy Φhrstrφm, Britth Sandin, Rebecca Stellato, Helena Svensson, Irene van Kamp.
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Centre for Psychiatry, Wolfson Institute of Preventive Medicine, Barts and the London School of Medicine and Dentistry, Queen Mary University of London, Old Anatomy Building, Charterhouse Square, London EC1M 6BQ
Source of Support: European Community QLRT-2000-00197, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3]
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