Julie Gonneaud

Grégoria Kalpouzos

Laetitia Bon

Fausto Viader

Francis Eustache

Béatrice Desgranges
email: <desgranges-b@chu-caen.fr

Gr Goria Kalpouzos

B Atrice Desgranges

Distinct and shared cognitive functions

Keywords: Adolescent, Adult, Aged, Aged, 80 and over, Aging, psychology, Cognition, Executive Function, Female, Humans, Inhibition (Psychology), Male, Memory Disorders, psychology, Middle Aged, Psychomotor Performance, Time Factors binding, episodic memory, executive functions, normal aging, prospective memory '

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Introduction

elderly individuals. However, this hypothesis is not supported by , as these authors D Ydewalle, Luwel and Brunfaut s results (1999) observed greater accuracy in the time-based condition than in the event-based one for older subjects. They attributed this result to the decrease in processing speed in adulthood and suggested that older subjects are quite capable of performing TBPM tasks in a satisfactory manner, if the timing constraints of the task allow them to dedicate sufficient time to time monitoring after processing ongoing items.

Moreover, the assumption that TBPM undergoes greater deterioration than EBPM is a fragile one for two reasons. First, it is difficult to make strict comparisons of EBPM and TBPM, as the tasks that are used have different levels of complexity and vary from one study to the next (see , for a review). One way of overcoming this issue would therefore be to assess performance on comparable Henry et al., 2004 EBPM and TBPM tasks. Second, it is not obvious that TBPM tasks do indeed involve a greater number of self-initiated processes, as EBPM tasks may actually require the sustained allocation of resources in order to detect PM cues under monitoring conditions (as opposed to spontaneous retrieval, see ;

). According Einstein, McDaniel, Thomas, Mayfield, Shank, & Morrisette, 2005[START_REF] Mcdaniel | Strategic and automatic processes in prospective memory retrieval: A multiprocess framework[END_REF] to the Multiprocess Theory of PM (see , ), EBPM relies on either automatic or controlled processes, [START_REF] Mcdaniel | Strategic and automatic processes in prospective memory retrieval: A multiprocess framework[END_REF][START_REF] Sperling | Functional MRI studies of associative encoding in normal aging, mild cognitive impairment, and Alzheimer s disease[END_REF] depending on the characteristics of the prospective memory task (i.e. target cue distinctiveness, associative strength between the target cue and the intended action, level of emphasis given to the PM task, length of the PM retention interval), the nature and demands of the ongoing task (i.e. focal processing of the target, degree of engagement and demands of the ongoing task), and the characteristics of the individuals. More precisely, non-distinctive target cue, non-focal target cue, weak association between the target cue and the intended action, highly demanding and absorbing ongoing task would necessitate the involvement of strategic monitoring. By contrast, PM would be based on spontaneous retrieval when the task contains the opposite characteristics. Hence, PM performance may differ according to the amount of attentional resources that are required. investigated, in young [START_REF] Okuda | Differential involvement of regions of rostral prefrontal cortex (Brodmann area 10) in time and event-based prospective memory[END_REF] subjects, TBPM and EBPM in a positron emission tomography study. They found that different subregions of the rostral prefrontal cortex were involved in EBPM and TBPM, and reported additional activation of several frontal, parietal and temporal cortices, as well as the cerebellum in the TBPM task, suggesting the engagement of additional processes in TBPM. The identification of distinct neural substrates for the two tasks supports the idea that different cognitive processes are involved in EBPM and TBPM.

In previous studies, the cognitive load induced throughout the ongoing activity has been manipulated in order to influence the availability of attentional resources in PM. While some studies have reported a deleterious effect of cognitive load on PM performance in young adults ( ; ; ; but see also M ntyl , Del Missier, & Nilsson, 2009 the target time has been found to have a beneficial effect (

). Moreover, some authors have suggested that Kvavilashvili & Fisher, 2007 reduced time-monitoring in older adults could be responsible for TBPM impairment ( ; ). Einstein et al., 1995[START_REF] Park | Effect of age on event-based and time-based prospective memory[END_REF] Other studies have attempted to link PM performance to metamemory, but results are far from clear-cut. While some of them point to a significant relationship between PM and reports of prospective and retrospective memory failures in daily life ( ; Kliegel & J ger, 2007 ä Mä ), other reveal no such links (

). Using the Prospective and Retrospective Memory Questionnaire (PRMQ; ntyl , 2003 ä [START_REF] Zeintl | Prospective memory complaints can be predicted by prospective memory performance in older adults[END_REF] ; see Method for its description), some [START_REF] Crawford | The Prospective and Retrospective Memory Questionnaire (PRMQ): Normative data and latent structure in a large non-clinical sample[END_REF][START_REF] Smith | Prospective and retrospective memory in normal ageing and dementia: A questionnaire study[END_REF] studies pointed to a significant relationship between PM and reports of prospective and retrospective memory failures in daily life (Kliegel ;

), while others did not reveal such a link ( ).

did not find & J ger, 2007 ä M ntyl , 2003 ä ä Zeintl et al., 2006 Meeks, Hicks and[START_REF] Meeks | Metacognitive awareness of event-based prospective memory[END_REF] any significant link between PM and the PRMQ, but they showed a significant correlation between PM and task-related prediction and postdiction scores as well as between those task-related metamemory indices and the PRMQ. Thus, both self-knowledge of memory from daily-life and memory task self evaluation are indices of metamemory but may be differentially implicated in PM outcomes. Meeks et al. suggested that individual memory self efficacy beliefs may modulate the way in which an individual approaches a task and, by (2007) extension, the way he or she performs it. In the present study, we chose to use the PRMQ as an index of metamemory.

Finally, some authors have suggested that PM performance depends on the quality of the association between the PM cue and the action that has to be carried out ( ; ). According to the theory of [START_REF] Cohen | Modulation of the prospective and retrospective components of memory for intentions in younger and older adults[END_REF]Guynn & McDaniel, 2007 reflexive-associative processes ( ; ), in EBPM tasks, when the [START_REF] Mcdaniel | Prospective memory: An overveiw and synthesis of an emerging field[END_REF][START_REF] Mcdaniel | Cue-focused and reflexive-associative processes in prospective memory retrieval[END_REF] cue and the action are strongly associated, seeing the cue will reflexively bring the intended action to awareness. If the association is strong enough, the mere perception of the cue will rapidly and automatically trigger the retrieval of the associated action. Involvement of binding processes in TBPM tasks may be less critical than in EBPM tasks. As evoked earlier, TBPM mostly requires self-initiated processes such as strategic monitoring. However, binding could also contribute to TBPM performance since an association between the target time and the intention exists. Since a binding impairment in aging has already been demonstrated (Mitchell, Johnson, Raye, Mather, ), especially in episodic memory ( Hobbel, 2008[START_REF] Naveh-Benjamin | Adult age differences in memory performance: Tests of an associative deficit hypothesis[END_REF][START_REF] Overman | The associative deficit in older adult memory: Recognition of pairs is not improved by repetition[END_REF] can be hypothesized that age-related difficulties in PM are strongly linked to a binding impairment. However this hypothesis has never yet been tested. To our knowledge, only a very recent study (

; ; ), it & d Esposito, 2000 ' Kessels &
) have investigated PM and associative memory [START_REF] Wang | Effects of aging and prospective memory on recognition of item and associative information[END_REF] conjointly, but their study was focused on the latter and designed to evaluate the effect of divided attention (induced by a PM task) on it.

They found an age-related impairment both in associative memory and PM and a trend toward a correlation between PM and negative associative memory, probably reflecting the use of different individual strategies (e.g. focus on one task to the detriment of the other) rather than a true relationship between these two cognitive functions.

Previous studies have assessed only a limited number of cognitive functions, preventing any firm conclusion on the cognitive bases of PM decline in aging. Furthermore, the cognitive functions involved in PM may vary according to the characteristics of the task and the amount of cognitive resources required by the task. In order to unravel this issue, we assessed EBPM and TBPM, manipulating the cognitive load, in 72 young, middle-aged and older healthy participants. We also assessed a wide range of cognitive functions, including executive functions, processing speed, sustained attention, retrospective episodic memory, metamemory, and binding, in order to find out which processes contribute the most to the deleterious effects of aging on different measures of PM. First of all, in order to determine whether age, the nature of the PM task and the cognitive load of the ongoing task affected PM performance, we performed an analysis of variance. Second, we performed correlations between PM and all the other cognitive scores in order to determine which of them were linked to PM. Then, to find out exactly which cognitive functions predicted PM variability and contributed to PM impairment in aging, we performed stepwise regressions on the different measures of PM. Finally, mediation analyses were performed to identify the existence of variables that could be responsible for the age-related PM decline.

Method Participants

Twenty-nine young, 20 middle-aged and 23 older subjects were enrolled in this study. All were screened by means of a health questionnaire for any history of neurological or psychiatric conditions, head trauma and alcohol or drug abuse. The adults over 50 years old were screened for cognitive impairment using the Dementia Rating Scale (Mattis, 1976). All the subjects included in the present study had normal performance for their age and education. All subjects had had at least eight years of schooling. The mean number of years of schooling was higher for young than for middle-aged and older groups ( < 0.05), while no difference was found between the ps middle-aged and older groups ( 0.94) ( ). Given the reorganization of the French educational system, the number of school years p = Table 1 is not a good index of the education level when considered alone. Thus, the mean number of school years was combined with the vocabulary level, assessed by the Mill Hill test ( ) to obtain a more accurate index (by calculating z-scores and averaging Deltour, 1993 them). While the older group had a vocabulary level higher than the younger one ( < 0.01; non significant for the other comparisons), p ps there was no difference between the three groups for the composite index of the education level.

Procedure

The experimental protocol featured two sessions, separated by a one-week interval. The protocol included laboratory EBPM and TBPM tasks, a prospective and retrospective everyday memory failures self-report questionnaire allowing us to assess metamemory, and a set of complementary cognitive tasks. The EBPM task took place during the first session and the TBPM task during the second session, in order to avoid any time estimation occurring during the EBPM task (see below).

Prospective memory tasks

The tasks were inspired by the one devised by . The design is illustrated in . In both event-based and D Ydewalle et al. (2001) ' Figure 1 time-based PM conditions, mental arithmetic constituted the ongoing activity. Two levels of difficulty characterized the cognitive load factor: during the first 12 minutes, the subjects had to add up three 2-digit numbers (high-load condition), and during the following 12 minutes they had to add up two 2-digit numbers (low-load condition). The numbers were displayed one at a time on the computer screen, and when the equals sign appeared, the subjects had to mentally add them up and, if successful, say the result out loud. After the ' ' presentation of the sign, which duration varied from one to the next, an answer appeared on the screen for a duration of five seconds equal and the subject had to say whether or not it was the correct one. Half the answers proposed by the computer were correct and half were incorrect. Calculation times had previously been determined on the basis of a pilot study with a different group of young and older subjects: we selected calculations for which there was no age-related difference in resolution time, and the presentation time for each of these calculations corresponded to the subjects mean resolution time in the pilot study.

'

In the EBPM task, subjects had to press the D key when the answer displayed on the screen was over 100 and the L key when the number was made up of two identical digits (e.g., 77). In the high-load condition, four prospective cues appeared in the same order every three minutes (the subjects were not informed about the three-minute interval): D, L, D, L. The low-load condition was similarly constructed.

In the TBPM task, instructions were to press alternatively the D and L buttons every three minutes, regardless of what was happening " on the screen, starting with D at the third minute ; the experimenter also stressed not to respond before the target number of minutes. A full " point was provided if the subjects answered within 5 seconds following the targeted minute, and half a point, if the answer was comprised between 6 and 10 seconds after the targeted minute. In order to check the time, a clock displaying minutes and seconds was positioned to the right of the subject, at an angle of 90 relative to his or her gaze toward the screen. Consequently, the subject had to turn his or her °head round to check the time. Frequency of time monitoring was recorded by the experimenter, constituting an additional and specific measure of TBPM processing.

Altogether, there were 16 PM trials, 8 for the EBPM task (four in each load condition), and 8 for the TBPM task (again, four in each load condition). The dependent variable was the number of correct answers given during the PM trials.

At the end of each PM task, participants were asked to recall the instructions (what they had to do under which conditions). No subject failed to remember any component of the PM tasks.

Complementary cognitive tasks

In the light of the literature and in order to test our hypotheses, we measured several cognitive functions thought to play a role in PM.

On the basis of Miyake et al. s findings (

), we assessed the following three executive functions: shifting, inhibition, and updating. We ' 2000 also measured binding, which refers to the mechanism supported by the episodic buffer component of working memory and defined as the system allowing for the integration and maintenance of different items of information in an episodic representation ( ), [START_REF] Baddeley | The episodic buffer: A new component of working memory?[END_REF] sustained attention, processing speed, retrospective episodic memory and metamemory.

Metamemory

To evaluate memory self-efficacy beliefs, subjects were asked to complete the Prospective and Retrospective Memory Questionnaire (PRMQ; ;

). The PMRQ is composed of 16 Crawford, Smith, Maylor, Della Sala, & Logie, 2003[START_REF] Smith | Prospective and retrospective memory in normal ageing and dementia: A questionnaire study[END_REF] questions about retrospective and prospective memory failures in everyday life. Subjects had to rate the frequency of occurrence of their memory failures on a five-point Likert scale (from 1 never, to 5 very often). This questionnaire was administered prior to the other = = cognitive tasks.

Shifting

Task-set switching, was used to test the ability to shift between two different task sets (for full description, see ). [START_REF] Mayr | Task-set switching and long-term memory retrieval[END_REF] This task consisted in an alternate-runs paradigm. Within each block, participants had to perform alternating four-trial runs of two tasks (aaaabbbb, etc.), so that a shift occurred every four trials. Sixteen words were used, repeated eighteen times. Each item was accompanied by a sign that changed every four trials: when the item was accompanied by a heart, subjects had to make a living/non-living judgment on each word; when the item was accompanied by a cross, subjects had to decide whether it represented something larger or smaller than a soccer ball. To answer living or larger subjects had to press the D key, while answer non-living or smaller required the subjects to

" " " " " " " "
press the L key. The dependent variable for this task was the switch cost, that is, the difference in response times between the situation where there was a shift and the situation where there was none.

Inhibition

Inhibition was assessed using the random letter generation task ( ). Participants were asked to say consonants out loud [START_REF] Baddeley | The capacity for generating information by randomization[END_REF] in a random sequence, attempting to produce a set of 50 letters at the rate of one letter per second. They were told to avoid repeating the same letter sequence and avoid producing alphabetical sequences (forward and backward), acronyms and parts of words. The score was the number of letters produced minus errors (vowels and prohibited sequences).

Updating

The running span task (

) was used to assess updating. Sixteen Quinette, Guillery, Desgranges, de la Sayette, Viader, & Eustache, 2003 strings of consonants of variable length (4, 6, 8 or 10 letters) were provided orally, with no prior information about the length of the consonant string, and subjects were required to recall the four most recent items in the right order.

Binding in working memory

The binding task measured the ability to associate verbal and spatial features in working memory (Quinette, Guillery-Girard, No l, de ë ). Four colored uppercase letters were displayed in the center of a 5 4 grid, accompanied by la Sayette, Viader, [START_REF] Quinette | The relationship between working memory and episodic memory disorders in transient global amnesia[END_REF] × four colored crosses displayed randomly in the other squares of the grid. Subjects had to mentally associate each of the four colored letters with the location of the cross of the same color. After a five-second presentation phase, a black fixation cross appeared at the center of the blank screen. The grid then appeared again, but this time with only one black lowercase letter in one of the squares. Subjects had to decide if the letter was in the correct square in relation to the letter-location match they had performed mentally. A total of 20 trials were administered.

Attention and processing speed

The BAMS-T (

) was used to obtain two scores, one for sustained attention, the other for processing speed. The BAMS-T is Lahy, 1978 a crossing-out task, which involves a single page filled with rows of eight different symbols. Subjects were instructed to cross out every instance of the three target symbols, which are indicated at the top of the page, within the space of five minutes. Two scores are calculated: the speed and the accuracy of the perceptual processing. The speed score was calculated as follows: (total number of crossings-out)/(time in seconds). The accuracy score was calculated as follows: (number of correct crossings-out/number of omissions)/(number of correct crossings-out/number of errors).

Retrospective episodic memory

Lastly, we tested retrospective episodic memory using the Encoding, Storage, Retrieval task (ESR), described elsewhere (Kalpouzos, ; see also ), where the subjects had to Ch telat, Landeau, Clochon, Viader, Eustache et al., 2009 é Eustache, Desgranges, & Lalev e, 1998 é memorize two lists of 16 words and immediately recall them. For the first list, subjects were asked to say, for each word, if the first and last letters were in alphabetic order, which constituted shallow incidental encoding. The encoding of the second list took the form of deep intentional encoding: subjects were asked to memorize the list and generate a sentence containing each word. The score retained for analysis corresponded to the sum of words correctly recalled in both lists.

Statistical Analyses

First of all, in order to determine whether age, the nature of the PM task, and the cognitive load of the ongoing task affected PM performance, we performed a Group (young vs. middle-aged vs. older) Nature of PM (event-based vs. time-based) Cognitive load were performed on the complementary cognitive tasks to explore their sensitivity to age (young vs. middle-aged vs. older). Third, Pearson correlations between PM measures and the scores obtained on the complementary cognitive tasks were assessed for the whole sample in order to search for links between PM and cognitive functions. The same correlations were then assessed again, controlling for age (partial correlations), in order to find out whether they were mainly due to age or not. For both series of correlations, the Benjamini and Hochberg procedure was used to control the false discovery rate. This method consists in correcting the -values according to the total number of p tests performed (here correlations) and the -value ranks (see

F = MSE = p = F = MSE = p = '
performed in order to test our hypothesis of a differential effect of Cognitive load on EBPM and TBPM, we found that EBPM performance in the high-load condition was significantly lower than the scores in the other three conditions ( < 0.001 for all comparisons). In other p words, EBPM performance was sensitive to cognitive load, whereas TBPM performance was not.

= MSE = p F = MSE = p
analysis of the interaction using Tukey s HSD demonstrated that the Nature of memory failures effect was due solely to the subjects in the ' young group, who reported more prospective than retrospective memory failures ( 0.004; 0.98 and 1 for the middle-aged and p = p = p = older groups).

Cognitive Correlates of PM

Since no significant interaction was found between Nature of PM and Cognitive load, correlations between the cognitive scores on the complementary tasks and the PM scores were performed separately, according to the nature of PM and according to the cognitive load.

This also allowed us to retain a higher variability in PM measures (on 8 points). The scores in the high-and low-load conditions were added up for EBPM and for TBPM, while the EBPM and TBPM scores were added up for the high-load and the low-load conditions. It should be noted that in order to obtain a single measure reflecting the increase in time monitoring revealed by the ANOVA (see above), we subtracted the averaged number of fixations on the clock during the first and the second minutes from the averaged number of fixations during the third minute for each subject.

Pearson correlations between PM performance and the other cognitive functions for the whole sample are reported in . All Table 3 measures of PM were linked to inhibition, processing speed, and, except for TBPM, binding and retrospective episodic memory.

Additionally, shifting correlated marginally with EBPM, and updating was marginally linked to high-load condition performance. No significant correlation was found between TBPM and time monitoring. Finally, none of the measures of PM significantly correlated either with sustained attention or with PRMQ indices of prospective and retrospective memory failures. In sum, all measures of PM were linked to inhibition and processing speed, but we also found specific patterns of correlations according to the nature of PM. EBPM was additionally linked to binding, retrospective episodic memory and, marginally to shifting.

When we controlled for age, none of the above-mentioned correlations remained significant, except the relationship between EBPM and binding which became marginal after correction for multiple tests with the Benjamini and Hochberg procedure (r 0.36; 0.08),

= p =
suggesting that the links between PM and the other cognitive functions were due to age variability.

Stepwise Regressions

Twenty subjects were excluded from the stepwise regressions because of missing data for the retrospective episodic memory measure.

As a result, 21 subjects between 18 and 39 years, 12 subjects between 40 and 59 years and 19 subjects between 60 and 80 years were included in these analyses. To ensure that the exclusions did not affect our previous results and to be consistent across all the results, we performed ANOVAs on PM performance and complementary cognitive tasks without these 20 subjects. The analyses globally yielded the same pattern of results (see note below , and for further detail). Tables 2 3 Fig. 2 Forward stepwise regression analyses were performed for EBPM and TBPM, as well as for the high-load and low-load conditions (see ). Variance in EBPM was mainly accounted for by binding and retrospective episodic memory. When age was forced into the Table 4 model, we found that age explained a significant proportion of variance. Binding was included in the second step and retrospective episodic memory in the third. Both accounted for a significant proportion of PM variance and their inclusion in the model reduced age to a nonsignificant role. The only regressor included in the model for TBPM was inhibition. As with EBPM, when age was forced into the model, we found that it made a significant contribution to PM variability. No other factor explained a significant proportion of variance.

Finally, forward stepwise regressions for the high-and low-load conditions demonstrated that the only variable that could significantly account for performance was age.

Mediation analyses

Stepwise regressions underlie variables that could be predictive of PM performance. To go beyond the above results, mediation analyses were performed on the 52 subjects to evaluate the possibility that different cognitive functions mediate the effect of age on PM.

Mediation effects were tested following the procedure proposed by ; see also ). To [START_REF] Baron | The moderator-mediator variable distinction in social psychological research: conceptual, strategic, and statistical considerations[END_REF]Kenny (1986 Frazier, Tix &Barron, 2004 conclude that a specific function mediates relation between age and PM there must be a a significant relation between age and PM, b a

{ } { }
significant relation between age and the mediator, c a significant relation between the mediator and PM when controlling for age, and

{ }
finally d the strength of the relationship between age and PM must be reduced when the mediator is added to the model. When the { } relationship between the predictor (e.g. age) and the dependant variable (e.g. PM) becomes non significant, the mediation is complete;

when the relationship between the predictor and the dependant variable decreases but remains significant, the mediation is partial (Brauer, ;

). Results are presented in . 2000 Frazier et al., 2004 Figure 4 Regressions were assessed on the 52 subjects to test criteria a , b and c as described above. Binding and retrospective memory

{ } { } { }
influences on age-EBPM relationship ( 0.65; < 0.001) were the only ones responding to criteria a , b and c . Multiple regressions

β = - p { } { } { }
performed to test condition d showed that binding and retrospective memory partially mediated the effect of age on EBPM ( 0.31; A mediation of the age-TBPM relationship by binding processes could be expected from stepwise regression results. All the criteria were not entirely fulfilled to conclude to a mediation. In fact, despite a significant link between age and TBPM ( 0.39; < 0.005) and β = -p between age and inhibition ( 0.53; < 0.001), the relationship between inhibition and TBPM controlling for age (criterion c ) was

{ } β = - p < 0.
β = - p { }
only marginal ( 0.27; 0.08). Nevertheless, the multiple regression revealed that the age-TBPM link, controlling for inhibition,

β = - p =
became not significant ( 0.25; 0.10).

β = - p =

Discussion

The present study sought to assess the impact of aging and its cognitive correlates on PM, investigating the entire adulthood. Our results showed that PM decline started in middle age and, in contrast to the literature, EBPM was more affected by age than TBPM.

Cognitive load also affected PM, but only in the older group and in the EBPM condition. We also identified distinct cognitive correlates of PM, according to the condition. While inhibition and processing speed were common to all PM conditions, their implication in each of the PM conditions differs and additional cognitive functions were specifically related to EBPM in particular.

Prospective Memory in Aging

Using EBPM and TBPM tasks that were designed to be as comparable as possible, we found that age had a more deleterious effect on EBPM than on TBPM. One important feature of the present study is that we investigated PM across the whole adult lifespan, from 18 to 84 years. While a decrease in PM from 40 50 years onwards had already been demonstrated ( ), very few studies

- M ntyl & Nilsson, 1997 ä ä
had investigated performance in middle age. Interestingly, EBPM was not only the condition that was the most impaired with aging, but was also the one that was impaired the earliest. This result contradicts previous studies that assumed that EBPM is less sensitive to the effects of aging than TBPM because the former requires fewer self-initiated processes, which are known to be impaired in aging (Craik, ). However, and as predicted by the Multiprocess Theory ( , ), EBPM does not rely solely on 1986 [START_REF] Mcdaniel | Strategic and automatic processes in prospective memory retrieval: A multiprocess framework[END_REF][START_REF] Sperling | Functional MRI studies of associative encoding in normal aging, mild cognitive impairment, and Alzheimer s disease[END_REF] successful automatic detection of PM cues, as it may also require controlled processes. In the present experiment, it can be assumed that EBPM did indeed require controlled processes, as i) the subjects were not informed of the frequency of appearance of the prospective cues (they were only told that they were few and far between), and ii) these cues were not as distinctive as they have been in previous studies ( ). Thus, the EBPM task we used must have required controlled processes in [START_REF] Park | Effect of age on event-based and time-based prospective memory[END_REF]Vogel, Dekker, Brouwer, & de Jong, 2002 order to continuously monitor the items displayed on the screen and detect the cues. Although the same time intervals were used in both the EBPM and TBPM tasks, the fact that subjects knew when a response had to be provided in the TBPM may have resulted in more efficient resource-sharing in the multitasking. As a result, the older subjects may have performed better on the TBPM task because they were able to organize their time. This hypothesis has been put forward in naturalistic studies where older individuals outperform their younger counterparts, probably because they organize their daily lives more efficiently (

). [START_REF] Rendell | Aging and prospective memory: Differences between naturalistic and laboratory tasks[END_REF] As indicated in the methodological section, TBPM was always performed after EBPM, after a one-week interval. As a result, we cannot rule out the hypothesis that subjects better performance on TBPM in comparison to EBPM could be linked to a training effect on ' the ongoing task, making the TBPM task easier to achieve, due to lower attentional demands allocated to the ongoing task. Indeed, analyses on the ongoing task revealed that performance was higher in the TBPM condition than in the EBPM condition, but the lack of significant interaction between Nature of PM (EBPM versus TBPM) and Group on the ongoing task indicates that the effect of practice on this task was the same for all subjects (results not shown). Moreover, we assume that the training effect between the two sessions would be limited thanks to the one-week interval.

PM performance in high-load and low-load conditions displayed the same pattern of an age-related decrease in middle age that continued at the same pace into old age. However, the cognitive load of the ongoing task only had a deleterious effect in the EBPM task and for the older group. The increase in task complexity forced subjects to engage more controlled processes in order to perform the ongoing activity, thus reducing the amount of available resources for the PM task. This result goes against the findings of Khan et al.

, who observed the opposite pattern, that is, a greater effect of cognitive load on TBPM than on EBPM. One explanation for this is (2008) that Khan et al. manipulated cognitive load in a different way, introducing a second ongoing task to increase load.

There was thus a differential effect of age on PM according to the condition. The correlation analyses showed that age-related PM decline stemmed from various cognitive impairments, and specified which of these were either systematically or specifically involved in this decline.

Systematic Cognitive Correlates of Age-related PM Decline

Inhibition and processing speed correlated with PM in all conditions, that is, EBPM, TBPM, and the low and high cognitive load conditions. As these cognitive functions are impaired in older people, their involvement may have mediated the age-related PM impairment. Successful multitasking relies on the ability to inhibit one task in order to perform another task. Our results consistently suggested that the older subjects had difficulty inhibiting the processing of the ongoing activity in order to perform the PM task (Kliegel et ;

). In line with the hypothesis of a general slowdown with age, contributing to a decline in accuracy in many al., 2003West & Craik, 2001 different cognitive functions ( ), processing speed also seems to mediate age-related PM decline ( Salthouse, 1996 Salthouse, Berish, & ). Our findings are consistent with previous results highlighting the implication of inhibitory and processing speed deficits Siedlecki, 2004 in PM decline with aging (

). Kliegel & J ger, 2006 ä While the mechanisms of inhibition and processing speed subserved PM decline in aging in all conditions, their respective contributions differed for each of them, and other cognitive functions were found to be specific to each condition.

Specific Cognitive Correlates of Age-related EBPM and TBPM Impairment

Taking into account all the analyses, it seems that our EBPM task required several controlled processes, in opposition to the TBPM one that involved a reduced range of cognitive functions. While, additionally to inhibition and processing speed, EBPM was linked to binding, retrospective memory and marginally to shifting, no supplementary correlation was found between TBPM and the other cognitive functions.

A novel contribution of the present study is that PM impairment, more specifically EBPM one, may also be due to a deficit in binding.

That is, PM impairment could result from a deficit in associating the action to be performed with the PM cue. Binding processing is mainly subserved by the frontal cortex ( ; ; ), and the [START_REF] Mitchell | fMRI evidence of age-related hippocampal dysfunction in feature binding in working memory[END_REF][START_REF] Prabhakaran | Integration of diverse information in working memory within the frontal lobe[END_REF][START_REF] Sperling | Functional MRI studies of associative encoding in normal aging, mild cognitive impairment, and Alzheimer s disease[END_REF] dysfunction of this brain area in aging (

) may contribute to an Kalpouzos, Ch telat, Baron, Landeau, Mevel, Godeau et al., 2009 é age-related binding deficit ( ; ). This is in line with the reflexive-associative theory of PM [START_REF] Mitchell | fMRI evidence of age-related hippocampal dysfunction in feature binding in working memory[END_REF][START_REF] Sperling | Functional MRI studies of associative encoding in normal aging, mild cognitive impairment, and Alzheimer s disease[END_REF] Guynn & ; ;

). found that intention retrieval is mediated [START_REF] Mcdaniel | Prospective memory: An overveiw and synthesis of an emerging field[END_REF][START_REF] Mcdaniel | Prospective memory: An overveiw and synthesis of an emerging field[END_REF][START_REF] Mcdaniel | Cue-focused and reflexive-associative processes in prospective memory retrieval[END_REF][START_REF] Mcdaniel | Cue-focused and reflexive-associative processes in prospective memory retrieval[END_REF] by a reflexive-associative process when the target cue and the action are closely associated. However, when they are not, retrieval of intentions seems to be more consistent with a discrepancy-plus-search model. As older subjects have greater difficulty binding several elements together in memory, they cannot fall back on a reflexive-associative process and are forced to allocate more resources to the task.

As shown by the regression analyses, binding accounted for the greatest proportion of variance in EBPM performance, whereas it was not predictive of TBPM performance. Here, the greater difficulty displayed by the older subjects in EBPM can be explained by the weak association between the target cue and the action to be performed.

As suggested above, more controlled processes may have been required in the EBPM task than in the TBPM task. The fact that additional cognitive functions were found to be related to EBPM scores reinforces this idea. After binding, retrospective episodic memory was the second factor that accounted for EBPM variability. Thus, besides the deleterious effect of age on both aspects of memory (prospective memory and retrospective episodic memory; shown through the analyses of variance), our results support the hypothesis of a deleterious effect of retrospective episodic memory decline on EBPM. Several studies have failed to find a significant relationship between PM and retrospective episodic memory (see, for example, ;

). Nevertheless, as PM contains Einstein & McDaniel, 1990[START_REF] Park | Effect of age on event-based and time-based prospective memory[END_REF] a retrospective component (the intention content, i.e., what the subject has to do once a cue is detected), searching for such link is not counterintuitive. In these studies, the retrospective component of PM was limited to a simple action, whereas in the present one, subjects had to remember two prospective actions and their associated targets. Although shifting also correlated with EBPM, it did so to a lesser degree than binding and retrospective episodic memory, as it did not account for a significant proportion of EBPM variance.

The major implication of binding and retrospective memory in age-related PM performance has been confirmed by the mediation analyses which showed that, beyond being predictive of EBPM performance, binding and retrospective memory age-related decline were responsible for age-related EBPM impairment. It is likely that the more the task involves binding and retrospective memory (e.g. task with low link between prospective memory cue and intention, and with several information to memorize), the more it will be sensitive to the age effects. We can hypothesize that the controlled processes, likely responsible for the greater impairment of EBPM than of TBPM in aging, correspond to these two age-related impaired cognitive functions.

In contrast with EBPM, which had specific correlations with supplementary cognitive functions, TBPM had not. Nevertheless, inhibition was the only process to account for a significant proportion of variance in TBPM performance, while it was not predictive of EBPM performance. Inhibition processes therefore seem to play a particular role in TBPM. The fact that TBPM variability could be explained either by inhibition or by age, depending on the model, suggests that these two factors shared a common variance and were jointly responsible for TBPM difficulties. However, the mediation role of inhibition on TBPM age-related decline has to be confirmed.

Regarding time monitoring, we found a classic J-shaped distribution in all groups (i.e., more frequent time checking as targeted minutes approached) which was greater in the low-load condition. This is consistent with Occhionero, Esposito, Cicogna, and Nigro s ' that time monitoring is an index of resource allocation in TBPM tasks, insofar as a time-monitoring decrease can be suggestion (2010) observed when more attentional resources have to be allocated to the ongoing activity. Moreover, when the ongoing task is sufficiently low-demanding to enable subjects to allocate resources to time monitoring, older subjects can compensate for their TBPM impairment by increasing their time-monitoring frequency ( ; see also ). In our study, the older subjects did M ntyl et al., 2009 ' not show any increase in comparison with the young subjects, resulting in reduced TBPM capabilities. However, and in contrast to previous studies, we failed to find a significant link between TBPM scores and time monitoring, which may be due to a lack of statistical power in our correlation analyses.

In sum, the present study clearly demonstrated that different cognitive processes are involved in PM, depending on the nature of the task, EBPM involving a larger range of cognitive functions than TBPM.

Specific Cognitive Correlates of Age-related PM Impairment for High and Low Cognitive Loads

The distinction between the cognitive correlates of low-load and high-load conditions in PM was not as demonstrative as it was for EBPM and TBPM. Indeed, the only distinction between these two conditions concerned updating, which has been shown to decline during aging and tended to correlate with the high load condition. As a result, the greater deficit in the high-load condition compared with the low-load one, for the older subjects, may have been linked to the implication of updating in the former. The absence of distinct cognitive correlates, with the exception of the updating factor, could be due to the fact that the cognitive correlates of EBPM and TBPM were shared in both conditions, thus preventing the emergence of specific correlates for the high-and low-load conditions. In future studies, the number of cues will have to be increased, in reasonable ratio, in order to assess the cognitive correlates of PM in the high-load and low-load conditions separately for EBPM and for TBPM. Moreover, it would be interesting to test whether PM performance is subserved by shared or distinct cognitive correlates when cognitive load is manipulated either via an increase in the complexity of the ongoing task itself or via the addition of a second ongoing task.

Prospective Memory and Metamemory in Aging

Reports of prospective and retrospective memory failures in daily life, assessed via the PRMQ ( ), proved not to be [START_REF] Smith | Prospective and retrospective memory in normal ageing and dementia: A questionnaire study[END_REF] sensitive to the effects of aging. However, further analyses demonstrated that younger subjects reported more prospective memory failures than retrospective memory failures, whereas middle-aged and older subjects reported prospective and retrospective memory failures to be equally frequent. More importantly, these reports were not linked to PM performance. This has previously been demonstrated by several authors ( ) who concluded that reports of difficulty in everyday remembering tasks were not predictive of their [START_REF] Rendell | Aging and prospective memory: Differences between naturalistic and laboratory tasks[END_REF]Henry et al., 2004 we can assume that PM performance in the laboratory is most closely linked to PM metamemory when the assessment refers to the experimental task itself ( ).

Meeks et al., 2007

Conclusion

By assessing different PM conditions and several cognitive functions in healthy individuals covering the entire adult lifespan, we were able to gain an integrative view of PM impairment in normal aging, specifying its cognitive correlates. This study showed that, with advancing age, PM decline is linked to the impairment of several cognitive functions, notably inhibition and processing speed, which are particularly vulnerable to the effects of age. Concerning the nature of the PM task, we found that EBPM was more affected than TBPM. This finding goes against several previous studies, which have suggested a more deleterious effect of age in TBPM tasks. Consistent with the Multiprocess Theory of PM, our findings support the hypothesis that EBPM relies on controlled processes, as well as automatic ones, resulting in the greater sensitivity of EBPM to the effects of aging. While inhibition and processing speed correlate with all PM measures, their respective contributions differed according to the conditions, and other cognitive functions were found to be specific to one particular condition. EBPM appears to be strongly dependent upon binding and retrospective episodic memory and, to a lesser extent, upon shifting, whereas TBPM relies mainly on inhibition. Further research is needed to clarify whether cognitive load influences the relations between distinct cognitive functions and age-related PM impairment or whether load influences the strength between those cognitive functions and PM decline.

05). p p

There was no age effect on TBPM. The effect of nature was only significant for the older group ( < 0.01). p When correlations were assessed on 52 subjects, correlations of processing speed became no significant with TBPM and low load condition (r 0.24, ns; and r 0.35, ns; respectively) and marginal with = = EBPM (r 0.41; p < 0.10). Moreover, correlation between TBPM and inhibition became marginal (r 0.40; p < 0.10) as well as the low load condition and retrospective memory one (r 0.41; p < 0.10).

= = =

We assumed that these weaker effects were due to the reduced number of subjects, resulting in less statistical power.

  . low) repeated-measures ANOVA. Regarding TBPM, we also analyzed time monitoring, using a Group (young vs. middle-aged vs. older) Cognitive load (high vs. low) Period (first minute vs. second minute vs. third minute) ANOVA. Second, one-way ANOVAs × ×

3

 3 Concerning time monitoring, a Group (young vs. middle-aged vs. older) TBPM Cognitive load (high vs. low) Period (first minute × × vs. second minute vs. third minute) ANOVA was performed. As subjects were supposed to press a key every third minute, an increase in time monitoring was expected in the run-up to every third minute. None of the effects involving the Group factor was significant.However, the main effects of Cognitive load, (1significantly increased between the first and third minutes. There was a greater increase in frequency in the low-load condition than in the high-load one. Tukey s HSD revealed that the difference between the high-load and low-load conditions for the first ' minute was the only one that did not reach significance ( 0.97; < 0.001 for other comparisons). ( about here) p = p FigureAge Effects on the Complementary Cognitive TasksResults on the complementary cognitive tasks are shown in . One-way ANOVAs demonstrated an age group effect on

Figure 1

 1 Figure 1 Design of the event-based (a) and time-based (b) prospective memory (PM) tasks (see Method).

Figure 2 p

 2 Figure 2 Interaction effects between age (young, middle-aged and older groups) and nature of PM (event-based and time-based conditions) and between age and cognitive load (high and low load conditions). The Y-axis displays the mean scores obtained by each group in the EBPM and TBPM conditions (on the left) and in the low and the high load PM conditions (on the right). t: trend; <.05; <.01. *p **p

Figure 3

 3 Figure 3Interaction effect for time monitoring between Load condition (high vs. low load) and period (first minute vs., second minute vs. third/critical minute). The Y-axis displays the time-checking frequency (number of times the subjects checked the clock, averaged over the three groups).

Figure 4

 4 Figure 4 Mediation analyses on the relationships between age and event-based prospective memory (a; b; c) and between age and time-based prospective memory (d). Numbers refer to betas of the regressions and numbers in italic in parentheses represent the betas of the regressions controlling for the mediator. t: trend; <.05; <.01; <.001. *p **p ***p

  For the PRMQ, a Group (young vs. middle-aged vs. older) Nature of memory failures (prospective vs. retrospective) ANOVA failed

											Table 2
	inhibition and updating, (2, 69) 9.23; F =	MSE =	29.13; < 0.001 and (2, 69) 6.26; p F =	MSE =	10.91; < 0.01, but not on shifting, (2, p F
	69) 1.53; =	MSE =	106559;	p =	0.22. Processing speed, (2, 69) 8.42; F =	MSE =	270.85; < 0.01, binding, (2, 69) 24.14; p F =	MSE =	0.02;	p
	< 0.001, and retrospective episodic memory, (2, 49) 15.99; F =	MSE =	9.29; < 0.001, were significantly impaired by aging, whereas p
	sustained attention was not, (2, 69) 1.25; F =	MSE =	0.02;	p =	0.29.
											×
	to reveal an effect of Group on memory self-efficacy beliefs, (2, 69) 0.84; F =	MSE =	26.04;	p =	0.44, but did reveal an effect of Nature of
	memory failures, (1, 69) 6.33; F	11.61; < 0.05, which interacted with age group, (2, 69) 3.47;	11.61; < 0.05. An

  absence of a correlation between PRMQ and PM performance could be explained by a lack of correspondence between the experimental task and the context of the prospective and retrospective memory failures. The fact that the young group was the only one to report prospective rather than retrospective memory failures is in line with findings of naturalistic studies, which have observed better PM

	Hertzog & Hultsch, 2000
	objective difficulties in laboratory tasks, notably for PM (	Zeintl et al., 2006	; but see also	Kliegel & J ger, 2006 ä	;	M ntyl , 2003 ä ä	). The
	performance for older subjects than for younger ones (		; see	, for a review). More specifically,

Table 1 Population

 1

	Young group	Middle-aged group	Older group

Table 2

 2 Complementary cognitive tasks scores for young, middle-aged and older groups.

	Young group	Middle-aged group	Older group

± b ′ c ′ ′

′ express a trend for the same contrasts ( < 0.07). ANOVAs on 52 subjects showed the same pattern of results, except that effect of age on updating was marginal, (1, 49) 2.85; p F

Table 3

 3 Correlations between complementary cognitive tasks and PM for the whole sample.

	EBPM	TBPM	High Load	Low Load

* p<0.05; ** p<0.01.

[START_REF] Occhionero | The effects of ongoing activity on time estimation in prospective remembering[END_REF]

[START_REF] Occhionero | The effects of ongoing activity on time estimation in prospective remembering[END_REF]

[START_REF] Occhionero | The effects of ongoing activity on time estimation in prospective remembering[END_REF]

[START_REF] Occhionero | The effects of ongoing activity on time estimation in prospective remembering[END_REF]

[START_REF] Occhionero | The effects of ongoing activity on time estimation in prospective remembering[END_REF]

[START_REF] Occhionero | The effects of ongoing activity on time estimation in prospective remembering[END_REF]

[START_REF] Occhionero | The effects of ongoing activity on time estimation in prospective remembering[END_REF]

[START_REF] Occhionero | The effects of ongoing activity on time estimation in prospective remembering[END_REF]

[START_REF] Occhionero | The effects of ongoing activity on time estimation in prospective remembering[END_REF]

Acknowledgements:

The authors are grateful to Mr. Julien Ansot for his help in this study. This research was funded by the G.I.S. Institut de la long vit et du é é vieillissement, Inserm.