Executive functioning

1.5.1.1. Executive functioning definition

Executive functioning refers to goal-directed (Anderson, 1998), organisational/control-based (e.g. Ardila, Pineda & Rosselli, 2000; Carlson, 2005) and regulatory cognitive processes.

Executive functioning includes many cognitive abilities: the withholding of pre-potent

behavioural responses (inhibition), recall of information and concurrent processing (executive-loaded WM), the monitoring and updating of WM representations in response to constantly changing stimuli (updating), the generation of novel verbal or non-verbal examples

(verbal/design fluency), attending to specific stimuli while ignoring distracters (selective attention), moving between representational sets and/or task goals (switching/set-shifting) and planning (see Miyake et al., 2000; Pennington & Ozonoff, 1996; Singer & Bashir, 1999).

Furthermore, Hayes, Gifford and Ruckstuhl (1996) suggested that EF abilities are important for novel task performance (tasks to which there are no learned automatic response patterns).

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Executive function (EF) has been associated with frontal cortical brain regions (e.g. Welsh, Pennington & Groisser, 1991).

1.5.1.2. Frontal lobes / executive functions and prospective memory

The frontal lobes have been suggested to support the self-initiated processing that is required for PM tasks (McFarland and Glisky, 2009). For example, forming a strong association between a PM cue and the intention, retaining the intention during the time interval between intention formation and its execution, dividing attention over ongoing and PM task demands, monitoring for cues, and interrupting and inhibiting ongoing activities have all been found to be impaired in frontal lobe patients (Fuster, 1997; Stuss & Benson, 1984). In addition, prospective memory tasks require participants to switch between the ongoing task and PM activity. Departing from the ongoing activity which is required in PM tasks has also been argued to require executive processes i.e. breaking out from an ongoing activity (Van den Berg, Aarts, Midden &

Verplanken, 2004). Moreover, McDaniel and Einstein (2000) stated that executive resources need to be allocated to a PM task in order to bring the intended action to mind periodically.

Studies employing electrophysiological and functional imaging measures have shown that PM task performance involves frontal lobe activation. For instance, Burgess et al. (2001)

administered four event-based PM tasks to eight healthy adult participants in order to

investigate the involvement and roles of brain structures in PM tasks. The four tasks employed a variety of ongoing and PM tasks requiring decisions to be made on the basis of shapes, colours, semantic categories and numbers of letters. Each of these tasks was administered under three conditions. The first condition was a baseline condition where no PM trials were included. In the second condition called the “expectation” condition, participants were told that there would be PM trials but no PM trials were actually shown to participants. In the third condition

participants were told that PM trials might occur, and they did. This condition involved the actual execution of the PM tasks. A positron emission tomography (PET) and MRI scanning methods were used in order to investigate regional cerebral blood flow (rCBF) in order to distinguish between brain activity related to maintaining a PM intention and the manifestation or realisation of it. The results from the comparison of the baseline condition and two other conditions where PM trials were expected, showed an increase in the rCBF in the frontal pole bilaterally (especially Brodmann’s area; BA 10), right lateral prefrontal cortex (PFC), the right parietal lobe, and the precuneus bilaterally. These increases in rCBF in these areas were argued by Burgess at al. (2001) to be related to the maintenance of the prospective intention in ones

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mind. The authors of this study argued that it is the process of monitoring which supports the retrieval of PM that is dependent on the PFC.

Burgess, Veitch, de Lacy Costello and Shallice (2000) investigated multitasking abilities which included RM, PM and planning in a sample of 60 patients with focal cerebral lesions. On the basis of the results the authors argued that PM and planning rely strongly on processes supported by left Brodmann’s areas 8, 9 and 10 and the right dorsal prefrontal cortex. Rule-breaking and task-switching was found to be related to medial and more polar parts of Brodmann’s areas 8, 9 and especially 10.

A study conducted by Okuda (2007) used PET to investigate activation of the brain regions while performing time-based and event-based PM activities. The rostral prefrontal region of the brain was shown to be activated while participants were performing prospective memory tasks.

More specifically, the medial frontal lobe among two other regions of the rostral prefrontal area (the right superior frontal gyrus and anterior cingulate gyrus) has been found to be more active while participants were performing time-based PM tasks compared to event-based PM tasks.

Okuda (2007) concluded that these findings suggest that there are different processing demands produced by time-based and event-based PM tasks. This is in line with Einstein and McDaniel (2007) who stated that time-based tasks encourage monitoring processes, as there are no PM cues and one needs to self-initiate checking of time. This was compared to event-based PM tasks where spontaneous retrieval processes are more likely to be employed, as there are PM cues embedded in the PM task.

Little is known about the cognitive functions of the rostral prefrontal region also known as Area 10 of the brain (Burgess, Gilbert & Dumontheil, 2007; Burgess, Gilbert, Dumontheil & Simons, 2001). This large area of the brain is known to be involved in cognitive processes supporting attending self-generated/-maintained thought (independent attending) or stimulus-orientated attending (Burgess at al., 2001; 2007). Both of these cognitive processes are very important and relevant to PM memory as stimulus-independent attending is crucial for time-based PM whereas the stimulus-orientated attending for event-time-based PM. Namely, time-time-based PM tasks rely more on self- initiation and thus are more stimulus-independent whereas the event-based PM relies more on environmental cues and thus is more relevant to stimulus-orientated attending.

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A number of behavioural measures also revealed relationships between frontal functioning and PM. McDaniel, Glisky, Rubin, Guynn and Routhieaux (1999) investigated the relationship between PM and frontal functioning. They administered an event-based PM task to two groups of older adults. One group was characterised by high frontal function and the other by low on the basis of a battery of neuropsychological tests (Glisky, Polster & Routhieaux, 1995). The PM task in this study involved responding to the word president by pressing a specific key on a keyboard. The ongoing task involved a general knowledge test. Participants with high frontal functioning were found to be significantly better than participants with low frontal functioning on the event-based PM task. The authors argued that this provided evidence for frontal lobes involvement in event-based PM.

Martin, Kliegel and McDaniel (2003) investigated the involvement of processes which are mediated by prefrontal executive systems in different PM tasks varying in the amount of involvement of executive functions during intention formation and intention execution stages.

They argued for the possibility that the amount of EF required for a PM task is dependent on the extent to which the PM task focuses on the intention formation and/or intention execution stages, as opposed to the intention retention. Namely, the more relative weight is directed towards intention formation and intention execution stages, the greater the involvement of executive processes. Unlike event-based PM tasks (which are believed to involve relatively few strategic retrieval processes), time-based PM tasks have been hypothesised to involve greater amounts of EF. Since these tasks involve self-initiated monitoring in the intention execution stage and monitoring has been reported to be controlled by prefrontal function (Shallice &

Burgess, 1991, Shimamura, Janowsky & Squire, 1991).

Martin et al. (2003) used a standard clinical PM measure of event-based PM named the

Rivermead Behavioural Memory Test (RBMT; Wilson, Cockburn & Baddeley, 1985), in which participants needed to remember to request a return of an item at the end of the testing session.

This task was not expected to involve any strategic executive control processes in the intention execution phase, as it only involved a single event-based task which focussed on the retention and reinstantiation stages in a less demanding setting. Researchers also used standard laboratory event- and time-based PM measures (e.g. Einstein et al., 1997; Kliegel et al., 2001), as they would likely involve moderate involvement of EF, as these paradigms focus on the intention execution in a demanding task setting. The last PM task was a complex PM task developed on the basis of a task used by Kliegel et al. (2000) and focused on intention formation and intention execution stages in a demanding task setting. This multitask PM paradigm (MTPM) involved a

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mixture of six time- and event-based PM activities, which required planning during the intention formation phase and was argued by the researchers to rely on the greatest amount of prefrontal functioning.

Three standard neuropsychological tests were used to measure EF, these were the Wisconsin Card Sorting Test (WCST; Heaton, Chelune, Talley, Kay & Curtiss, 1993), a colour-word version of the Stroop task (Stroop, 1935), and the Tower of London (TOL; Morris, Evenden, Shakian & Robbins, 1987). These EF measures allowed the researchers to measure specific aspects of frontal functioning which are theoretically related to PM performance e.g. planning, inhibition, cognitive and response flexibility, and monitoring. The results from the principal components analysis with z-scores from all EF tests showed that there was a single executive factor which represented common variance of the three tests.

Martin et al.’s (2003) first goal was to investigate further the extent of the involvement of prefrontal executive processes in the four standard PM tasks employed in this study. They were also interested in investigating whether age related differences in different PM tasks can be explained by individual differences in EF. They employed forty young and 40 old adults, as frontal functioning has been argued to decline with age (e.g. Schretlen, Pearlson, Anthony, Aylward, Augustine, Davis & Barta, 2000; Wecker, Kramer, Wisniewski, Delis & Kaplan, 2000). The results showed age related performance differences on the experimental time- and event-based PM tasks, and on the complex PM measure only. On the three frontal/EF measures, age related differences were evidenced, with age related differences increasing as the

involvement of frontal/executive functions in the four different PM tasks increased. Also a series of corrections employing the four PM measures and EF factor scores for young and old adults computed separately, indicated that the laboratory time- and event-based PM tasks as well as the MTPM task correlated significantly with EF in older adults, but not in younger adults.

Additional analysis involving a stepwise regression revealed that individual differences in EF explain a large amount of variance of PM performance on the majority of the PM tasks used by Martin et al. (2003), with the exception of the RBMT task. After addition of age into the regression, the researchers found that there was no significant increase in explained variance in performance in the laboratory event- and time-based PM tasks. Interestingly, in the MTPM task it was not only the EF measures that explained variance in PM, but also age and nonexecutive measures such as RM and health. These results showed no significant effects of group

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membership on any of the four PM tasks. Also post hoc tests showed no significant mean differences in the RBMT, but there were similar performances in the young adults and old adults with high EF compared to old participants with low EF in the event- and time-based experimental measures. There were also significant differences between all three groups in the MTPM task. The authors argued that the results demonstrated a clear relationship between the extent to which frontal/executive functions are involved in PM task performance, with EF predicting the PM performance on the experimental time- and event-based PM tasks. In the most complex measure of PM (MTPM), both EF and age were good predictors of PM performance. In general, it was argued that frontal/executive functions are related to PM performance in a range of PM paradigms.

McFarland and Glisky (2009) further supported the claim that the frontal lobe is involved in PM task performance. They used a laboratory time-based PM task on a group of 32 younger and 32 older adults. They divided their older participants ortoganally into four groups based on composite measures of frontal lobe and medial temporal lobe function. In addition, this study also investigated age effects with each of the four groups of older adults being compared to a control group of younger adults. The results showed that older participants who were in the high frontal lobe group performed significantly better on the time-based PM task compared to low frontal lobe group. It was also found that older adults from the high medial temporal lobe group were better than participants from the low medial temporal lobe group when it came to PM performance, but only if they were also high in frontal lobe functioning. In addition, frontal lobe functioning was found to predict the quality of plans which participants generated in order to assist PM performance, patterns of time monitoring and accuracy of time estimation. Medial temporal lobe was not found to predict any of these. Similarly to the PM performance results, the results from older adults with high frontal lobe function across all of these measures were similar to these of younger adults. The authors concluded that it is frontal functioning which determined PM performance rather than age alone.