Over the decades, empirical studies on Working Memory have revolutionised the way that we think about the human mind, by providing us with a greater understanding of the cognitive processes involved in executive functions such as attention, problem solving, reasoning and decision making.
Nevertheless, despite this significant progress, certain questions still remain unanswered as to how specific cognitive mechanisms are organized. Perhaps one of the most prominent models to have emerged from the study of executive functions and cognitive control in working memory is that which was proposed by Baddeley and Hitch (1974).They described a three component model of working memory in which executive functions operate under a unitary core structure called the Central Executive (CE). However, whilst the central executive is considered to be the most important component of working memory, due to the significant role it plays in manipulating and integrating visual and linguistic resources to distinct executive tasks, the notion that it forms a unitary structure still remains tentative.Instead, more recent studies have argued that the central executive may in fact be fractionated into further sub-components, thus suggesting that this theoretical construct is more diverse than previously thought.
This paper will therefore attempt to examine and evaluate some of the evidence put forth in recent findings, in an attempt to reconcile these opposing perspectives.Prior to the 1980’s Baddeley’s model of working memory appeared to depict the central executive as a solitary isolated unit which controls and regulates the other sub-systems or ‘slaves’ of working memory, namely the phonological loop and the visuo-spatial sketchpad. However, there remained much ambiguity surrounding the exact role of the central executive, because Baddeley failed to provide a definitive explanation as to how the central executive actually controls and regulates such cognitive mechanisms.In fact, Parkin (1998) argues, that the central executive is nothing more than an inscrutable concept which is frequently used to explain executive task performance that cannot be explained by the other ‘slave’ systems, and suggests that more rigorous empirical testing is required before it can be considered more than a homunculus that deals with unanswered questions. In acknowledging some of the shortcomings of their previous model, further analysis by Baddeley et al. (1986) led to the conclusion that the central executive operated in a similar fashion to Norman and Shallice’s (1986) supervisory activating system, consequently rejecting any notion that it was involved in the storage of information.
Instead, they posited that the central executive was like an ‘attention controller’, which seeks to resolve conflicts by selecting, initiating and terminating cognitive processes triggered by schemata, and as such, it plays a vital role in executive functions such as, task co-ordination, decision making and planning.In recent years, attempts to study patients with frontal lobe damage have continued to provide researchers with numerous insights into the functions and diversity of the central executive system. According to Baddeley (1996) cited in Eysenck and Keane (2010, p. 218) patients with pre-frontal damage, often experience substantial impairments to central executive functioning, such as the ability to plan, organise, monitor and initiate behaviour. The term he used to describe such impairments was called Dysexecutive Syndrome (also referred to as Frontal Lobe Syndrome).
One tool which is commonly used to assess patients with brain damage is the Behavioural Assessment of the Dysexecutive Syndrome (BADS). The BADS test forms a battery of six sub-tests and a 20 item questionnaire, each of which is aimed at tapping into and assessing specific executive functions, such as inhibition, problem solving, planning, temporal judgement, organization and self-monitoring. However, as Baddeley (1998a) points out, whilst the frontal lobes are often implicated in deficits of executive functioning, this is not absolute.Undoubtedly, neuropsychological studies using repetitive transcranial magnetic stimulation (rTMS) do provide evidence to suggest that the pre-frontal cortex plays a dominant role in executive functioning (Eysenck & Keane, 2010, p. 217), however, upon analysing performance on the Winsconsin Card Sorting test (WCST), (a test which is commonly used to measure function of the pre-frontal lobes), Anderson, Damasio, Jones and Tranel (1991) found that executive deficits were also evident in individuals, even in the absence of prefrontal cortex damage, thus suggesting several anatomical locations may be implicated in executive functions.
With further progress being made in an attempt to understand the complexities of executive functions, Baddeley et al. , (1996) extended further support to the idea of fractionating the central executive into individual subcomponents, by identifying four separate functions of the central executive; selective attention; divided attention (timesharing) in dual tasks; switching of attention and activation of long term memories.According to Baddeley, each of these executive functions plays an important role in guiding our cognitive processes and activities (Eysenck & Keane, 2010, p. 218), and evidence to support the distinction between separate functions can be seen in a variety of experimental studies involving random generation and dual coordination tasks (Baddeley, 1996a; Baddeley, Della, Papagno & Spinnler, 1997; Baddeley, Emslie, Kolodny & Duncan 1998).In particular, comparative studies between patients with Alzheimer’s and healthy adults, have accrued strong evidence for a separate function of divided attention, with numerous studies showing that Alzheimer’s patients have greater difficulty conducting two tasks simultaneously (such as keeping track of several conversations at the same time or dual tasking with visuospatial tracking and digit sequence recall) than when presented with the same tasks separately.
Coinciding with these findings, a study of individual differences was conducted by Miyake et al. (2000) which employed a latent variable analysis in an attempt to identify common underlying factors in executive processes. By administering a variety of tests including the Wisconsin Card Sorting test (WCST), Tower of Hanoi (TOH), Random Number Generation (RNG), and dual-tasking (e. g.
, operation span), they were also able to identify three moderately related but dissociable executive functions, called inhibition, shifting and updating. Inhibition refers to one’s self control and the ability to restrain or stop habitual impulses or behaviour.Shifting typically requires flexible thinking which allows the successful transition back and forth between activities, and updating is concerned with the ability to keep track of and revise working memory representations (Eysenck & Keane, 2010, pp. 218-219).
Similar findings have been produced by Lehto, Juujarvi, Kooistra, and Pulkkinen (2003) who performed their own factor analysis to examine the heterogeneity of executive functions in children between the ages of 8 and 13 years old.They also provided further evidence to suggest fractionation of the central executive, by distinguishing the related but separable functions of inhibition, shifting and working memory. However, whilst there has been considerable evidence put forth in support of fractionation of the central executive, much of what has been concluded has stemmed mainly from investigating the functional dissociations in cognitive processing using executive tasks, rather than focusing on the underlying brain structures involved in executive processes.That said, neuroimaging techniques have since provided converging evidence that appears to reflect a disparate view of the unitary construct, and instead provides evidence in support of distinct brain regions relating to executive functioning.
By analysing neural activation in a dual task paradigm, Johnson and Zatorre (2006) unequivocally identified the middle dorsolateral prefrontal cortex as playing a key role in dual task processing.They investigated the executive processes of focused and divided attention by subjecting participants to simultaneous visual and auditory stimuli, and found that activation in the middle dorsolateral prefrontal cortex was more predominant when conducting the tasks simultaneously, whereas activation was more prevalent in the sensory cortex when using focused attention to conduct the tasks separately.Despite this, other studies using Positron emission tomography (PET) have also attempted to identify anatomical structures recruited during tasks which involved various processes such as updating, inhibition, shifting, divided attention and regulation. As a result, Collette and Linden (2002) found that most executive functions not only made use of the pre-frontal areas of the brain, but also relied heavily on posterior regions, more specifically the parietal lobes.These results appear to imply that executive functions cannot necessarily be limited to specific regions of the brain, but instead involve an intricate interplay of neural networks to enable the execution of specific executive processes.
More recently, however, technical advances in neuroimaging have now made it possible to produce three-dimensional mappings of cortical lesions, thus making it easier to link behavioural deficits against localized lesions in the brain.As such, Robinson, Calamia, Glascher, Bruss and Tranel (2013) were able to analyse hemodynamic responses (blood flow) whilst they administered a battery of executive tests tapping into both metacognitive goal directed behaviours, such as planning, response, inhibition and working memory, and emotional/motivational executive processes, which deal with the co-ordination of social and emotional cognition.By assessing patients who presented with lesions in either prefrontal or extraneous regions, they were able to identify disparate anatomical regions within the frontal lobes, each of which could be related to certain impairments in metacognitive and emotional/motivational executive processes.For example, they found that deficits in cognitive processes such as planning, inhibition and working memory were strongly linked to the dorsolateral pre-frontal cortex (dlPFC) and the anterior cingulate, whereas impairments in social and emotional processes were affiliated mainly with the ventromedial pre-frontal cortex (vmPFC). However, despite implicating these structures in various executive processes, they also acknowledge some limitations of their study.Firstly, to augment the use of data produced, they acknowledge that previous studies have shown that the anterior temporal lobes (Eslinger, Moore, Anderson, & Grossman, 2011; Pennington, Hodges, & Hornberger, 2011), superior parietal lobule and intraparietal sulcus (Collette, et al.
, 2005; Koenigs, et al. , 2011) have all been linked to similar executive processes, and as such, it may be beneficial to replicate their study, by focusing on patients with lesions outwith the frontal lobes, to obtain a more comprehensive view on the diversity of brain regions involved.Secondly, increasing the sample size would enable further assessment of individual differences so that other factors such as gender could be considered. To conclude, whilst the concept of a unitary central executive has been hindered by recent advancements in neuroscience, much of the research put forth still provides an abundance of evidence which attributes the frontal lobes as playing a critical role in executive processes.
However, there still remains much controversy as to the extent in which they dominate other anatomical regions of the brain in carrying out executive functions. Moreover, there is consistent evidence to suggest that there are distinct sub-components which are responsible for executive processes such as inhibition, selective attention, dual task co-ordination and updating of long term memory representations.That said, perhaps the most recent advancements in neuroscience yield the biggest breakthrough in understanding the complexities and mechanisms involved in executive functioning, and as such, whilst a unitary concept of the central executive is useful as an umbrella term to describe executive processes, the evidence would suggest that such processes are undeniably facilitated by the reciprocity and interactivity of complex neural networks and various brain structures.
