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Role of executive functions in p

Role of executive functions in psychiatric disorders

Abhishek Pathak * Jai Singh Yadav **, Samiksha Kaur***

*Junior Resident, Dept of Psychiatry, IMS,BHU

**Assistant Professor, Dept of Psychiatry, IMS,BHU,*** Psychologist SRLNMH Varanasi

Introduction

Executive function is cognitive processing requiring the co-ordination of several sub processes to achieve a particular goal. Neurobiological evidence suggests that executive processing is intimately connected with the function of the frontal cortices. Co-ordination, control and goal orientation is the basic concept of executive function. Executive function as ‘a product of the co-ordinate operation of various processes to accomplish a particular goal in a flexible manner. When these systems break down, behaviour becomes poorly controlled, disjointed and disinhibited1. Patients with damage to the prefrontal cortex show impaired judgments, organization, planning and decision making, as well as behavioral dis-inhibition and impaired intellectual abilities2


It is clear that prefrontal cortex is a vital component of the circuitry sub serving executive function, posterior cortical regions and subcortical structures collaborate with prefrontal cortex to mediate successful executive processes.3,4 There is also evidence that patients with subcortical ischaemic vascular disease show selective deficits on tests of executive function (Cummings, 1994). In all disorder the executive deficits are attributed to either frontal lobe damage or dysfunction or to disruption in fronto subcortical connectivity.In most cases of executive dysfunction, deficits are attributed to either frontal lobe damage or dysfunction or to disruption in fronto-subcortical connectivity3. Neuroimaging with PET and fMRI has confirmed the relationship between executive function and functionalontal pathology3. Neuroimaging studies have also suggested that some constituent functions are not discretely localized in prefrontal regions.5

Functional imaging studies using different tests of executive function have implicated the dorsolateral prefrontal cortex to be the primary site of cortical activation during these tasks.6In addition, PET studies of patients with Parkinson’s disease have suggested that tests of executive function are associated with abnormal function in the globus pallidus and appear to be the genuine result of basal ganglia damage.3 With substantial cognitive load, fMRI signals indicate a common network of frontal, parietal and occipital cortices, thalamus, and the cerebellum.7 This observation suggests that executive function is mediated by dynamic and flexible networks that are characterized using functional integration and effective connectivity analyses.8The complete circuit underlying executive function includes both a direct and an indirect circuit.

The neural circuit responsible for executive functioning is, in fact, located primarily in the frontal lobe 6. The emerging view suggests that cognitive processes materialize from networks that span multiple cortical sites with closely collaborative and overlapping functions.5 A challenge for future research will be to map the multiple brain regions that might combine with each other in a vast number of ways, depending on the task requirements.9

Etiology: - The etiology of executive dysfunction is heterogeneous10, as many neurocognitive processes are involved in the executive system and each may be compromised by a range of genetic and environmental factors. Learning and development of long-term memory play a role in the severity of executive dysfunction through dynamic interaction with neurological characteristics.

Executive processes are closely integrated with memory retrieval capabilities for overall cognitive control; in particular, goal/task-information is stored in both short-term and long term memory, and effective performance requires effective storage and retrieval of this information (Verbruggen & Logan 2008). Certain genes have been identified with a clear correlation to executive dysfunction and related psychopathologies. According to Friedman et al (2008), the heritability of executive functions is among the highest of any psychological trait. The dopamine receptor D4 gene (DRD4) with 7'-repeating polymorphism (7R) has been repeatedly shown to correlate strongly with impulsive response style on psychological tests of executive dysfunction, particularly in clinical ADHD2

Testing and Measurement:-There are several measures that can be employed to assess the executive functioning capabilities of an individual.

Clock Drawing Test

The Clock Drawing Test (CDT) is a brief cognitive task that can be used by physicians who suspect neurological dysfunction based on history and physical examination, it is relatively easy.

Stroop Task

The cognitive mechanism involved in the Stroop task is referred to as directed attention. The Stroop task requires the participant to engage in and allows assessment of processes such as attention management, speed and accuracy of reading words and colours and of inhibition of competing stimuli4. The stimulus is a colour word that is printed in a different colour than what the written word reads.     

Wisconsin Card Sorting Test

The Wisconsin Card Sorting Test (WCST), is used to determine an individual's competence in abstract reasoning, and the ability to change problem-solving strategies when needed.4 These abilities are primarily determined by the frontal lobes and basal ganglia, which are crucial components of executive functioning making the WCST is a good measure for this purpose11.

Trail-Making Test

Executive Dysfunction in Clinical Populations The Clinical presentation of severe executive dysfunction that is unrelated a specific disease or disorder is classified as a dysexecutive Syndrome, and often appears following damage to the frontal lobes of the cerebral cortex12. As a result, executive dysfunction is implicated etiologically and/or co-morbidly in many psychiatric illnesses, which often show the same symptoms as the dysexecutive Syndrome. It has been assessed and researched extensively in relation to cognitive developmental disorders, psychotic disorders, affective disorders, and conduct disorders, as well as neurodegenerative diseases and acquired brain injury (ABI).

Individuals with schizophrenia

It may demonstrate amnesia for portions of their episodic memory. Observed damage to explicit, consciously accessed, memory is generally attributed to the fragmented thoughts that characterize the disorder.

These fragmented thoughts are suggested to produce a similarly fragmented organization in memory during encoding and storage, making retrieval more difficult. However, implicit memory is generally preserved in patients with schizophrenia (Ward. 2006).

Persons with schizophrenia also tend to demonstrate deficits in response inhibition and cognitive flexibility (Thoma et al.2008).

Patients often demonstrate noticeable deficits in the central executive component of working memory as conceptualized by Baddeley and Hitch. However, performance on tasks associated with the phonological loop and visuospatial sketchpad are typically less affected Ward, 2006.

More specifically, patients with schizophrenia show impairment to the central executive component of working memory, specific to tasks in which the visuospatial system is required for central executive control (Oram et al. 2005).

Attention Deficit/Hyperactivity Disorder

       Individuals with ADHD often experience problems with organization, discipline, and setting priorities, and these difficulties often persist from childhood through adulthood10. In both children and adults with ADHD, an underlying executive dysfunction involving the prefrontal regions and other interconnected subcortical structures has been found.10As a result people with ADHD commonly perform more poorly than matched controls on interference control, mental flexibility and semantic verbal fluency10.

Autism Spectrum Disorder

Autism is diagnosed based on the presence of markedly abnormal or impaired development in social interaction and communication and a markedly restricted repertoire of activities and interests. Individuals with autism commonly show impairment in three main areas of executive functioning: a) fluency; b) planning; and c) cognitive flexibility 8.

Fluency.

Fluency refers to the ability to generate novel ideas and responses, findings have suggested that children with autism generate fewer novel words and ideas and produce less complex responses than matched controls.

Planning.

Persons with autism demonstrate impairment on tasks requiring planning abilities relative to typically functioning controls, with this impairment maintained over time. As might be suspected, in the case of autism comorbid with learning disability, an additive deficit is observed in many cases.

Flexibility.

Poor mental flexibility, as demonstrated in individuals with autism, is characterized by perseverative, stereotyped behaviour, and deficits in both the regulation and modulation.  In general individuals show relatively spared performance on tasks that do not require mentalizing. These include: use of desire and emotion words, sequencing behavioural, pictures and the recognition of basic facial emotional expressions. In contrast, individuals with autism typically demonstrated impaired performance on tasks.

These include: false beliefs, use of belief and idea words, sequencing mentalistic pictures, and recognizing complex emotions such as admiring or scheming (Ward, 2006).

Bipolar Disorder

Executive dysfunction in bipolar disorder associated particularly with the manic state, and is largely accounted for in terms of the formal thought disorder that is a feature of mania (Dixon et al., 2004). Individuals affected by bipolar disorder exhibit deficits in strategic thinking, inhibitory control, working memory, attention and initiation that are independent of affective state15. However patients with bipolar disorder with a history of psychosis demonstrated greater impairment on measures of executive functioning and spatial working memory compared with bipolar patients without a history of psychosis suggesting that psychotic symptoms are correlated with executive dysfunction15. Functional neuroimaging studies have implicated abnormalities in the dorsolateral prefrontal cortex and the anterior cingulate cortex as being volumetrically different in individuals with bipolar disorder 13

Parkinson's Disease

Persons with PD show deficits in the areas of: a) spatial working memory; b) central executive aspects of working memory; c) loss of episodic memories; d) locating events in time. Parkinson’s disease (PD) primarily involves damage to subcortical brain structures and is often associated with movement difficulties, in addition to problems with memory and thought processes (Ward, 2006). In the PD often demonstrate difficulties in working memory, a component of executive functioning. Cognitive deficits found in early PD process appear to involve primarily the fontro-executive functions 14. Moreover, studies of the role of dopamine in the cognition of PD patients have suggested that PD patients with reduced dopamine supplementation are more impaired in their performance on measures of executive functioning (Grossman et al. 2002). This suggests that dopamine may contribute to executive control processes. Increased distractibility, problems in set formation and maintaining and shifting attention sets, deficits in executive functions such as self-directed planning, problems solving, and working memory have been reported in PD patients14.

Spatial working memory PD patients often demonstrate difficulty in updating changes in spatial information and often become disoriented. They do not keep track of spatial contextual information in the same way that a typical person would do almost automatically. Similarly, they often have trouble remembering the locations of objects that they have recently seen, and thus also have trouble with encoding this information into long-term memory.

Central executive aspects PD is often characterized by a difficulty in regulating and controlling one’s stream of thought, and how memories are utilized in guiding future behaviour. However, some recent research suggests that PD patients may actually be less persistent in pursuing goals than typical persons and may abandon tasks sooner when they encounter problems of a higher level of difficulty 14.

The loss of episodic memories in PD patients typically demonstrates a temporal gradient wherein older memories are generally more preserved than newer memories. Also, while forgetting event content is less compromised in Parkinson’s than in Alzheimer’s, the opposite is true for event data memories.

Locating events in time, PD patients often demonstrate deficits in their ability to sequence information, or date events. Part of the problems is hypothesized to be due to a more fundamental difficulty in coordinating or planning retrieval strategies, rather than failure at the level of encoding or storing information in memory. This deficit is also likely to be due to an underlying difficulty in properly retrieving script information. PD patients often exhibit signs of irrelevant intrusions, incorrect ordering of events, and omission of minor components in their script retrieval, leading to disorganized and inappropriate application of script information.

Conclusions and future directions

Executive function is, by definition, complex, and this review has only scratched the surface of the current debates concerning the neuronal basis of executive function and dysfunction. It is clear that although the prefrontal cortex is a vital component of the circuitry subserving executive function, posterior cortical regions and subcortical structures collaborate with prefrontal cortex to mediate successful executive processing. The advent of functional neuroimaging techniques has provided the means to study the neuronal basis of human executive function directly. Early imaging experiments attempted to dissociate component processes of executive function and attribute them to discrete prefrontal foci. Although there is some evidence for both functional and material specificity in prefrontal cortices, it appears that this is more a matter of degree than reflecting fixed and fundamental dissociations. It is also clear that the same prefrontal regions mediate very different executive functions. An increasingly influential view is, therefore, that we should not be looking for one-to-one mappings between structure and function. Rather, we should be using more sophisticated analysis approaches to study flexible and dynamic changes in effective and functional connectivity between brain regions. This has dramatic implications for our understanding of normal and abnormal executive functions. From a clinical perspective, a dynamic network approaches is synchronous with the rising trend for disconnection models of neurological and psychiatric disorders. Important challenges for the future will be to gain a more through understanding of these models, and to construct theoretical frameworks for understanding the mechanisms of both disconnection and functional re-organisation.

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