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Disorders of executive functioning_Turner.docx

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University of Toronto Scarborough
Kris Romero

Disorders of executive functioning and self awareness. By Turner and Levine Executive functions and self awareness are the highest achievements of brain evolution. They distinguish us from other species. These depend on multiple of integrating brain regions which make them highly sensitive to brain injury  imprecision and inconsistencies in the definition/assessment create a clinical challenge. Even tho neuroscience had advanced our understandings of these abilities there is no theoretically driven rehab program Models of executive functioning and self-awareness A/ Defining executive functions and self-awareness: -executive functions: collection of abilities that help us plan goal, weight alternatives, select a course of action, sequence intervening activities, and regulate behavior so that a goal can be achieved.ALL of these share the common element of control and direction of basic low lever abilities. - autonoetic awareness/ self-awareness: overlaps/supports executive functions. The human ability to maintain a consistent sense of self across time, from past to present to future. Damage to the prefrontal lobe causes deficits in autonoetic awareness and personality changes (personal agency- see themselves as separate from the environment yet free to act on it).  the following models are useful but none fully account for all mental capacities, they all have considerable heterogeneity but yet deal with some aspect of the PFC (least understood area that occupies 30% of cortical matter). - executive and self-awareness functions has been historically researched backwards compared to other constructs. They first noticed these functional deficits in brain injured ppl and then defined the constructs. Thus prefrontal functions and executive functions have ben treated synonymously - this is problematic for both clinics and research: i.) theory: defining executive functions as what the frontal lobes do, creates as infinite regression. Since theres so much functional heterogeneity of the frontal lobes. This makes the constructs vulnerable to shift depending on which area of the frontal/prefrontal lobe is being examined. Especially since these areas have a lot of interconnections with other areas of the brain. - posterior, subcortical, and diffuse damage can mimic prefrontal cortical damage thru frontal systems disconnection. ii.) Clinical: pts with neurobehavioral deficits can have any or all of these combination w/focal prefrontal damage, complicating lesion-behavior specificity. Thus these abilities are not specifically the domain of the PFC Models of executive (prefrontal) function: A/ Supervisory Attention System (Norman & Shallice): - thought/action are controlled by distinct processing mechanisms in routine vs novel situations. In routine situations responses are considered automatic (without conscious awareness), while action is controlled by individual schemas (automated programs capable of achieving relevant goal-states effectively/efficiently once initiated by environmental triggers. - routine action selection: controlled by a contention scheduling mechanisms whereby an schema whose activation value exceeds an established threshold is brought online while competing schemata are suppressed thru lateral inhibition processes. - supervisory attentional system (SAS): used when deliberate attention is needed for planning/decision making, trouble shooting, novel or technical difficult action, or resisting habitual tendencies/temptations. In these situations, routine environmental triggering of thought/action schemata are insufficient to generate an appropriate response. - The SAS involves multiple processes that guide behavior thru 3 stages of action control: strategy generation, implementation, & monitoring - disturbances of the SAS resulting from frontal lobe lesions causes: i.) behavioral rigidity: unmodulated contention scheduling caused by diminished activity in the SAS, produces perseverative behavior in situations whenever a single action schema is highly activated as a result of salient environmental triggers ii.) distractibility: results when multiple environmental triggers are similarly activated ---this model is useful in explaining strategy application disorder (pts behavior in naturalistic, unstructured situations is not governed by higher-level control mechanisms). However the PROBLEM with this model is the underspecification of the SAS (in models its represented as a unitary concept, but its better understood as a collection of control processes that modulate contention scheduling ---Staus divided SAS 5 discrete control processes: energerization/inhibition of action schemata, adjustment of contention scheduling, monitoring of schema activity, and control of if-then logical processes. PRO: divides SAS into frontal and posterior subregions ---Shallice divided SAS into 4 discrete processes that mediate behavior in non-routine situation: (a) top-down modulation of schemata in contention scheduling- localized within left DLPFC; (b) monitoring of behavior- medicated by right DLPFC; (c) specification of a required memory trace (retrieval mode- right ventromedial PFC); and (d) establishment of future intentions- localized with the frontal pole (BA10) B/ Goal selection and goal neglect (Duncan): - posits that the central role for the PFC is goal formulation, action selection and goal monitoring.Action is thought to be directed by goal lists (series of task requirements that must e completed to achieve a desired goal state). - these list in turn guide the formation of action structures (component actions/mental operations that provide a procedural map to the goal state). Finally, a continual means-ends analysis is carried out to assess the discrepancy that exists btwn current and desired goal state. This latter stage guides the selection of new goal lists & action structures. - the major dysfunction of the PFC is goal neglect: tendency for these pts to disregard the requirement of a given task even tho they can retain these requirements thru out task performance. Found to be inversely correlated w/ the ability to pursue multiple goals simultaneously and fluid intelligence. These functions are thought to be related to DLPFC. C/ Working memory (Baddeley): this research started on animals - working memory: using info held in the brain to complete complex goals - memory cells: located in the PFC, they fire only during the delay portion of a standard delay-task paradigm (physiological location for working memory). Later work identified separate ramp-up cells (firing coincides w/the upcoming response rather than the onset of the delay period, physio location for anticipatory attention or motor set). This lead to the conception of prefrontal function as cross-temporal mediation of behavior - this view posits the DLPFC is atop of the perception-action cycle and acts as a bridge for the temporal separation of perception and action, especially in situations that are new, ambiguous & complex - this model is teleological: represents an ability to use info and knowledge drawn from past exper to influence future actions - Representational memory: online maintenance of info no longer in the environment and its properties are distributed thro out the PFC in modality-specific working memory circuits - Baddeleys Executive-slave systems model: auditory-linguistic and visuospatial slave systems (articulatory loop and visuospatial sketchpad) maintain representations in posterior cortices, while the central executive (PFC) coordinates working memory activity. Recently a 4 component was added, episodic memory buffer (integrative temporary store for multimodal temporally extended episodic representations that serve as a in btwn long term episodic memory and online operation of working memory). D/ Executive Knowledge (Structured Event Complexes): - the PFC principal role is in the neural representation of knowledge or symbols. Higher order cognition is controlled by hierarchically organized knowledge domains in the PFC. - the basic knowledge unit represents a set of events, actions or ideas that are sequentially linked. Then these are linked together in a temporally extended structured event complexes (SECs): contain event themes, boundaries and consequences (similar to action schema). - managerial knowledge unit (KUI): the SEC specified for cog planning, social behavior and the management of knowledge. They are hierarchically organized from abstract to those direct behavior in specific context. - the technique they use to research this is script generation task (pp generate an action sequence describing a new/familiar even and/or build coherent, logical action sequences from a list of actions) - CON: describe the WHAT of these processes not the HOW E/ Somatic Markers (Damasio): - the ventromedial prefrontal cortex is involved with integrating info from all sensory modalities (external world) with info about body states (internal world- muscoskeletal), and this info is used to decide if future outcomes are advantageous/disadvantageous. - somatic markers bias decision makers - ppl with ventromedial PFC damage make risky & socially inappropriate - acquired sociopathy
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