Organizational Skills in Individuals with Autism Spectrum Disorders
Organizational skills are a set of techniques used by an individual to facilitate the efficiency of future-oriented learning, problem-solving, and task completion. Organization requires the integration of several elements to reach a planned goal. Dawson and Guare (2010) define organization as “the ability to design and maintain systems for keeping track of information or materials” (p. 1). The broader domain of executive functioning plays a large role in the development and execution of organizational skills. Executive functioning is the multifaceted construct used to describe higher order, goal-directed thinking involving planning, inhibition, flexibility, organized search, and working memory and is thought to be impaired in individuals with ASD (Gyori 2006). Organization can be considered a subcomponent of the larger executive function construct.
Evidence suggests that individuals with autism spectrum disorders (ASD) have deficits in executive functioning abilities. For instance, impairments in flexibility, attention, and planning have been documented (O’Hearn et al. 2008). Those with ASD also have been noted to evidence dysfunction in their organizational skills (Kenworthy et al. 2005). This disorganization may be due to the presence of the aforementioned executive functioning deficits and related regulatory sequelae, such as the fact that individuals with ASD have a greater tendency to overfocus on irrelevant details to the exclusion of conceptualizing the “big picture” (Kuschner et al. 2009). In contrast, people with ASD have been anecdotally noted to be highly organized regarding their special interests, such that they know every single detail about a topic and have organized the information quite well (Baron-Cohen 2009). To date, however, there is a relative dearth of research specifically focusing on the subcomponent of organization in individuals with ASD, their relationship to ASD symptoms and characteristics, their developmental unfolding, and their importance to developmental functions and outcomes. What literature is available has tended to address higher functioning individuals, with a focus on broader executive functions (e.g., Hill 2004). In this chapter, we provide an overview of the history of research on organizational skills within the context of executive functions in individuals with ASD; compare and contrast the development of organizational skills in typically developing children and children with ASD, along with potential etiological agents; and present emergent interventions that aim to improve organizational skills.
Organizational skills lie within the broader cognitive domain of executive functions. The empirical investigation of executive functions began in the 1980s, and research using samples of individuals with ASD soon followed, suggesting the presence of an array of difficulties as compared to typically developing individuals. In one of the first studies examining executive functioning in individuals with ASD, Rumsey (1985) used the Wisconsin Card Sorting Test (WCST) to compare the cognitive flexibility of adult men with high-functioning autism to the flexibility of a sample of age-matched typical adults. Rumsey found that individuals with autism demonstrated significant perseveration when compared to the control group, a finding that has since been replicated with children and adolescents (Ozonoff et al. 2005; Prior and Hoffman 1990). Additional studies in the early 1990s revealed that individuals with high-functioning ASD had executive functioning deficits relative to other disorders, such as attention-deficit/hyperactivity disorder and learning difficulties, and across different cultures, suggesting that selected executive dysfunctions likely were core deficits in individuals with ASD (Ozonoff et al. 2005).
The study of executive functioning has evolved to better address the multidimensional nature of the construct, using a wider variety of outcome measures to consider other abilities such as categorization, working memory, inhibition, selective attention, and managing verbal feedback (Hughes 2001; Ozonoff et al. 2005). Indeed, a variety of models of executive function have now evolved (e.g., Stuss 2007; Friedman and Miyake 2017), and these are ripe for continued examination of their relevance for this population. Despite these advances, there remains a paucity of research specifically examining organizational skills in individuals with ASD.
The scientific examination of organization skills also is complicated by the tendency for individuals with ASD to be overly focused on details rather than conceptualizing the broad whole. Does this represent intact organization or, rather, disorganization due to the lack of attention to peripheral but important environmental details? One result of this tendency to overfocus on selected tasks is the difficulty with the generalization or the appreciation that skills learned in one context may be applied to other contexts (Volkmar and Wiesner 2009). Further, this approach to tasks may indirectly contribute to social skills difficulties, as to extract social information from various contexts requires both flexibility and the ability to organize this information in a meaningful way (Hughes 2001). Organizational skills also directly relate to an individual’s success with academic and vocational demands, as well as activities of daily living, such as paying bills, grocery shopping, and keeping up with doctor’s appointments. As such, the area of organizational skills is a critical one for clinical investigation and day-to-day functioning. For individuals with ASD, organization and related executive functions have been associated with adaptive behavior (Pugliese et al. 2016), social functioning (Freeman et al. 2017), driving (Cox et al. 2016), and affective regulation (Wallace et al. 2016).
The Development of Executive Functions and Organization in Typically Developing Children
In a typically developing child, the development and application of organizational strategies begins in infancy. As children remember information, they store it in an orderly manner through the use of categorization (Berk 2002). Three- to five-month-olds have been found to categorize stimuli by shape, size, number, and other physical characteristics. Berk (2002) cited a number of studies providing evidence that infants not only organize their physical world through the use of categorization but their emotional and social worlds as well. This includes infants sorting people and voices by gender and age and the emerging ability to discriminate between emotional expressions. During early childhood (ages 2 through 6 years), children begin to use planning and sequencing in order to reach a goal with familiar or simple tasks. Children also begin to use strategies in problem-solving and move from less to more efficient strategies through practice, reasoning, and adult assistance, among other factors (Berk 2002). Generally, during these years, children will engage in a variety of executive functions, but they tend to be task or situation specific, with generalization of these skills being limited. By middle childhood (ages 6 through 11 years), children make more use of planning skills and begin to engage in generalizing these skills to other tasks and situations. For instance, they will decide how to order steps so as to accomplish a complex task and then utilize those strategies for another task or situation. In addition, children are able to utilize multiple strategies concurrently, and their long-term knowledge base becomes organized into more sophisticated, hierarchically structured networks (Berk 2002). Finally, adolescence to adulthood marks the development of more focused and regulated attention, more effective and efficient strategy development and use, and an expansion of metacognition (Berk 2002). Taken together, these lead to more highly developed organizational capabilities and skills.
The Development of Executive Functions and Organization in Individuals with ASD
Although executive functions have been shown to be pervasively affected in many neurodevelopmental disorders across a range of ages and functional levels, less is known about the development of organizational skills in children with ASD or, more broadly, the developmental course of executive functions (O’Hearn et al. 2008). It has been theorized that although selected executive function deficits (i.e., set shifting and flexibility) have been demonstrated in young children with ASD (mean age = 5 years) relative to a control group of typically developing peers, these deficits are not autism-specific; but, instead, they are related to developmental delays. Another perspective with some empirical support is that executive functioning in preschool ASD is similar to that of young children with developmental delays matched on chronological and mental age, and suggests that executive dysfunction may emerge over the course of development and is not yet present (or observable) in early childhood. In that regard, these deficits are secondary in nature (Ozonoff et al. 2005). There is evidence that executive dysfunction and impairments may be most prominent during adolescence (Tsatsanis 2005; Rosenthal et al. 2013), particularly the application of planning and the goal-directed organization of information (Ozonoff et al. 2004). Kuschner et al. (2009) discovered that while typically developing individuals demonstrated improvements in organizational and planning skills with age when reproducing the Rey-Osterrieth Complex Figure (ROCF), no similar qualitative differences in performance were displayed between children and adolescents/adults with HFA. In contrast, other findings have demonstrated improvements in executive function from childhood to adolescence in individuals with ASD, perhaps secondary to neuropsychological maturation; nevertheless, executive functioning in adulthood was found to remain relatively impaired despite these developmental gains (Brady et al. 2017; O’Hearn et al. 2008). Longitudinal research is needed to better illuminate the developmental trajectory of the various dimensions of executive functions in children with ASD, with a specific need to include carefully considered covariates (e.g., nonverbal learning difficulties) (Hagberg et al. 2015).
Although much is to be learned regarding the development of executive functions, evidence related to planning and organization deficits has begun to surface over the past 10–15 years. Geurts et al. (2004) compared children with high-functioning ASD and attention-deficit/hyperactivity disorder (ADHD) on measures of executive functioning. These investigators reported that the errors made by the ASD group were related to difficulties with planning and cognitive flexibility, whereas errors by the ADHD group were better attributed to deficits in inhibition and fluency. Problems with planning efficiency and self-monitoring, which are closely related to organizational skills, have been documented in a number of studies (Tsatsanis 2005). Impaired performance on the Tower of London task was reported for children with ASD relative to typically developing controls, and was attributed to impaired planning skills and reduced strategy formation (Robinson et al. 2009).
Examining a sample of individuals with subthreshold autism traits, Christ et al. (2010) found a pattern of deficits in a number of executive functioning domains including planning, organizing, and setting goals as measured by the Behavior Rating Index of Executive Function (BRIEF). In contrast, Christ et al. (2010) did not find a relationship between autism symptomatology and the tendency to keep one’s work space and materials orderly, as measured by the Organization of Materials Scale of the BRIEF. Overall, their work suggested that even individuals with subthreshold autism traits exhibit profiles of executive dysfunction, including deficits in organization/planning, similar to those of individuals with diagnoses of ASD. These findings suggest that executive dysfunction may appear on a continuum of impairment, although it appears that problems with organizational skills of children with ASD may not be clearly dissociable from other childhood conditions (e.g., Tourette Syndrome, ADHD) (Hovik et al. 2017) – or even different subgroups of ASD (Blijd-Hoogewys et al. 2014). Adults with ASD who completed self-ratings of their broader executive functions did report their planning and organization skills to be mildly impaired, and significantly lower than typical controls (Davids et al. 2016).
Additionally, individuals with ASD have been noted to have difficulty copying and remembering the Rey-Osterrieth Complex Figure (ROCF), a task which requires visuospatial abilities, planning, and organization. Kenworthy et al. (2005) considered difficulty reproducing the ROCF, along with verbal organization deficits, to suggest impairments in organization. Kenworthy et al. (2005) expanded upon this suspicion to document deficits in the organization skills of individuals with high-functioning autism (HFA) and Asperger’s disorder (AD) across several verbal and visual tasks. For example, although both high-functioning ASD groups were found to have intact ability to interpret discrete data, relative impairments were noted in their ability to integrate and organize complex information. Another discrepancy that the authors attributed to a deficit in organizational skills was the participants’ ability to copy abstract visual information from a model but comparative difficulty with perceiving a visual gestalt when organizing puzzle pieces. Despite mean scores within the average range on several general cognitive measures, these investigators found relative impairments in a number of domains of executive functioning in the high-functioning ASD groups, including tasks specifically measuring organization [e.g., the Planning/Organization Scale of the BRIEF, the Story Memory subtest from the Wide Range Assessment of Memory and Learning (WRAML), and the Object Assembly subtest from the Wechsler Intelligence Scale for Children-III (WISC-III)]. In sum, while many previous studies have emphasized deficits in planning, a construct closely linked with organization, this study provided more direct evidence that individuals with ASD have executive deficits in flexibility and organizational skills.
Despite these findings, however, the deficits do not appear to be directly linked to the core symptoms present in ASD, although recent theories have made an effort to assert this linkage (Baron-Cohen 2009). Further, from a clinical perspective, many clinicians will agree that their clients with ASD can be highly organized with respect to their restricted interests, perhaps being overorganized with respect to the intensity of their focus on these interests and their various components. For example, clinicians and researchers working with this population no doubt have been favorably impressed with the amount of focus, logic, and organization of information pertaining to a restricted interest. In many instances, these individuals actually have a knowledge base that would rival many experts in the targeted area, and their attention to the details pertaining to this information is exquisite. Anecdotally, this indicates that individuals with ASD are capable of goal-directed organization, although this skill is not always generalized and applied outside of their area of restricted interest and, for many individuals with ASD, herein lies the challenge.
Explaining Organizational Dysfunction
There are a number of possible underlying causes of, or contributors to, the impairments noted in organizational skills and other related aspects of executive functioning. Frith’s theory of weak central coherence (Frith 1989) posits that individuals with ASD tend to process information in fragments rather than in terms of the coherent whole. Consequently, individuals with ASD may focus on minor, irrelevant details and fail to process the underlying meaning of the cohesive information (Hughes 2001). Without an understanding of the broader concept, it is challenging to effectively organize and process presented information. Mesibov et al. (2004) proposed that in addition to focusing on minute details, individuals with ASD are less capable of assessing and prioritizing their relative importance. For example, Mesibov et al. (2004) described a young man with ASD who recalled having difficulty following a conversation that went from red balloons to birthday parties to the children enjoying themselves, because he became focused on “red balloons” and was unable to organize a schema for a birthday party. Although not stated directly, it is likely that this individual, or others like him, could tell you an enormous amount of information about (red) balloons in a detailed and organized fashion.
In addition, differences in executive functioning in individuals with ASD have been attributed to abnormalities in brain maturation (e.g., decreased synaptic pruning early in development, structural differences in regions of the brain such as the cerebellum, and decreased connectivity within the brain due to volumetric differences of white matter in the corpus callosum; Han and Chan 2017; Hazneder 2011; O’Hearn et al. 2008). For example, Just et al. (2007) found evidence of reduced intracortical connectivity resulting in a lower degree of integration of information across certain cortical areas. The investigators concluded the presence of deficits in the integration of information at the neural level which, in turn, contributed to cognitive impairments. While a fuller discussion of these neurobiological features of ASD is beyond the scope of this chapter, it is important to note that the neurological bases of executive dysfunctions in autism have yet to be fully explained (Han and Chan 2017; O’Hearn et al. 2008).
Impairments in executive function skills, such as planning, prioritizing, and organizing, may become more apparent when children begin school. In this situation, they are required to keep track of their personal items, follow multistep directions, engage in multistep assignments, and complete tasks in a set amount of time. Despite a need for structure and order, children with ASD may have messy desks and turn-in incomplete assignments (Moore 2002). In this regard, there does appear to be a significant need for specific interventions to improve selected types of organization skills. There is some notion that various executive functions can be improved through intervention; however, there is a lack of strong empirical research looking at remediating executive functioning in children with ASD (Ozonoff et al. 2005).
One randomized controlled trial has emerged examining the utility of the Unstuck and On Target intervention versus social skills training to address executive dysfunction in children with high functioning ASD (Kenworthy et al. 2014). Forty-seven third to fifth grade students were assigned to the Unstuck and On Target intervention, and they were matched by age, gender, and, race, IQ, ASD symptomotolgy, medication status, and parents’ education to 20 students who were randomized to the social skills training. The interventions were implemented in 28 small group sessions (30–40 min in length) and were conducted in the school setting. Findings revealed significant improvements in organization and planning for the Unstuck and On Target group, with effect sizes being in the small to medium range. Improvements also were noted in problem solving and behavioral flexibility, with these findings translating into improved rule-following behaviors and significantly better capabilities to make transitions. Given the potentially pervasive nature of these problems, interventions may require collaboration between schools and families, organizational aids, and significant day-to-day structure (Volkmar and Wiesner 2009). Although there are some medical treatments that have shown promising early results (e.g., repetitive transcranial magnetic stimulation) (Ameis et al. 2017), educational and behavioral strategies currently remain the best options for improving organizational skills.
To date, from a best practice perspective, a number of strategies have been offered to address organizational problems. Volkmar and Wiesner (2009) recommended a stepwise approach, consistency and predictability, the use of functional routines, providing time to process/prepare, and the use of aids such as visuals, organizers, and computers. Moore (2002) suggested physically organizing the classroom and using color coding, visuals, daily schedules, checklists, teaching children to find the main idea rather than focusing on irrelevant details, and using an assignment notebook, among other aids. Dawson and Guare (2010) proposed that children lacking intact organizational capacity should be provided with organizational schemes. The children should be cued to use the organizational scheme and receive reinforcement for using it. An example for organizing a backpack provided by Dawson and Guare (2010) is to use specific pockets and compartments, which can be labeled or color coded for lunch, permission slips, money, and homework. This latter example also may capitalize on the cognitive rigidity, or need for “sameness,” typically evidenced by many individuals with ASD. More generally, this approach also suggests the need for a thorough evaluation of various executive functions, including organizational capabilities.
Many of the aforementioned suggestions to remedy the difficulties associated with executive dysfunction are related to concepts from structured teaching. Structured teaching, a cornerstone of the TEACCH (Treatment and Education of Autistic and Communication handicapped CHildren) program, is based on the premise that clear structure and organization make the environment more navigable for individuals with ASD by clarifying expectations and enabling communication. This is achieved through the use of physical organization, visual support, and schedules (Hartford and Marcus 2011). Goals of physical organization (e.g., dividers, clearly delineated areas for specific activities) include the addition of contextual clues, clarification of boundaries, and the minimization of distractions and stimulation. Visual supports are utilized to capitalize on the tendency for individuals with ASD to have relatively strong visual learning abilities and include picture prompts, video modeling, ordered pictures, or step-by-step written directions, which can help in attention and sequencing. Schedules can be used to establish functional routines and add predictability to the day, and they can indicate a sequence through words, pictures, or objects to facilitate smooth transitions and temporal understanding. In general, built-in organization through structured teaching methods can help individuals with autism effectively understand and participate in the world around them (Hartford and Marcus 2011).
Direct instruction interventions with empirical support, such as providing organizational skills training, can also be used to improve the skills of children with ASD in the classroom. For example, in one of the few evidence-based studies related to organization training in ASD, elementary-age children with high-functioning autism and Asperger’s syndrome were found to increase the percentage of items correctly filed and decrease the number of seconds needed to retrieve a requested item following an intervention teaching the children self-monitoring skills and the use of a file box system to enhance organization (Dorminy et al. 2009).
This encyclopedia entry has addressed an important but targeted area of executive functioning in the ASD population; namely, organizational skills. Despite the range of studies that have been focused on executive functions in this population, few empirical studies have specifically examined the subcomponent of organization. The empirical literature that does exist in the domain of organization, and even executive functions more broadly, typically has studied individuals with high-functioning ASD, although it is noteworthy that more recent studies have begun to include individuals with ASD who have a broader range of overall functioning. It would be worthwhile for future research to not only examine profiles of executive functions that would include organization, but also to examine these profiles in lower functioning populations. While some might argue that these problems may not be high on the priority list of needs for many of these individuals, such research could shed light on how organizational deficits contribute to the presence of aggressive, self-injurious, and repetitive behaviors as well as to the overall adaptation of individuals with ASD who are higher functioning.
For instance, it has been theorized that individuals engage in repetitive behaviors when interpreting social situations as unpredictable as a means to impose order (Hughes 2001); thus, an understanding of how individuals with ASD differentially order and organize both social and nonsocial information might provide additional strategies for implementing successful interventions. In addition, although there is a breadth of research on neurodevelopmental constructs such as attention, memory, language, and temporal-sequential ordering, there is limited research specifically investigating organization in individuals with ASD, the developmental course of organizational skills throughout the life span in this population, or how this cognitive function contributes to ASD symptom manifestation.
Finally, there remain a myriad of additional questions related to the organizational skills of individuals with ASD. In addition to examining the multidimensionality of these skills across all ages and levels of functioning, questions pertaining to their generalized versus specific nature, associated measurement strategies, their relationships to core and associated symptoms in ASD, and their linkages to evidence-based practices all remain unanswered at present. Specific studies addressing the organizational skills of individuals with ASD will continue to advance our knowledge of how these skills facilitate our overall understanding of ASD.
References and Reading
- Ameis, S. H., Daskalakis, Z. J., Blumberger, D. M., Desarkar, P., Drmic, I., Mabbott, D. J., Lai, M.-C., Croarkin, P. E., & Szatmari, P. (2017). Repetitive transcranial magnetic stimulation for the treatment of executive function deficits in autism spectrum disorder: Clinical trial approach. Journal of Child and Adolescent Psychopharmacology, 27, 413–421.CrossRefPubMedGoogle Scholar
- Berk, L. E. (2002). Infants, children, and adolescents (4th ed.). Boston: Allyn and Bacon.Google Scholar
- Cox, S. M., Cox, D. J., Kofler, M. J., Moncrief, M. A., Johnson, R. J., Lambert, A. E., Cain, S. A., & Reeve, R. E. (2016). Driving simulator performance in novice drivers with autism spectrum disorder: The role of executive functions and basic motor skills. Journal of Autism and Developmental Disorders, 46, 1379–1391.CrossRefPubMedGoogle Scholar
- Dawson, P., & Guare, R. (2010). Executive skills in children and adolescents: A practical guide to assessment and intervention (2nd ed.). New York: The Guilford Press.Google Scholar
- Dorminy, K. P., Luscre, D., & Gast, D. L. (2009). Teaching organizational skills to children with high functioning autism and Asperger’s syndrome. Education and Training in Developmental Disabilities, 44(4), 538–550.Google Scholar
- Frith, U. (1989). Autism: Explaining the enigma. Oxford: Blackwell Scientific.Google Scholar
- Gyori, M. (2006). Autism and cognitive architecture: Domain specificity and psychological theorising on autism (neurocognitive development and impairments). Budapest: Akedemiai Kiado.Google Scholar
- Hartford, D., & Marcus, M. (2011). Educational approaches. In E. Hollander, A. Kolevzon, & J. Coyle (Eds.), Textbook of autism spectrum disorders (pp. 537–553). Washington, DC: American Psychiatric Publishing.Google Scholar
- Hazneder, M. M. (2011). Structural neuroimaging. In E. Hollander, A. Kolevzon, & J. Coyle (Eds.), Textbook of autism spectrum disorders (pp. 385–394). Washington, DC: American Psychiatric Publishing.Google Scholar
- Hovik, K. T., Egeland, J., Isquith, P. K., Gioia, G., Skogli, E. W., Andersen, P. N., & Øie, M. (2017). Distinct patterns of everyday executive function problems distinguish children with Tourette Syndrome from children with ADHD or autism spectrum disorders. Journal of Attention Disorders, 21, 811–823.CrossRefPubMedGoogle Scholar
- Hughes, C. (2001). Executive dysfunction in autism: Its nature and implications for the everyday problems experienced by individuals with autism. In J. Burack, T. Charman, N. Yirmiya, & P. Zelazo (Eds.), The development of autism, perspectives from theory and research (pp. 255–275). Mahwah: Lawrence Erlbaum Associates.Google Scholar
- Kenworthy, L., Anthony, L. G., Naiman, D. Q., Cannon, L., Wills, M. C., Werner, M. A., Alexander, K., Strang, J., Bal, E., Sokoloff, J. L., & Wallace, G. L. (2014). Randomized controlled effectiveness trial of executive function invention for children on the autism spectrum. Journal of Child Psychology and Psychiatry, 55, 374–383.CrossRefPubMedGoogle Scholar
- Kuschner, E. S., Bodner, K. E., & Minshew, N. J. (2009). Local vs. global approaches to reproducing the Rey Osterrieth Complex Figure by children, adolescents, and adults with high-functioning autism. International Society for Autism Research, 2, 348–358.Google Scholar
- Moore, S. T. (2002). Asperger syndrome and the elementary school experience, practical solutions for academic and social difficulties. Shawnee Mission, KS: Autism Asperger Publishing.Google Scholar
- Ozonoff, S., Cook, I., Coon, H., Dawson, G., Joseph, R. M., Klin, A., et al. (2004). Performance on Cambridge neuropsychological test automated battery subtests sensitive to frontal lobe function in people with autistic disorder: Evidence from the collaborative programs of excellence in autism network. Journal of Autism and Developmental Disorders, 34, 139–150.CrossRefPubMedGoogle Scholar
- Ozonoff, S., South, M., & Provencal, S. (2005). Executive functions. In F. R. Volkmar, R. Paul, A. Klin, & D. Cohen (Eds.), The handbook of autism and pervasive developmental disorders (pp. 606–627). Hoboken: Wiley.Google Scholar
- Pugliese, C. E., Anthony, L. G., Strang, J. F., Dudley, K., Wallace, G. L., Naiman, D. Q., & Kenworthy, L. (2016). Longitudinal examination of adaptive behavior in autism spectrum disorders: influence of executive functions. Journal of Autism and Developmental Disorders, 46, 467–477.CrossRefPubMedPubMedCentralGoogle Scholar
- Stuss, D. T. (2007). New approaches to prefrontal lobe testing. In B. Miller & J. Cummings (Eds.), The human frontal lobes: functions and disorders (2nd ed., pp. 292–305). New York: Guildford Press.Google Scholar
- Tsatsanis, K. D. (2005). Neuropsychological characteristics in autism and related conditions. In F. R. Volkmar, R. Paul, A. Klin, & D. Cohen (Eds.), The handbook of autism and pervasive developmental disorders (pp. 365–381). Hoboken: Wiley.Google Scholar
- Volkmar, F. R., & Wiesner, L. A. (2009). A practical guide to autism: What every parent, family member, and teacher needs to know. Hoboken: Wiley.Google Scholar
- Wallace, G. L., Kenworthy, L., Pugliese, C. E., Popal, H. S., White, E. I., Brodsky, E., & Martin, A. (2016). Real-world executive functions in adults with autism spectrum disorder: profiles of impairment and associations with adaptive functioning and co-morbid anxiety and depression. Journal of Autism and Developmental Disorders, 46, 1071–1083.CrossRefPubMedPubMedCentralGoogle Scholar