Attention deficit-hyperactivity disorder (ADHD) is a chronic disorder that affects the nervous system and inhibits the attention while promoting impulsive behavior. Some of the most common symptoms include restlessness, impulsiveness, short-term memory impairment, confusion, slow information processing, and difficulty in following a single assignment. In addition to ADHD, children usually develop several comorbid disorders, such as borderline personality disorder, mood disorders, substance abuse, or sleep disorders (Castel, Lee, Humphreys, & Moore, 2011).
The outcomes of ADHD usually affect the socioeconomic future and academic achievements of children. Studies show that 30 percent of the children diagnosed with ADHD will fail a grade, and 30 pecent of them will drop out of high school while the others will less likely going to complete their education (Huang-Pollock & Karalunas, 2010). In addition, they usually have lower occupational and socioeconomic statuses as adults than children who were never diagnosed with ADHD (Huang-Pollock & Karalunas, 2010).
ADHD was generally considered a childhood disorder because the onset was before the age of seven, and sci. However, studies proved that up to 70 percent of the children diagnosed with ADHD still show symptoms of the disorder in adulthood (Alderson, Hudec, Patros, & Kasper, 2013). In the US, ADHD affects approximately 4 percent of adults (Engelhardt, Nigg, Carr, & Ferreira, 2008).
The main determinants for the development of ADHD are structural abnormalities and the reduced activity of the prefrontal cerebellum, caudate, and cortices because they impair the executive function among children (Huang-Pollock & Karalunas, 2010). The prefrontal and frontal cortical regions are responsible for the execution, planning, regulation, and inhibition of behavior (Alderson et al., 2013).
The control processes which are usually impaired in people with ADHD are called executive functions, and one of the most significant areas of interest for researchers investigating ADHD is the working memory. Working memory is an executive function that is responsible for temporary or short-term memory storage and the active manipulation of information. Besides the central executive system, it also contains the phonological buffer/loop and the visual-spatial sketchpad (Alderson et al., 2013).
Neurophysiological and neuroimaging research showed that all of those systems that comprise the working memory are independent systems (Alderson et al., 2013). While the central executive system was responsible for dividing attention and organizing internal information, the other two subsystems were responsible for short-term memory. In contemporary ADHD research, working memory is an important domain because it is evident that cognitive inhibition in ADHD patients is insignificant while behavioral inhibition, which also reflects in the working memory, is significantly higher than it is in individuals without ADHD.
In a literature review on the topic of ADHD, Barkley (1997) proposes a theoretical model that links the deficit in behavioral inhibition, which is associated with ADHD, to four neurological functions. Those functions include working memory, reconstitution, self-regulation, and the internalization of speech (Barkley, 1997). The four functions were adopted from Bronowski’s theory while including segments of Fuster’s theory and several other components, such as including several motivational theories to explain the self-regulation domain.
In the review, Barkley (1997) aimed to find evidence that would support the claims that deficit in those four domains has a strong correlation with ADHD and that the model can be used in future ADHD-related research. When it comes to working memory, the model suggests that children with ADHD are influenced by current events and situations rather than being able to practice long-term planning. In addition to traditional deficits, the model also predicted that sense of time, memorized information organization, inability to maintain goal-oriented behavior, deficiencies in practicing social skills, and several other working memory deficits.
While some research on the topic existed at the time of writing the review, the research on working memory in children was limited to digit retention and oral repetition, mental arithmetic, memorizing sequences of information and executing an assignment, or other assessments (Barkley, 1997). There was little research that aimed to test and differentiate between impulsive individuals and individuals with ADHD.
Although the proposed model lacked research support at the time its concept was created, most contemporary studies explore the four domains proposed by Barkley (1997) and recognize them under the common term “executive functions.” However, researchers also frequently isolate a single domain when they want to explore a single aspect of ADHD. For example, a researcher who wants to understand how ADHD impacts memory will investigate the working memory efficiency and deficits in patients diagnosed with ADHD.
The model proposed by Barkley (1997) suggested that behavioral inhibition was linked to working memory deficits, but the distinction between cognitive inhibitions and behavioral inhibitions remained unclear and was heavily studied in ADHD patients (Engelhardt et al., 2008). In a study with 288 adult and adolescent participants, Engelhardt et al. (2008) aimed to understand if cognitive inhibition exists in ADHD.
The participants were divided in two groups. One group contained 115 people with ADHD. The other group contained 173 people who did not have ADHD because they were required for comparing the data and results obtained in this cross-sectional study. The participants had to complete an IQ assessment and other subtests, so the researchers could evaluate their competence to participate in the study.
The results showed little differences between the two groups. The sentence processing task accuracy for the control group was 85 percent while the group with ADHD showed an average accuracy of 79 percent, and in both instances, the results were significantly above chance with p < .01 (Engelhardt et al., 2008). In the working memory domain, the ADHD participants were significantly less accurate than the control group. According to Engelhardt et al. (2008) the results suggested that people with ADHD have problems with interferences that impair their working memory.
In conclusion, the sentence processing results did not support the ideas that cognitive inhibition deficit impacts ADHD patients (Engelhardt et al., 2008). However, the study by Engelhardt et al. (2008) also proved that working memory is a significant weakness for people with ADHD because their limited attention span does not allow them to block out interferences while completing the given assignment.
A cross-sectional study by Castel et al. (2011) examined how children with ADHD compare to children without ADHD when it comes to efficient memory usage. The study tested the data obtained from 116 children were required to memorize different items, which were assigned different values. The goal of the test was to accumulate as many points as possible, and the aim was explained to the children before conducting the study.
According to the results, the two groups did not show a significant difference in their memory capacity. However, Castel et al. (2011) found that children with ADHD were not as selective as children from the healthy control group when it came to memorizing items based on value. In children with ADHD Combined, it was possible to notice that their strategic allocation of memory and it efficient usage was the lowest in contrast to the ADHD Inattentive group and the healthy group.
A lot of current research is directed towards ADHD in adults because a lifelong trajectory of the diagnosis has been established (Alderson et al., 2013). Alderson et al. (2013) conducted a study that used a regression approach for the purpose of examining the impact of the central executive processes and storage/rehearsal processes on the working memory of adults diagnosed with ADHD.
The sample for the study consisted of 37 undergraduate students. They were between the ages of 18 and 24. The study was designed as a cross-sectional study, so 21 participants met the criteria for ADHD diagnosis while 16 of them were the healthy control group. Participants with comorbid disorders, psychosis, previous seizures, and gross neurological impairments were discontinued while participants who took psychostimulants were asked to discontinue their medication 24 hours before attending the study.
Alderson et al. (2013) measured phonological working memory, visual-spatial working memory, participants’ psychosocial backgrounds, intellectual functions, and ADHD rating. They also conducted clinical interviews with the participants. The study was the first that used a regression approach, which aimed to examine how the components of working memory independently contribute to adults with and without ADHD.
According to Alderson et al. (2013), their approach identified central executive and phonological process as the main deficits that determine working memory impairments in adults who are diagnosed with ADHD. It is also possible to suggest that their findings may have clinical implications because they emphasize the importance of age as a treatment variable.
There are many comorbid disorders and similar symptoms related to ADHD that overlap with other disorders. For example, children may suffer from reading disabilities or they may be slow learners. Although ADHD can be comorbid with both conditions, they can be distinguished because of the cognitive differences among children. A study by Swanson, Mink, and Bocian (1999) aimed to investigate the phonological and executive processing in several groups of children, who were identified as slow learners, diagnosed with ADHD, diagnosed with reading disabilities, or diagnosed with both ADHD and slow learning or ADHD and reading disabilities.
Overall, the study by Swanson et al. (1999) aimed to investigate several aspects of hypotheses. First, they predicted that the phonological system functions better in children with ADHD than children with reading disabilities. Second, the executive functions were considered more intact among children with reading disabilities than among children with ADHD. Third, children with comorbid ADHD and reading disability have a similar profile to children with reading disability without ADHD.
The participants were 90 children from 25 elementary schools, and the groups were created after assessing the IQ, reading, and spelling scores. The phonological processing tasks included the Word Attack subtest, phonological oddity, and phonological deletion, and their aim was to collect data on cognitive inhibitions. The working memory was assessed with sentence span and visual matrix tasks. Swanson et al. (1999) also administered two short-term memory tasks, which included the backward and forward digit spans.
The three main findings by Swanson et al. (1999) showed that most of the initial assumptions were wrong. First, there is no support for the prediction that phonological systems are functioning better in either of the groups participating in the study. Second, the executive functions in children with reading disabilities are not working better than they are in children with ADHD. Third, rather than showing a similar cognitive profile to children with reading disabilities, children with both reading disabilities and ADHD showed a similar profile to children who were identified as slow learners.
When it comes to academic achievement, children with ADHD often lack attention and have difficulty in executing goal-oriented tasks and acquiring new skills and information. Huang-Pollock and Karalunas (2010) investigated how children with ADHD obtain complex cognitive skills despite the inhibitions of their attention and working memory.
The three groups in this cross-sectional study comprised of 48 children without ADHD, 32 children with ADHD Combined, and 21 children with ADHD Inattentive. The alphabet arithmetic and finger math tasks were used were used to measure cognitive skill acquisition among those groups. The alphabet arithmetic required more working memory than finger math.
During both tasks, it was evident that the population with ADHD has been more prone to errors than the non-ADHD population since the beginning of the study. Even though the children practiced extensively, children with ADHD were not able to reach the same level of cognitive skill as children without ADHD. However, the hypothesis by Huang-Pollock and Karalunas (2010) was confirmed because the children with ADHD performed better in finger math, which required less working memory to complete.
It is possible to notice that the similarity between various disorders exists, and it is also evident that ADHD is frequently comorbid with other disorders. However, while it is evident that certain similarities exist and that cognitive profiles from children with learning or reading disorders are often similar, a distinct set of criteria is used to determine which disorder is prevalent in children. Furthermore, the neurophysiologic aspects of ADHD and its impact on memory are currently clear because it is evident that the prefrontal cortex is responsible for the functions that are impaired in individuals with ADHD. While there is no way to cure ADHD with drugs, the understanding of the executive functions and the subsystems involved in memory impairment is a significant finding in ADHD research.
Future research needs to address several important aspects of ADHD and transition from cross-sectional studies to longitudinal studies because it is not clear how the trajectory of its development persists from childhood to adulthood. It is also not clear why some people appear to lose the symptoms of ADHD by adulthood while others continue to display some symptoms.
Some of the neglected aspects of research include the lack of understanding how ADHD persists and changes in adults. Because the prevalence of ADHD among adults was only recently confirmed, and because recent assumptions suggested that its onset is before the age of seven, it is evident that new diagnosis criteria are required to account for the differences in age groups.
Finally, clinical trials are required to introduce brain-imaging technology in ADHD diagnosis because it would be accurate and useful for subsequent management of the condition. Research is also required to systematize treatments and improve the collaboration between the educational system and healthcare for the purpose of providing children with ADHD with safer medication management, better therapy coordination, and prevention or early intervention strategies.
References
Alderson, R. M., Hudec, K. L., Patros, C. H., & Kasper, L. J. (2013). Working memory deficits in adults qith attention-deficit/hyperactivity disorder (ADHD): An examination of central executive and storage/rehearsal processes.
Barkley, R. A. (1997). Behavioral inhibition, sustained attention, and executive functions: constructing a unifying theory of ADHD. Psychological Bulletin, 121(1), 65-94.
Castel, A. D., Lee, S. S., Humphreys, K. L., & Moore, A. N. (2011). Memory capacity, selective control, and value-directed remembering in children with and without attention-deficit/hyperactivity disorder (ADHD). Neuropsychology, 25(1), 15-24.
Engelhardt, P. E., Nigg, J. T., Carr, L. A., & Ferreira, F. (2008). Cognitive inhibition and working memory in attention-deficit/hyperactivity disorder. Journal of Abnormal Psychology, 117(3), 591-605.
Huang-Pollock, C. L., & Karalunas, S. L. (2010). Working memory demands impair skill acquisition in children with ADHD. Journal of Abnormal Psychology, 119(1), 174-185.
Swanson, H. L., Mink, J., & Bocian, K. M. (1999). Cognitive processing deficits in poor readers with symptoms of reading disabilities and ADHD: More alike than different?. Journal of Educational Psychology, 91(2), 321-333.
