Cognitive Processing and Intelligence In Children With Attention Deficit Hyperactivity Disorder
By Vicki Schwean, Ph. D.

SSTA Research Centre Report #92-12: 26 pages, $11.

Preamble A multidisciplinary team from the University of Saskatchewan, representing the fields of psychiatry, psychology and education, report this study of the cognitive processing capabilities of attention deficit hyperactivity disordered (ADHD) children and how methylphenidate impacted on those processes. Also described is the behavioral, social, and learning/achievement factors associated with responsivity to methylphenidate.
Background to the Research Program
Research Objectives
Study One
Study Two

Back to: Students - Diverse Needs

The SSTA Research Centre grants permission to reproduce up to three copies of each report for personal use. Each copy must acknowledge the author and the SSTA Research Centre as the source. A complete and authorized copy of each report is available from the SSTA Research Centre.
The opinions and recommendations expressed in this report are those of the author and may not be in agreement with SSTA officers or trustees, but are offered as being worthy of consideration by those responsible for making decisions.


In the fall of 1989, a multidisciplinary team comprised of University of Saskatchewan researchers who represented the fields of psychiatry, psychology, and education, made application for and were granted a Saskatchewan School Trustees Association Grant-in-Aide of Research. The Grant-in-Aide of Research was to support a research program designed to examine the cognitive processing attention deficit hyperactivity disordered (ADHD) children and to explore how methylphenidate impacted on those processes. A secondary purposes was to describe the behavioral, social, and learning/achievement factors associated with responsivity to methylphenidate. Sixty clinical diagnosed ADHD children between the ages of 8 and 11 years participated in this project.

Table of Contents


While earlier attempts at conceptualizing the underlying mechanisms of ADHD focused on excessive motor activity theoretically arising from minimal brain damage or dysfunction theoretically arising from minimal brain damage or dysfunction (Strauss & Lehtinen, 1947; Wender, 1971), current arguments reference central deficiencies in self-regulation of behavior that exert a negative effect on attention and effort (Barkley, 1981; Douglas, 1972, 1988; sergeant, 1988; Swanson et al., 1990). Self-regulation involves the higher-order cognitive processes of fine-tuning and modifying one's actions (Brown, Branford, Ferrars, & Campione, 1983). Although there is growing evidence that ADHD children perform poorly on various cognitive tasks (e.g., Matching Familiar Figures Test), there continues to be considerable debate about the nature of the cognitive processes assessed by such measures (e.g., to what extent they tap control processes) and hence, to what degree findings support a central deficiency in self-regulation of behavior (see Douglas, 1988).

Table of Contents

Self-Regulation and Ritalin Therapy

Ritalin (Methylphenidate) is the most commonly used drug and the drug of choice in medical management of ADHD (Pelham et al., 1990). There is general agreement that Ritalin enhances the performances of about 70% if ADHD children on tasks assessing the primary symptoms of attention, effort, and impulsivity (Werry & Aman, 1984). Whether Ritalin achieves its effect on these primary symptoms by redressing faulty self-regulation remains unknown. Moreover, research has underscored the need to remain sensitivity to the issue of drug-response curves. Reports indicate that the dosage effect for Ritalin may not be consistent across cognitive and social performances. For example, Douglas, Barr, Aman, O'Neill and Britton (1988) found similar dose-effect patterns across most behavioral, academic, and cognitive measures but noted that some learning tasks appear to be detrimentally affected at high stimulant dosages.

Growing evidence indicates that intact self-regulatory abilities underlie efficiency in reading and mathematics (Wong, 1986). Douglas' (Douglas, Barr, O'Neill, & Britton, 1986; Douglas et al., 1988) assertion that stimulants enhance the cognitive performance of AHD children by activating self-regulatory processes suggests that academic gains should be expected as a function of stimulant therapy. The research findings, however, are equivocal. While a number of reviewers have concluded that methylphenidate does little to improve academic performance (Gadow, 1983; ottenbacher & Cooper, 1983), others argue that methodological and conceptual weaknesses (e.g., failure to differentiate between new and previously acquired learning) have led to faulty conclusions (Das, personal correspondence; Douglas et al., 1986).

Table of Contents


One of the objectives of our research was to enrich the understanding of the role of self regulation in producing the primary symptoms of ADHD by systematically exploring the attentional, coding, and self-regulatory abilities of ADHD children through the use of a theoretically driven information processing battery (i.e., Cognitive Assessment System; Das & Naglieri, in press). We selected the Das PASS model (i.e., planning, attention, simultaneous and successive processing), because, as Ashman and Schroeder (1986) point out, there is a considerable body of literature that uses its nomenclature to diagnose cognitive deficits and their relationship to educational performance. A preliminary report in Naglieri, Das, & Jarman (1990) suggests that ADHD children show deficits in attention, planning, and successive processing; however, we felt it was necessary to replicate those findings. As well, the PASS model afforded us an opportunity to examine the effects of varying dosages of Ritalin (0.15 mg, 0.3 mg, & 0.6 mg of racemic Ritalin per kg of body weight) on the cognitive processes of ADHD children, and in turn, on established and newly learned academic skills. In addition, we administered the WISC-III (Psychological Corporation, 1991) and the four subtest short forms of the Stanford-Binet: FE (Thorndike, Hagen, & Sattler, 1986) to arrive at a psychometrically sound description of general mental ability.

In the following sections, two independent studies funded in part through an S.S.T.A. Grant-in-Aid of Research are presented.

Table of Contents

Study One: Cognitive Processing in ADHD Children


The central purpose of this double-blind study was to examine the cognitive processing capabilities of attention deficit hyperactivity disordered (ADHD) children. A further objective was to explore how methylphenidate (Ritalin) affects these behaviors. Forty children, ages 8 to 11 years, meeting diagnostic criteria for ADHD were administered the Cognitive Assessment System (CAS: Das & Naglieri, in press). After a four-week methylphenidate trial, the CAS was readministered to children on either a 10 mg. clinical dose of methylphenidate or placebo. Data analysis indicated that methylphenidate enhanced performance on selected planning, attention, and simultaneous processing tasks.


Attention deficit hyperactivity disorder (ADHD) is a relatively common developmental disability which affects approximately 9% of school-age Canadian boys and 3% of girls (Szatmari et al., 1989). Primary difficulties with attention, impulsivity, arousal modulation, hyperactivity, and the delay of gratification (Douglas, 1983, 1988) profoundly impact upon the ability of these children to meet situational demands in an age-appropriate fashion (Routh, 1978) and in turn, lead to serious impairments in academic and social skills (Barkley, 1989).

While earlier attempts at conceptualizing the underlying mechanisms of this disorder focused on excessive motor activity theoretically arising from minimal brain damage or dysfunction (Strauss & Lehtinen, 1947), and more recently, constitutional impairments in sustained attention and impulse control (Douglas, 1972), current arguments reference central deficiencies in executive functioning or self regulation of behavior that exert a negative effect on attention and effort (Douglas, 1988; Sergeant, 1988). Executive functioning or self regulation involves the higher-order cognitive processes of fine tuning ones planning, monitoring, evaluation, and testing activities (Brown et al., 1983). Although there is growing evidence that ADHD children perform poorly on various higher-0order cognitive tasks (e.g., Matching Familiar Figures test), there continues to be considerable debate about the nature of the specific cognitive activities assessed by such measures and hence, to what degree the findings support a central deficiency in self regulation of behavior.

Mehtylphenidate (Ritalin) intervention is the most widely used treatment for ADHD. While a comprehensive understanding of methylphenidate effects on cognition has been evolving in recent years, to this point, the efficacy of methylphenidate within a context that takes into account our understanding of the relationship between basic cognitive processes and performance on problem-oriented tasks has not been established.

This research addresses this gap by assessing the impact of methylphenidate on cognition using a theory-drive information processing battery, the Cognitive Assessment System (CAS: Das & Naglieri, in press). This battery, based on Das' operationalization of Luria's neuropsychological model, is comprised of planning (perceptual and logical search), attention (selective and sustained), and basic coding (simultaneous and successive tasks (PASS model). A schematic representation of the PASS model is presented in the figure below.

Table of Contents



From amongst those children, ages 8 to 11 years, referred to Children's Services, Royal University Hospital, University of Saskatchewan, because of overactivity, short attention span, restlessness, distractibility, impulsivity, lack of effort, and disruptiveness, thirty subjects meeting the following criteria were selected for participation.

a) met criteria for DSM-III-R diagnosis of ADHD, as assessed by a child psychiatrist.

b) received clinical ratings on the Hyperactivity scales of either the parent or teacher Conners Rating Scales or Revised Problem Behavior Checklists.

c) obtained a full scale IQ of 85 or above on the abbreviated Stanford-Binet: 4th Edition.

d) not currently receiving drug therapy.

Children who exhibited symptoms of psychosis or primary visual, auditory or language deficits, brain-damage, or restless behavior attributable to emotional problems or a stressful home environment were excluded from participation.


Cognitive performance was measured with the Cognitive Assessment System (CAS; Das & Naglieri, in press). This battery is based on Das' operationalization of Luria's neuropsychological model and attempts to examine how knowledge is organized and accessed in the memory system, as well as the mental operations by means of which various intellectual tasks are actually accomplished. The CAS test battery consists of a number of tasks designed to measure three kinds of cognitive processes: basic coding (simultaneous and successive), arousal and attention, and planning.

At initial assessments, a battery of higher-order cognitive tasks, including the CAS, were administered. Each child was then placed on a four-week methylphenidate trial. Following this trial, children were randomly assigned to either a 10 mg methylphenidate or placebo condition and were reassessed one-half hour after taking either methylphenidate or placebo on the CAS by a clinician blind to their condition.


Our understanding of ADHD has gradually evolved from a focus on hyperactivity to an emphasis on deficits in attentional mechanisms and most recently, to an examination of potentially serious impairments in self regulation and planning. Using the PASS model, Reardon and Naglieri (1989) found evidence for a planning deficiency in ADHD children, in addition to deficits in attention and successive processing. The present study assessed the effects of methylphenidate on the cognitive processes of ADHD children as described by this model.

Our results revealed that methylphenidate enhanced performance on the Planned Connections and Matching Numbers planning tasks. The Planned Connections, a variant of the Trail Making task, purports to assess the planning activities involved in selective attention, while the Matching Numbers task, a task requiring the child to find two out of six numbers in each row that are the same, provides a measure of verbal planning that requires coding at the perceptual level. By using ration scores (time/score), we were able to determine methylphenidate improved both efficiency and accuracy on these planning tasks. Other researchers have argued that planning behavior is unaffected directly by methylphenidate (e.g., Reid & Borkowski, 1984). However, since both of our groups received methylphenidate for a four-week trial period prior to reassessment, group differences on these tasks appear to be directly related to the experimental methylphenidate condition.

Only the Auditory Selective Attention task discriminated between the drug and placebo groups. This task requires the child to identify specific categories of words according to whether they are presented in a male and/or female voice. Methylphenidate improved the total score, reflecting changes in both focused and divided attention. Interestingly, methylphenidate did not enhance performance on the remaining attentional tasks, although previous research has shown similar tasks to be sensitive to methylphenidate intervention (e.g., Stroop test). Among other explanations, there is some indication that for the Expressive Attention task, ceiling effects may account for the failure to find differences.

Scores on both the Matrices and Figure Memory tasks, measures of simultaneous processing, were sensitive to methylphenidate. The Matrices task is very similar to the Raven's Progressive Matrices, while the Figure Memory task requires the child to locate and outline a geometric figure that is embedded within a more complex design. Although these findings suggest improvement in selective simultaneous information processing, we would argue that what has been enhanced is the ability to inhibit particular behaviours (e.g., on the Matrices task, inhibition of being misled by salient but less informative cues).

Despite Readon and Naglieri's findings that successive processing deficits are observed in ADHD children, we did not find differences on any of three successive processing tasks as a function of medication condition.

Table of Contents


What our research program is principally concerned with is determining the nature of ADHD children's cognitive functioning and examining the effects of methylphenidate on the complex cognitive functions that underlie interpersonal and academic learning. In this study, we have shown that methhylphenidate has an impact on planning, simultaneous coding, and attentional aspects of cognition. Given recent conceptualizations of ADHD as a regulatory disorder, our findings are encouraging in that they show methylphenidate has a positive impact on self-regulation.

Furthermore, the tasks contained in the CAS battery have been used to relate the strengths and weaknesses of cognitive processes to varied academic tasks and intellectual abilities. Results from our ongoing research program should further enrich our understanding of the relationship between the basic cognitive processes of ADHD behavioral measures, and the relationship to treatment interventions.

Table of Contents

Study Two: The Performance of ADHD Children on the WISC-III


This study examined the performance of ADHD children on various indices of intelligence as a function of methyulphenidate therapy. Forty-three clinically diagnosed ADHD children (ages 7 to 13 years), all of whom were favorable methylphenidate responders, were administered the WISC-III. Subjects were randomly assigned to either a placebo or methylphenidate condition at the time of testing. Results indicated that both groups performed similarly on the subtests, Verbal IQ, Performance IQ, Freedom from Distractibility Index, and Processing Speed Index.

Table of Contents


In 1949, Wechsler developed the Wechsler Intelligence Scale for Children (WISC). This scale was revised and restandardized in 1974; its present form is called the Wechsler Intelligence Scale for Children - Revised (WISC-R). Within the past year, the WISC-R has undergone a full revision and restandardization and was recently published as the Wechsler Intelligence Scale for Children - Third Edition (WISC-III, 1991). The Wechsler Intelligence Scales are the most frequently used measure of intelligence for children ages six to sixteen.

A number of studies have attempted to determine whether ADHD children show recognizable patterns of test scores on the earlier versions of the Wechsler scales. In general, studies show that relative to controls, ADHD children show more subtest variability (e.g., Douglas, 1972), exhibit more verbal-performance discrepancies (Lambert & Sandoval, 1980), evidence greater cognitive deficits (Palkers & Stewart, 1972; Carlson, Lahey, & Neeper, 1986), and demonstrate poor performance on subtests tapping attention (Keogh, Wetter, McGinty, & Donlon, 1973; Milich & Loney, 1979). With the recent publication of the Wechsler Intelligence Scale for Children - Third Edition (WISC-III; Wechsler, 1991), a further opportunity to examine the cognitive profiles of children with ADHD on a refined version of the earlier tests was provided.

Although methylphenidate (Ritalin) is the most commonly used pharmacological treatment for ADHD children, to date, there is a paucity of studies examining the short-term effects of methylphenidate on measures of intelligence. This study attempted to address this gap.

Table of Contents


The proposed research was exploratory in nature and sought to describe the intellectual competencies of ADHD children on the experimental version of the WISC-III. Particular focus was directed to exploring whether ADHD children were characterized by a distinguishable test profile. Moreover, the effects of methylphenidate on subtest, Index, and IQ scores was also examined.

Table of Contents



Subjects for this study were 43 clinically diagnosed ADHD children between the ages of 7 and 13 years presently receiving methylphenidate (Ritalin) therapy prescribed by a psychiatrist in Children's Services, Department of Psychiatry, Royal University Hospital.


Parents of children receiving methylphenidate therapy were mailed a letter inviting their child's participation in the study. Information regarding the research procedures and instrument were explained fully to parents prior to obtaining signed consent. Each child was randomly assigned to either a drug or drug holiday condition and administered the WISC-III. Test administration was conducted by an experienced clinician, blind to the child's drug condition. Administration time was approximately 90 minutes.

Children assigned to the drug condition were assessed on a school day when medication was clinically prescribed, approximately one-half hour after ingesting a 10 mg tablet of methylphenidate. Children assigned to the placebo condition were tested on a drug holiday but were given a placebo. At no time did the research project interfere with the previously prescribed schedule for methylphenidate administration.

Table of Contents


Initial analysis was conducted to ascertain whether methylphenidate exercised any short-term effects on WISC-III scores. The results id not reveal any significant treatment effects for the subtest, Index, or Verbal and Performance IQ scores. Since no treatment effect was found, subsequent analysis were performed on the total ADHD sample (n = 43).

Correlational analysis was undertaken to determine whether the pattern of correlations for ADHD children were similar to that of the standardization sample. Results revealed that the Verbal subtests clustered together as did the Performance subtests. Correlations of the subtests with the Index scores were in line with the four factor description of the WISC-III. Similarly, the Verbal and Performance subtests correlated more highly with their respective composite IQ scores, while showing moderate but significant correlations with the Full Scale IQ. Correlation patterns between the Index scores, Verbal IQ. Correlation patterns between the Index scores, Verbal IQ, Performance IQ, and Full Scale were consistent with trends reported for the standardization samples.

A comparison of WISC-III subscale, Index, and scale scores revealed that the Perceptual Organization Index (POI) score of ADHD children was significantly higher than their Freedom From Distractibility Index (FDI), Processing Speed Index (PSI), and Verbal Comprehension Index (VCI) scores. A comparison of the WISC-III scores of ADHD children with a representative sample of children was also undertaken. Results revealed that nine of the subtest scores showed significant differences between ADHD children and the normative sample with the former scoring higher on the Picture Completion, Block Design, and Mazes subtests but significantly lower on the Information, Coding, Picture Arrangement, Arithmetic, Symbol Search, and Digit Span subtests. Furthermore, the ADHD subjects scored significantly lower on the PSI and FDI scores, as well as on the Verbal IQ, relative to the normative group.

Table of Contents


These results clearly show that methylphenidate did not enhance the intellectual performance of ADHD children. However, several factors may account for these findings. First, it may simply be the case that traditional measures of intelligence are not sensitive enough to detect subtle medication effects. Alternatively, it may be that the methylphenidate dosages in the present study were too moderate to effect performance improvements.

Given that the Wechsler scales are the most commonly used measure of intellect with children and are frequently employee in making diagnostic decisions, it was important to determine the validity of the WISC_III for ADHD children. Examination of the patterns of correlations between WISC-III subtests, factor Indexes, and IQ's supported the convergent and discriminate validity of the WISC-III for ADHD children.

The diagnostic utility of the WISC-III was examined through a comparison of the performance of the ADHD sample with that of the WISC-III normative sample. Results suggested that relative to normal same-age peers, ADHD children are more likely to show relative strengths on tasks assessing perceptual organization but weaknesses on tasks measuring verbal comprehension, freedom from distractibility, and processing speed.

Table of Contents

Back to: Students - Diverse Needs