Studies of Attention

Attention has been a major focus of neuropsychological research into ADHD and deficits on various tasks designed to measure attentional aspects of performance have been found in children with ADHD. However, task performance in these children has been found to be affected by many factors other than attention and it is generally concluded that poor performance cannot be attributed to a specific deficit in attention (Barkley, 1997; Schachar, 1991; Swanson et al., 1990; Van der Meere, 1996). Instead, the cognitive deficit in ADHD appears to be at the output or motor stage of information processing rather than at input or attentional stages (Barkley, 1997; Van der Meere, 1996).

In a series of studies that manipulated task parameters in order to isolate different aspects of information processing Van der Meere, Sergeant and colleagues found no evidence of specific deficits in selective, focussed, sustained or divided attention, encoding, search or decision processes, or acquisition of automatic processing (Sergeant & Scholten, 1983, 1985a,b; Van der Meere & Sergeant, 1987, 1988a,b,c). They did find that children with ADHD consistently made more errors and had slower and more variable reaction times than normal controls. They concluded that this task inefficiency in children with ADHD could not be explained in terms of an information processing or attention deficit, but was due to a deficit in output or motor processes (Van der Meere, 1996; Van der Meere & Sergeant, 1988a). Deficits in motor timing, preparation and control may better explain the poor performance of children with ADHD on attentional tasks. In terms of behavioural symptoms, children with ADHD are consistently reported by parents and teachers to demonstrate inattention (Barkley, 1990, 1997). This behavioural inattention and distractibility may also arise from deficits in inhibition and self-regulation and the effects these deficits have on task persistence and interference control (Barkley, 1997; Schachar et al., 1995; Van der Meere, 1996).

Sustained Attention—the Continuous Performance Task

The continuous performance task (CPT) was originally developed by Rosvold et al. (1956) as a measure of sustained attention or vigilance. The CPT has been widely used as both a research and a diagnostic instrument in the field of ADHD (Corkum & Siegel, 1993). These tasks require the subject to view sequences of randomly ordered stimuli, such as letters, and to respond to a particular infrequent stimulus, such as the letter X, or to a particular sequence of stimuli, such as X

preceded by A. Omission errors (missed targets) are considered a measure of inattention, while commission errors (responses to non-targets or false alarms) are considered a measure of impulsivity (Burke, 1990; Corkum & Siegel, 1993). Reaction time (time between stimulus onset and response) to target stimuli is considered a measure of alertness (Levy, 1980).

Reaction time is often not reported in CPT studies, which tend to concentrate on the number and/or type of errors made (Chee et al., 1989). However, children with ADHD have been found to have slower reaction times in response to target stimuli than normal controls in several studies (Chee et al., 1989; Klorman et al., 1979; Overtoom et al., 1998; Schechter & Timmons, 1985; Strandburg et al., 1996; Wood et al., 1999). This finding has been interpreted as suggesting an inability to process and respond to information quickly in children with ADHD (Wood et al., 1999). Children with ADHD also have more variable reaction times than their normal peers (Klorman, 1991; Van Leeuwen et al., 1998).

Children with ADHD have been shown to make significantly more errors of omission than normal controls in many studies and this finding has been interpreted as evidence for inattention and deficient arousal in children with ADHD (Corkum & Siegel, 1993; Losier et al., 1996). Children with ADHD also make more errors of commission than their normal peers and this is thought to result from poor inhibition and more impulsive responding (Barkley, 1997; Halperin et al., 1993; Kupietz, 1990; Van Leeuwen et al., 1998). Losier et al. (1996) performed a meta-analytic review of error rates in CPT studies of ADHD and found that across 11 studies children with ADHD made twice as many omission errors and more than twice as many commission errors as normal controls. Both these differences were statistically significant. Increased CPT error rates in children with ADHD is a fairly consistent result, although many studies find significant group differences for only one type of error (omission or commission), and some studies find no significant difference in either type of error between ADHD and control subjects (e.g. Werry, 1987; Wood et al., 1999).

While performance of the CPT requires sustained attention, it also taps many other processes including arousal, motivation and inhibition (Corkum & Siegel, 1993; Klorman, 1991). Therefore, poor performance on the CPT does not necessarily reflect a specific sustained attention or vigilance deficit. Several other explanations for CPT performance deficits in children with ADHD have been proposed including momentary concentration problems (Corkum & Siegel, 1993; Oades, 1998), compromised allocation of effort (Dinklage & Barkley, 1992; Van der Meere & Sergeant, 1988), or an inability to modulate activation according to task demands (Van der Meere, 1996). The cognitive processes measured by the CPT and therefore the nature of the deficits revealed by poor performance may also depend on task and external variables, which vary greatly between studies (Corkum & Siegel, 1993; Losier et al., 1996).

Performance on the CPT has been shown to be improved by stimulants, which generally reduce reaction times and the number of errors made (Klorman, 1991; Klorman et al., 1979, 1991; Losier et al., 1996; Michael et al., 1981; Rapoport et al., 1980). Losier et al. (1996) performed a meta-analytic review of the effects of methylphenidate on the CPT performance of children with ADHD. They found that across 15 studies omission errors were reduced by 39% after methylphenidate administration, and commission errors were reduced by 29%. Klorman et al. (1991) suggested that this improved accuracy results from more efficient stimulus evaluation. Stimulants have been found to also improve CPT performance in normal children and adults (Klorman, 1991; Rapoport et al., 1980; Strauss et al., 1984), so this effect is not specific to ADHD.

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Understanding And Treating ADHD

Understanding And Treating ADHD

Attention Deficit Disorder or ADD is a very complicated, and time and again misinterpreted, disorder. Its beginning is physiological, but it can have a multitude of consequences that come alongside with it. That apart, what is the differentiation between ADHD and ADD ADHD is the abbreviated form of Attention Deficit Hyperactive Disorder, its major indications being noticeable hyperactivity and impulsivity.

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