Research Article From: Archives of Clinical Neuropsychology Volume 22, Issue 6, August 2007, Pages 763–771 DOI:
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By: Jens Egeland


Most previous studies of attention deficit in ADHD and schizophrenia have used overall measures of inattention that may disguise differences in underlying mechanisms. The present study investigated types of inattentive errors and applied a process view of attention in analyses of Conners’ Continuous Performance Test protocols from subjects with ADHD-combined (51), inattentive type (19) and schizophrenia (26). Subjects with ADHD-I had more omission errors and became more inattentive as a function of time on task. Subjects with ADHD-C made more errors of commissions as time passed. In contrast, the performance of the subjects with schizophrenia improved, indicating a training effect. There were no differences in overall attentiveness between the groups.

Although all groups were impaired on an overall level, they displayed three distinct patterns of inattention. The ADHD-I group displayed a lethargic inattention characterized by high fatigue. The ADHD-C group showed a hyperactive-impulsive pattern, while the schizophrenia group showed an inability to initially focus attention.

Keywords: ADHD-C, ADHD-I, Schizophrenia, Sustained attention, Hyperactivity-impulsivity, Continuous performance test
Topic: fatigue schizophrenia lethargy inattention adult attention deficit hyperactivity disorder

Impaired attention is a defining characteristic of ADHD and is typically also found among subjects with schizophre- nia (Egeland et al., 2003). Although the typical hyperactive-impulsive ADHD subject is easily distinguishable from a thought disturbed and hallucinating patient with schizophrenia, the two conditions may be difficult to separate in less prototypical cases. Many subjects with ADHD are not hyperactive. Some are admitted to hospital due to drug induced psychotic episodes. In fact, one study found a 10% prevalence of psychotic symptoms among patients with ADHD (Stahlberg, Søderstrøm, Rastam, & Gillberg, 2004) while another has shown significant thought disorder compared to normals among both adolescents with schizophrenia and ADHD (Caplan, Guthrie, Tang, Nuechterlein, & Asarnow, 2001). Many subjects with schizophrenia have prodromal symptoms dominated by attention deficit (Silverstein, Mavrolefteros, & Turnbull, 2003) and some subjects genetically at high risk of schizophrenia meet the criteria for an ADHD-diagnosis (Oner and Munir (2005). Thus, there clearly is an overlap in the symptomatology of the two disorders.

Nevertheless, they are considered to have a different etiology and prognosis and they are treated differently. While ADHD is treated with central stimulating medication which increases the level of dopamine in the brain, schizophrenia is treated with anti-psychotics which block the effect of dopamine. Still, the few studies that have compared subjects with ADHD and schizophrenia have generally failed to find differences (Barr, 2001). More specifically, studies have failed to find group differences in sustained attention (Øie & Rund, 1999), working memory (Karatekin & Asarnow, 1998) and vigilance (Seidman et al., 1998). There is, however, evidence of differences in the underlying neurophysiology of the two disorders (Rowe et al., 2004). Thus, the mechanisms of attentional disturbance may also be fundamentally different in the two illnesses (Ollincy et al., 2000). Studies preceding the DSM-IV did not differentiate between the inattentive, the hyperactive and the combined sub-type of ADHD. This may have obscured genuine differences between any one of the sub-groups and schizophrenia. Another reason for the lack of differences may be that many studies have applied a reductionistic view of attention as one homogeneous ability. This may obscure the possibility that subjects with ADHD and schizophrenia differ in the way they fail in attention rather than in the degree of attention deficit present. Specifically, few studies have applied a process view of attention, and investigated whether the two groups differ with regard to the way they attend over time. In a comparison of schizophrenia and depression, Egeland et al. (2003) found that subjects with schizophrenia performed better as performance became more automatic, while subjects with depression suffered from fatigue.

Studies focussing on sustained attention in schizophrenia or ADHD separately are often difficult to compare because they typically apply different methods. Although continuous performance tests (CPTs) are among the most popular tests applied, different versions are often used for the two groups. The CPT versions used in studies of schizophrenia typically require the subjects to uphold vigilance to a multitude of stimuli, while responding only to a minority of them. The “Identical Pairs CPT” (Cornblatt, Lenzenweger, & Erlenmeyer-Kimling, 1989), the “Degraded Stimuli CPT” (Nuechterlein, Edell, Norris, & Dawson, 1986) and “3-7 CPT” (Harvey et al., 1990) are examples of such low-signal-to- noise tests frequently used in schizophrenia research. The CPTs used in studies of ADHD on the other hand, typically require subjects to respond almost continuously (Riccio, Reynolds, & Lowe, 2001). The former tests are sensitive to attention dwindling off, while the latter tests are sensitive to difficulties inhibiting response impulses. The differences in methodology reflect presuppositions of an inergetic attention deficit in schizophrenia and a hyperactive/impulsive attention deficit in ADHD. There is however, a risk that this assumption could be self-fullfilled.

In the present study, we compare a group of young subjects with first episode schizophrenia and a group of adolescent and adult subjects with ADHD on the same attention measures. As other studies have failed to find differences between these groups, we expect that an overall measure of attention will not differentiate the groups. However, the subjects are expected to score in the impaired range compared to published norms. Such findings would testify to impaired attention in both ADHD and schizophrenia. By distinguishing between the types of inattentive errors and by applying a process view of attention we expect to be able to differentiate between the type of attention deficit characterizing the combined version of ADHD, the inattentive version of ADHD and schizophrenia. This analysis was motivated by our findings in a previous study of the Conners’ CPT (CCPT). In that study (Egeland & Kowalik-Gran, submitted for publication) we factor-analyzed the CPT performance of 368 persons with different neuropsychiatric and organic disturbances, finding no evidence of a “g-factor”, i.e. no general attention dimension. Instead, we found evidence of five distinct dimensions of attention with specific clinical groups differing in factor-profile. In the present study, we go beyond factor-scores, which represent aggregates of several measures, and investigate whether the three clinical groups known to be impaired in attention, can be differentiated more specifically to error type and process measures specifically.

We expect that subjects with the inattentive type of ADHD will have a larger deficit in focused attention. In the factor analysis study, the number of omissions and variability of reaction time to stimuli, loaded on this factor. Thus, we expect subjects with ADHD-I to be most impaired with regard to these two measures. While ADHD-I subjects exhibit lethargic inattention, we expect impaired inattention in subjects with ADHD-C to be mediated by hyperactivity and impulsivity. The factor analysis indicated that number of false positive errors (commissions) and a fast reaction time, measured this. Consequently we expect the ADHD-C-group to differ with regard to these two measures. At the process level, we expect that subjects with ADHD-I will be increasingly more inattentive during the test, i.e. they will have a specific deficit in sustained attention compared to the two other groups. Conversely, we expect that the subjects with ADHD-C will commit an increasing number of impulsive false positive errors as a function of time on task. Finding an increased number of omission errors among subjects with ADHD-I and commission errors among subjects with ADHD-C could be interpreted as two different mechanisms mediating the sustained attention deficit evident in the two groups. While lack of energy limits performance of the former group, increasing restlessness disturbes the performance of the latter. Egeland et al. (2003) found that the schizophrenia group was impaired in focused attention, but not specifically in sustained attention. Instead of becoming fatigued or restless, they profited from practicing the task, becoming more attentive as time passed. The present study can be viewed as a replication the Egeland et al. study (2003) using another CPT that is particularly sensitive to errors of commission. Nevertheless, the expectation is still that the subjects with schizophrenia will perform better as a function of time on task.

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