Contrasts in memory functions between adolescents with schizophrenia or ADHD
Introduction
Memory and learning impairments have been widely studied in schizophrenia and are regarded as a genuine neuropsychological deficit pattern associated with the disorder [37], [47], [48], [58], [59]. Some investigators have proposed that disruption in attention, motivation, and higher executive functions may serve as causal factors in the deficient performance on memory tasks in patients with schizophrenia [21], [29], [53].
Long-term memory in schizophrenia has been found to be mildly [41] or more substantially affected [11], [12], [14], [16], [22]. Short-term memory has been found to be intact by some investigators [41], [62] and impaired by others [13], [22], [45], [49]. In some studies, short-term memory in patients with schizophrenia has been found to be more impaired than in normal controls by the introduction of irrelevant auditory stimuli [50], [51], [54]; in others, not [26], [55], [56]. Short-term memory has been incorporated in a wider concept called working memory [5]. Working memory consists also of a central executive which monitors the functioning of short-term memory systems. In a review of neurocognitive deficits in schizophrenia Green [30] concludes that subjects with schizophrenia have well-documented deficits in several types of memory including explicit and working memory.
Most studies comparing verbal and visual memory in patients with schizophrenia have found comparable deficits in both modalities [42], [47]. Some indicate that verbal memory is more impaired than visual memory in patients with schizophrenia [37], [59].
Findings in the literature on recognition performance are mixed. Some investigators have concluded that patients with schizophrenia have impaired recall and normal recognition relative to normal controls [8], [41]. Others have found recognition deficits in patients with schizophrenia [27], [48], especially when more severely disturbed patients were assessed [11], [12], [15], [16]. Paulsen et al. [52] criticised most studies of learning and memory in schizophrenia in that only global measures of performance were used. These researchers [52] used the California Verbal Learning Test (CVLT) [23] which allows for the assessment of different aspects of verbal learning and retention. Adult patients with schizophrenia performed worse than normal comparisons on all learning, recall, and recognition memory measures [52].
As follows from the above review of the literature on schizophrenia and memory deficits, many questions still remain unanswered in spite of considerable research interest. While it has been established that adults with schizophrenia demonstrate memory deficits, relatively little effort has been directed at examining memory functioning in adolescents with schizophrenia. The first aim of the present study was to compare memory performance in adolescents with schizophrenia to a normal comparison group, in order to investigate whether memory deficits exist early in the course of schizophrenia. By studying adolescents with first-episode schizophrenia, we reduce the confounding influence of such variables as chronic neuroleptic treatment, disease process, and institutionalisation which often are factors in the study of adults with schizophrenia.
As stated, much of the previous research on memory and schizophrenia has relied mainly on tests measuring global memory functions, and has failed to show differences between different aspects of memory. A second purpose of the present study was to examine adolescents with schizophrenia on a comprehensive battery of memory tasks chosen to assess a broad range of memory functions, including measures of working memory, verbal learning and visual memory.
The majority of studies on memory function in patients with schizophrenia lack a relevant neuropsychiatric comparison group. Patients with schizophrenia have tended to perform significantly more poorly in comparison with affectively disturbed patients on measures of memory, suggesting that memory deviance is specific to the schizophrenia group [28], [32], [33], [41], [44], [50], [60].
Patients with Attention Deficit Hyperactivity Disorder (ADHD) are another neuropsychiatric group with cognitive dysfunctions as major symptoms. While some investigators have found no evidence of deficits in short-term memory in patients with ADHD [24], [38], others report impaired scores [2], [7], [9], [10], [20], [35]. Benezra [9] found few differences between hyperactive and normal subjects on visual and visual-spatial memory tasks. ADHD patients have been found to recall fewer words than controls on the CVLT [46].
Subtle attentional impairments play a central role in both schizophrenia and ADHD. Structural brain lesion has not been identified in either disorder, but dysfunctional frontal-subcortical systems subserving attention have been implicated in both schizophrenia [4] and ADHD [17]. By studying different neuropsychological components of the memory system we seek to elucidate the nature of cognitive dysfunctions in both disorders. This may eventually provide hypotheses about the neurochemical substrates as well as the neurodevelopmental pathophysiology of the two disorders. A final purpose of the present study was to compare these two groups on a comprehensive memory test battery.
Section snippets
Subjects
The study included 19 adolescents (ages 13–18) with a DSM-III-R [3] diagnosis of a schizophrenic disorder, 20 adolescents (ages 11–18) with a DSM-III-R diagnosis of ADHD, and 30 healthy adolescents (ages 12–18). Eleven of the adolescents with schizophrenia were recruited from the National Centre for Child and Adolescent Psychiatry (NCCAP) in Oslo, Norway, and eight patients were recruited from other hospitals in the region. Fifteen were inpatients. They had been hospitalised for approximately
Results
The means and SDs for all test variables are shown in Table 2. A significant main effect of group was found on the test battery (Wilks’ λ=0.47, F=2.73, df=18,108, P=0.001), and the results from the follow-up univariate ANCOVAs are listed in the table. The results on the CVLT recognition score did not change significantly after correcting for false positive errors. A trend toward impaired Digit Span Backward scores in the ADHD group disappeared when controlling for age differences between
Discussion
This study offers support for the presence of a variety of memory abnormalities in schizophrenia early in the course of the disease. This is consistent with the finding that adults with first-episode schizophrenia show deficits in memory [59], and that memory impairments are present in acute patients, not only in chronic patients [22], [31], [48].
Impairments were observed both in visual and verbal memory tasks. This is not in agreement with Saykin et al.’s [59] findings of selective memory
Acknowledgments
This study was supported by grants from the Norwegian Research Council (No. 377.94/013) and from Haldis and Josef Andresen’s Foundation, Anders Jahre’s Fund for the Promotion of Science, Eilertsen’s Legacy, and the Nansen Fund.
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