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8/11/2019 Intellectual Functioning and Memory Deficits in Schizophrenia
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Intellectual functioning and memory deficits in schizophrenia
Juan Carlos Ruiza,4, Mara Jose Solerb, Inma Fuentesc, Pilar Tomasd
aDepartment of Methodology, Facultad de Psicologa, University of Valencia, 46010 Valencia, SpainbDepartment of Basic Psychology, University of Valencia, 46010 Valencia, Spain
cDepartment of Personality, Evaluation, and Psychological Treatments, University of Valencia, 46010 Valencia, Spain
dCenter for Rehabilitation and Social Integration, Department of Social Welfare of the Regional Government of Valencia, 46001 Valencia, Spain
Abstract
Background: There is converging evidence about the existence of different subgroups of patients with schizophrenia in
relation to intellectual ability (intelligence quotient [IQ]). Studying cognitive deficits in such patients in relation to IQ, and more
specifically to memory, could help determine the patterns of preserved and impaired functioning in cognitive abilities in association withpatterns of preserved and compromised intellect. This information could serve to delimit the possibilities of treatment and rehabilitation in
those patients.
Methods: A total of 44 patients with schizophrenia completed a cognitive battery that included executive functioning, attention,
speed of information processing, working memory, explicit memory, implicit memory, and everyday memory. Their IQ was also measured
to identify 2 subgroups with an IQ of 85 as the cutoff point. Then, differences between the groups in the neurocognitive measures
were studied.
Results: Performance in executive functioning, attention, working memory, and everyday memory, but not that in speed of infor-
mation processing, explicit memory, and implicit memory, was associated with intellectual functioning. Patients performed at the same level
in perceptual implicit memory but at a lower level in conceptual implicit memory as did healthy control subjects.
Discussion: Cognitive deficits in schizophrenia are associated with intellectual functioning. Implicit memory should not be considered as a
unique entity. It is suggested that conceptual implicit memory deficit may be a core feature of schizophrenia.
D 2007 Elsevier Inc. All rights reserved.
1. Introduction
Memory deficits have been considered as a specific
dysfunction in schizophrenia[1,2]. Most empirical evidence
on such deficits come from studies that compared the
performance of patients with schizophrenia with that of
healthy control subjects in explicit verbal and nonverbal
recall and recognition, working memory, and semantic
memory. Their neural mechanisms have also been estab-
lished[3]. More recently, a specific pattern of dissociations
in schizophrenic memory functioning has been found with
impairment in episodic long-term memory and workingmemory coupled with preservation in areas such as short-
term memory, especially verbal short-term memory, and
procedural memory[2,4].
However, the literature has given less attention to the
study of everyday memory impairment and implicit memory
deficits in these patients.
According to McKenna et al [4], poor performance in
some memory forms could reflect the schizophrenic
tendency to general intellectual impairment. In fact, memory
deficits in schizophrenia commonly occur within some
degree of general intellectual impairment when intelligence
is measured using traditional psychometric instruments (ie,
an intelligence quotient [IQ] test). Consequently, the scores
on all cognitive tests could be depressed to some extent and
the memory performance pattern could be obscured.
However, results about the magnitude of intellectual
deficit in patients with schizophrenia do not show a clearpattern. Some studies have reported that patients with
schizophrenia have a lower IQ in comparison with healthy
control subjects, whereas other studies have shown that
intellectual decline after the onset of schizophrenia is not
general and that a relevant proportion of patients have a
normal IQ[5,6]. For example, in a cognitive evaluation of
221 patients with schizophrenia, Goldstein and Shemansky
[7]observed that 26% of the sample showed a normal IQ.
Similarly, Weickert et al[8]reported that, of 11 patients with
0010-440X/$ see front matterD 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.comppsych.2006.11.002
4 Corresponding author. Tel.: +34 963 864 414; fax: +34 963 864 697.
E-mail address: [email protected] (J.C. Ruiz).
Comprehensive Psychiatry 48 (2007) 276282
www.elsevier.com/locate/comppsych
8/11/2019 Intellectual Functioning and Memory Deficits in Schizophrenia
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schizophrenia consecutively admitted to a mental health
center, 24.8% had an IQ within normal limits. More
recently, in a sample of 109 patients, Badcock et al [9]
found that 41% had preserved intellectual functioning.
Parallel literature had also shown that patients with a
normal IQ also have substantial cognitive deficits [10],
although results do not always coincide. For example,
Rund et al [11] found that a high intellectually functioning
group of patients with schizophrenia who have IQ scores
between 100 and 125 scored lower than did control
subjects in executive functioning, visual memory, and
verbal memory. However, Weickert et al [8]only observed
bmild impairment in executive function and, possibly,
attention and encoding.Q
The heterogeneity of results observed in the general
intellectual functioning of patients has been afforded
through 2 strategies. The first strategy involves controlling
the influence of IQ on memory impairments using only
subjects with intact cognitive functions. Although this
strategy helps eliminate the effect of intellectual function-ing on different cognitive functions, it does not indicate
how memory deficits are related to intellectual deficit in
schizophrenia [10,11]. The second strategy is establishing
different subgroups of subjects using IQ level as the
criterion. This procedure could help elaborate the role that
intellectual functioning plays in cognitive functioning and
particularly in memory impairments [11]. If memory
performance is related to IQ, then we can expect different
scores on memory tests from patients who have a low IQ
and from those with a normal IQ. Studying these differ-
ences could make determining which memory aspects are
jointly impaired and which are uniquely affected for each
IQ group possible.
To draw more firm conclusions regarding the magnitude
and specificity of memory dysfunction in schizophrenia, we
analyzed the role of IQ in these deficits, assessing a wide
range of memory areas and the cognitive domains of
executive functioning, attention, and speed of information
processing. We examined the profile of the performance of
patients with schizophrenia evaluating different forms of
explicit and implicit memory. A battery of memory tasks
was administered, including explicit tests on working
memory capacity, recall and recognition tasks, implicit
memory tests such as word-fragment completion and word
production from semantic categories, and a memory test todetect and monitor everyday memory problems.
The general intellectual functioning of patients was also
measured using the Wechsler Adult Intelligence Scale
(WAIS-III). The IQ scores were used to classify subjects
in 2 groups: patients who were intellectually intact by
having IQ levels within 1 SD of the population mean
(IQ scores z85) and those who were intellectually impaired
by having IQ levels lower than 85. To determine the
possible influence of IQ on memory performance, we
compared the patterns of results of both patient groups on
the different cognitive and memory measures.
2. Subjects and methods
2.1. Participants
Forty-four outpatients from the Center for Rehabilitation
and Social Integration in Valencia, Spain, participated in the
study. They had a diagnosis of schizophrenia according to
Diagnostic and Statistical Manual of Mental Disorders,
Fourth Edition criteria. All the subjects provided written
informed consent before participating in the study. The
demographic andclinical characteristics of the patients are
shown inTable 1. All the patients were treated with typical
or atypical antipsychotic medication. A short form of the
Spanish version of the WAIS-III with documented psycho-
metric properties was administeredto estimate their current
IQ level. This short form [12] includes the following
subtests of the full scale: information, block design,
arithmetic, and digit symbols. Two groups were formed
according to the participants IQ; an IQ of 85 was used as
the cutoff point. Twenty-three subjects scored lower than
this point (mean IQ = 73.09; SD = 8.92), and 21 scoredhigher than it (mean IQ = 98.38; SD = 10.14).
2.2. Cognitive assessment
All the participants completed a battery of neuropsycho-
logic tests that had been selected to assess executive
functioning, attention capacity, speed of information pro-
cessing, working memory, explicit and implicit memory,
and everyday memory. All these tests were administered and
scored by trained psychologists.
The Wisconsin Card Sorting Test (WCST) [13] was
administered as a test of executive function. The variables
analyzed from this test were the number of categories
achieved, total trials to resolve the task, total errors, and
perseverative errors. Attention and speed of information
processing were assessed using the digit symbol substitution
and picture completion subtests of the WAIS-III. These
tasks require some level of attention, motor coordination,
and visual tracking.
The arithmetic calculation and digit span (forward and
backward) subtests of the WAIS-III were administered as
tests of working memory.
Explicit memory was measured with tests that assess
acquisition, storage, and retrieval deficits. The acquisition
mechanism was assessed with a general learning paradigm
(the immediate and delayed free recall of a short story) usingthe immediate and delayed subtests of the Rivermead
Table 1
Sociodemographic characteristics of all patients
IQ b85 (n = 23) IQ z85 (n = 21)
Sex (male/female) 20:3 20:1
IQ (mean F SD) 73.09 F 8.92 98.38 F 10.14
Age (y; mean F SD) 34.30 F 7.84 41.00 F 8.07
Education (y; mean F SD) 6.57 F 4.91 9.95 F 4.72
Duration of illness
(y; mean F SD)
15.39 F 8.71 19.33 F 8.38
J.C. Ruiz et al. / Comprehensive Psychiatry 48 (2007) 276282 277
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Behavioral Memory Test (RBMT) [14]. The storagemechanism was assessed using the information subtest of
the WAIS-III. This subtest is a recall test of information
learned over a delayed period. Finally, the retrieval
mechanism was measured using the immediate nonverbal
recognition task of the RBMT.
Implicit memory was assessed using a perceptual test
(word-fragment completion) and a conceptual test (word
production from semantic categories). In these tasks,
participants have to process a stimulus called a prime (eg,
to judge the familiarity of a list of words using a scale).
Then, they are told to perform another task (eg, to complete
word fragments or to generate words belonging to a specificcategory) that makes no reference to the first phase of the
test. The variables of interest in those memory tasks were
the proportion of correctly answered stimuli that are seen in
the first phase and the proportion of correctly answered
stimuli that are not seen in the first phase. The difference
between these 2 proportions is the priming score, which
reflects implicit memory.
Finally, everyday memory aspects were measured with
the RBMT. Performance on this test yields a total profile
score (range = 0-24), which weights each subtest equally in
relation to the others to provide comparable scores across
Table 2
Patients test scores in the neurocognitive measures and results of comparisons between the whole group, the low IQ subgroup (IQ b85), and the high IQ
subgroup (IQ z85) with normative data mean values
Normative data Whole group Low IQ subgroup (IQ b85) High IQ subgroup (IQ z85)
Mean Mean F SD t P Mean F SD t P Mean F SD t P P4
Executive functioning
WCST categories 5.57 3.84 F 2.11 5.37 .001 3.00 F 1.98 6.10 .001 4.71 F 1.93 2.04 .055 .006
WCST numberof trials
84.22 113.40 F 21.48 8.91 .001 123.45 F 11.55 15.93 .001 102.86 F 24.51 3.48 .002 .002
WCST total errors 17.05 45.16 F 26.32 7.01 .001 54.00 F 20.47 8.47 .001 35.90 F 28.95 2.98 .007 .024
WCST perseverative
errors
8.78 25.86 F 18.85 5.94 .001 30.77 F 14.41 7.16 .001 20.71 F 21.76 2.51 .021 .080
Attention and speed of information processing
WAIS-III picture
completion
10.7 8.00 F 3.18 5.64 .001 6.43 F 2.59 7.90 .001 9.76 F 2.68 1.60 .125 .005
WAIS-III digit
symbol substitution
10.6 6.75 F 2.19 11.66 .001 5.52 F 1.95 12.48 .001 7.76 F 2.02 6.43 .001 .001
Working memory
WAIS-III arithmetic
calculation
10.4 7.98 F 2.88 5.59 .001 6.30 F 1.69 11.62 .001 9.57 F 2.50 1.52 .145 .003
WAIS-III digits
(forward and backward)
10.3 8.82 F 3.00 3.27 .002 7.17 F 2.33 6.44 .001 10.57 F 2.84 0.44 .666 .001
Explicit memoryRBMT immediate
verbal recall
1.81 1.67 F 0.65 1.43 .161 1.52 F 0.73 1.89 .072 1.84 F 0.50 0.28 .783 .179
RBMT delayed
verbal recall
1.88 1.79 F 0.47 1.30 .201 1.70 F 0.47 1.88 .074 1.89 F 0.46 0.14 .890 .020
RBMT immediate
nonverbal recognition
1.90 1.86 F 0.35 0.78 .440 1.83 F 0.49 0.72 .478 1.84 F 0.37 0.67 .509 .529
WAIS-III information 10.5 9.70 F 3.22 1.64 .108 7.43 F 2.23 6.59 .001 12.00 F 2.45 2.81 .011 .001
Implicit memorya
WFCT studied
word fragments
62.18 57.14 F 18.14 1.57 .121 53.91 F 18.55 1.92 .066 60.90 F 17.92 0.29 .772 .234
WFCT nonstudied
word fragments
32.14 27.44 F 12.23 1.72 .090 22.28 F 10.47 3.23 .002 32.33 F 11.87 0.06 .955 .007
WFCT priming 30.04 29.70 F 15.18 0.10 .919 31.63 F 15.00 0.42 .679 28.57 F 15.39 0.37 .713 .528
WPSCT exemplars
from old categories
20.42 10.16 F 9.20 4.82 .001 9.30 F 7.19 4.55 .001 11.25 F 11.31 3.17 .002 .515
WPSCT exemplars
from new categories
4.70 1.58 F 3.68 2.26 .028 0.79 F 2.51 2.86 .006 2.53 F 4.61 1.32 .191 .156
WPSCT priming 15.72 8.58 F 8.04 3.66 .001 8.51 F 7.85 3.00 .004 8.72 F 8.65 2.74 .008 .936
Everyday memory
RBMT total
profile score
22.19 18.47 F 4.11 5.99 .001 17.39 F 3.73 5.67 .001 19.53 F 4.39 2.65 .016 .102
RBMT orientation 1.88 1.47 F 0.83 3.33 .002 1.09 F 0.95 4.01 .001 1.89 F 0.32 0.20 .841 .006
WFCT indicates word-fragment completion test; WPSCT, word production from semantic categories test.a Normative data from a previous experiment presented at the VII International Symposium on Schizophrenia (March 17-18, 2005; Bern, Switzerland;
Soler MJ, Ruiz JC, Fuentes I, Tomas P. Implicit memory in patients with schizophrenia) comparing patients with schizophrenia (n = 29) and control subjects
(n = 34).
4 Between the subgroups.
J.C. Ruiz et al. / Comprehensive Psychiatry 48 (2007) 276282278
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the battery of tests. The total profile score and the scores on
the subtest of orientation of the RBMT (this subtest is a
battery of 10 questions that make reference to aspects such
as the date of the day, the name of the patient, or the age of
the patient) were used to give objective measures of
everyday memory.
3. Results
Analyses were conducted using the SPSS statistical
package for Windows (version 11, SPSS, Chicago, Ill).
All statistical analyses were based on 2-tailed tests of
significance. The Bonferroni method was applied to control
for multiple comparisons; thus, the adjusted critical Pvalue
was .008 (.05/6), because implicit memory was scored using
6 measures, with the cognitive domain having the greater
number of measures.
The 2 groups of patients with schizophrenia showed
significant differences in age (t = 2.79; P = .008) and
intelligence (t = 8.80;P= .001). There was no statisticallysignificant difference between the 2 groups in length of
illness and level of education (Table 1). Because the 2 groups
differed in age, this variable was entered as a covariate in
the analysis of between-group differences in scores on the
neurocognitive tests.
Test scores on the various cognitive tasksfor the whole
group of patients are presented in Table 2. These scores
were compared with general population norms provided that
such normative data exist using t tests. As expected, the
results revealed that, in most of the measures, patients with
schizophrenia show performance levels lower than the
normative mean values for the same age group; this wasthe case for the executive function, attention, speed of
information processing, working memory performance and
capacity, conceptual implicit memory (word production
from semantic categories test), and everyday memory total
scores and orientation subtest score. There was no differ-
ence in the explicit memory tasks of immediate and delayed
verbal recall, immediate nonverbal recognition, general
information, and perceptual implicit memory task (word-
fragment completion test).
3.1. Group comparisons
Once the general neurocognitive profile of the group of
patients was established, data were analyzed looking fordifferences within the group associated with IQ. The 2 IQ
groups were compared with the normative data and with one
another on the different measures using the v2 test orttests
depending on the nature of the variable measured or using
analyses of covariance when age was correlated with the
dependent measure considered. The mean values and
standard deviations for the 2groupson the various cognitive
measures are presented in Table 2. The low intellectually
functioning group performed at levels lower than normative
data in the executive functioning (P= .001), attention and
speed of information processing (P = .001), working
memory (P = .001), WAIS-III information measure (P =
.001), conceptual implicit memory (P = .004), and
everyday memory (P= .001) scores. The high intellectually
functioning group showed significant differences in the
number of trials (P= .002) and total errors (P = .007) of
the WCST, in the digit symbol substitution (P = .001) of
the WAIS-III, and in the conceptual implicit memory
measure (P = .008).
When both groups were compared, significant differ-
ences appeared in the 4 subscales of the WAIS-III that were
used to evaluate working memory performance and capac-
ity, attention, and speed of information processing (P V
.005). In 2 WCST scores, differences between the groups
were significant: the number of categories achieved (P =
.006) and the number of trials (P= .002). The comparison
in explicit memory measures only showed differences in the
WAIS-III information score (P = .001). Both groups
performed at the same level in the 2 implicit memory tasks,
Table 3
Correlations between IQ and neuropsychologic measures in the 2 subgroups
of patients with schizophrenia and in the whole group
IQ b85 IQ z85 All patients
r P r P r P
Executive functioning
WCST categories 0.03 .885 0.51 .017 0.47 .002
WCST number of trials 0.11 .638 0.63 .002 0.63 .001
WCST total errors 0.09 .695 0.63 .002 0.51 .001
WCST perseverative errors 0.27 .228 0.57 .007 0.47 .001
Attention and speed of information processing
WAIS-III picture
completion
0.62 .002 0.34 .136 0.67 .001
WAIS-III digitsymbol substitution
0.62 .002 0.24 .287 0.62 .001
Working memory
WAIS-III arithmetic
calculation
0.48 .021 0.79 .001 0.81 .001
WAIS-III digits
(forward and backward)
0.60 .002 0.49 .024 0.72 .001
Explicit memory
RBMT immediate
verbal recall
0.11 .634 0.26 .285 0.22 .164
RBMT delayed
verbal recall
0.10 .672 0.31 .202 0.28 .070
RBMT immediate
nonverbal recognition
0.32 .139 0.18 . 473 0.05 .732
WAIS-III information 0.71 .001 0.62 .003 0.85 .001
Implicit memoryWFCT studied
word fragments
0.11 .652 0.29 .230 0.27 .090
WFCT nonstudied
word fragments
0.26 .261 0.51 .025 0.55 .001
WFCT priming 0.05 .830 0.06 .812 0.12 .481
WPSCT exemplars
from old categories
0.14 .547 0.08 . 767 0.03 .872
WPSCT exemplars
from new categories
0.36 .107 0.11 . 662 0.08 .629
WPSCT priming 0.01 .958 0.04 .864 0.01 .970
Everyday memory
RBMT total profile score 0.03 .906 0.37 .117 0.33 .036
RBMT orientation 0.10 .649 0.26 .279 0.34 .028
J.C. Ruiz et al. / Comprehensive Psychiatry 48 (2007) 276282 279
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except in one measure, the proportion of nonstudied word
fragments completed in the perceptual implicit memory task
(P = .007). The patterns of results were quite similar
in everyday memory. Differences only appeared in the
orientation scale when it was measured using profile scores
(P = .006).
3.2. Correlations between IQ and cognitive measures
Pearsons correlations were calculated between IQ and
the cognitive measures for the whole group of patients
(Table 3). The results showed that IQ was related to all
WCST scores: categories (r = 0.47; P = .002); number of
trials (r = 0.63; P b .001); total errors (r = 0.51; P =
.001); and perseverative errors (r =0.47;P= .001). Level
of IQ and the attention measures correlated significantly:
WAIS-III picture completion scores (r = 0.67; P = .001)
and WAIS-III digit symbol substitution scores (r = 0.62;
P= .001). The same pattern of results was observed in the
working memory measures: WAIS-III arithmetic calculation
(r = 0.81; P = .001) and digits (forward and backward;r = 0.72; P= .001). In the case of explicit memory, IQ was
correlated with WAIS-III information scores (r = 0.85;
P= .001). Implicit memory did not correlate significantly
with IQ. Two of the measures used to explore everyday mem-
ory also correlated significantly with IQ: orientation (r =
0.34;P= .028) and total profile score (r= 0.33;P= .036).
Pearsons correlation coefficients were also computed to
explore the relationship between IQ and the cognitive
measures in each patient group (Table 3). There was no
difference between the correlations obtained in both groups
when the correlations were compared using a Z test. The
absence of differences between groups in the correlations
obtained could probably be a result of the small sample size
of the groups.
4. Discussion
Schizophrenia and memory dysfunction are significantly
associated. Numerous studies have demonstrated that
memory deficits are core features of schizophrenia. The
evidence is so clear that the MATRICS group included
memory deficit among the 7 separable fundamental dimen-
sions of cognitive deficit in schizophrenia[15]. In contrast,
the nature of the disorder with respect to general intellectual
functioning is more heterogeneous. Patients with schizo-phrenia could show intact, mild, moderate, or severe
intellectual impairment. As a consequence, many studies
have been conducted to clarify the profile of cognitive
deficits associated with the different intellectual impairment
levels shown by patients with schizophrenia. In general,
results show that even intellectually intact patients have
deteriorated performance in some cognitive areas: executive
functioning, attention, speed of processing, and memory.
Specifically, in the neurocognitive memory domain,
although memory is globally described as impaired in the
literature, results are heterogeneous probably because of the
different aspects or types of memory that can be measured
and because of the great number of different tasks used. In
this study, we focused on the evaluation of 4 dimensions of
memory using different tasks for each one with the aim of
clarifying which aspects of memory are and which are not
deteriorated in schizophrenia. Then, the aim was to analyze
how patterns of memory function differ in relation to
intellectual functioning.
Standard intelligence tests place a relatively heavy
emphasis on measures of memory, including semantic
memory, episodic memory, and working memory. It is then
expected that IQ will correlate with these memory measures,
and the literature confirms this statement. Even in the case
of everyday memory, a measure not included in intelligence
tests, the correlation is significant [14]. In the case of
implicit memory, not much is known about its relation with
IQ. Precisely, one of the main interests of our study was to
clarify this relation.
In the whole sample of patients, we found evidence for
distinct patterns of memory dysfunction in schizophrenia.With respect to working memory, the patients with
schizophrenia performed at significantly lower levels as
compared with normative control subjects. Many of the
studies in this area have found impairment in schizophrenia,
although few patients have shownnormal function or only a
trend toward a poorer performance[4]. The studies that show
this result of preserved working memory tend to include
control subjects who are matched for IQ, age, or education;
however, there are some studies that have found a working
memory impairment using well-controlled designs[16].
Results showed preserved explicit memory in the group.
Globally, these results are not in line with some previous
results. Studies that have used explicit tests such as free and
cued recall, verbal fluency tests, and semantic processing
tests have reported that the performance of patients with
schizophrenia is impaired [1,17,18]. However, previous
work had demonstrated heterogeneity in the results dis-
played in schizophrenia inthe wide variety of tasks used to
measure memory[19,20].
Implicit memory appeared to be preserved when percep-
tual implicit memory was measured but appeared to be
impaired when the task was conceptual. Literature always
presents implicit memory as a normal cognitive function in
schizophrenia [4]. However, differences within implicit
memory should be considered. In a previous study, wefound a dissociation in priming between healthy and patient
groups depending on the task. In the word-fragment test,
priming was similar in the 2 groups. However, in the word
production test, the priming obtained from patients with
schizophrenia was lower than that obtained from healthy
control subjects. These results could be explained because
there are obvious differences in task demands posed by
different kinds of tests. Dissociations between implicit
memory tests can be accounted for by postulating a
continuum of tests, from those requiring more perceptual
information (data-driven tests) to those requiring knowledge
J.C. Ruiz et al. / Comprehensive Psychiatry 48 (2007) 276282280
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of meaning (conceptually driven tests) [21-23]. Word-
fragment completion is a data-driven task in which the
fragments evoke the perceptual identification record of the
target. In this test, the stimulus presented during the study
phase is shown in a perceptually related form, albeit
partially. The letters in the fragment, which are drawn
randomly from the word, are poor cues for locating words in
the lexicon. Thus, perceptual overlap between study and test
stimuli is sufficient to produce priming[24].
Our data show equal priming in healthy control subjects
and patients with schizophrenia (see the meta-analysis of
implicit memory tasks with nonclinical populations in
Reference[25]), which suggests that perceptual comparison
processes are apparently not affected by the cognitive
deficits of schizophrenia.
In contrast, word production from semantic categories is
a conceptually driven task in which the information
provided in the test phase is semantically related to the
information studied and requires a response based on the
meaning of the stimulus. This task involves an elaborativeprocessing that is predominantly conceptual and, therefore,
seems to be more affected by the cognitive deficits present
in schizophrenia.
Everyday memory was also impaired. The distribution of
profile scores for patients on the RBMT ranges from 17 to
21. This means that patients with schizophrenia have a poor
everyday memory and that their performance is poorer than
that of healthynormative control subjects of a comparable
age range[26]. Previous studies with healthy control subjects
have obtained a slight correlation between the RBMT and
intelligence, although this does not account for more than
approximately 10% of the variance in performance on the
RBMT. The results of our study with patients with
schizophrenia show that, although there was a significant
correlation between intelligence and everyday memory, there
was no significant difference in this form of memory when
the 2 IQ groups were compared. The 2 IQ groups obtained
significant differences only in the orientation subtest.
Finally, results in executive functioning weresimilar to
the findings of a recent study by Hughes et al [27] with
62 patients. They found differences between the perfor-
mance of patients with schizophrenia and that of control
subjects in the number of categories achieved and the
number of perseverative errors on the WCST (see also the
review of Laws[28]). The patterns of attention and speed ofinformation processing deficits in the whole group were also
similar to those obtained in another study [8]. Patients with
schizophrenia perform worse than do healthy control
subjects. Taken together, these results are in line with those
of authors who argue that executive deficits constitute a
necessary dysfunction in schizophrenia [8,9]. Nonetheless,
this statement should be taken with caution because, as our
results show, executive functioning deficits appear to be
associated with IQ. These results are not new, and, as
Landro et al [19] indicated, significant differences between
patients with schizophrenia and healthy control subjects, on
the number of categories achieved and on the perseveration
index of the WCST, disappear when the patients are selected
with normal general intellectual function.
When the profile of memory data was analyzed
separately in the 2 IQ subgroups of patients with schizo-
phrenia, the pattern of results changed. The low intellectu-
ally functioning group performed at lower levels as
compared with the healthy control subjects, and the high
intellectually functioning group performed at the same level
in working memory. Both groups did not show differences
in comparison with normative data and with one another in
explicit memory, except in the information scale of the
WAIS-III. The low IQ group, but not the high IQ group,
showed differences with normative data. Results in the
implicit memory tasks replicated those of the whole group.
Preserved perceptual implicit memory and compromised
conceptual implicit memory patterns were similar in both
groups. However, the patterns were different in everyday
memory. The high intellectually functioning group showed a
performance level similar to that of the healthy controlsubjects, but the performance in the low intellectually
functioning group was deteriorated.
In summary, patients with an IQ higher than 85 (mean
IQ = 98) perform at a standard level in working memory,
explicit memory, everyday memory, and implicit memory
when their performance is measured with a perceptual task.
They only perform at lower-than-standard levels in implicit
memory when their performance is measured using a
conceptual task. The profile of results changes when the
patients IQ is lower than 85 (mean IQ = 73). They show
deficits in working memory, everyday memory, and explicit
memory when their performance is measured using the
information scale of the WAIS-III and in implicit memory
when their performance is measured with a conceptual task.
These mean that, although intellectual functioning appears
to be related to working memory and everyday memory, it is
not associated with explicit and implicit memory.
Despite the fact that patients with schizophrenia have key
cognitive deficits, such as executive functioning, attention,
and memory, their IQ seems to be related to all of them but not
globally to memory. Within the memory domain, there are
differences associated with the explicit-implicit dichotomy
and with the specific measure used to assess memory.Our
results are in line with the hypothesis of Reber et al[29], who
considered that the implicit systems operate largely indepen-dently of standard measures of cognitive capability, such as
intelligence. In fact, Wagner and Sternberg [30] differentiated
between tacit and explicit intelligence. Although tacit
knowledge is practical, informal, and usually acquired
indirectly or implicitly, explicit intelligence is evaluated
primarily by tasks usually found on mental ability tests.
5. Conclusions
This study presents relevant data in relation to implicit
memory and everyday memory impairment in schizophrenia
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and its relation to IQ: (1) the type of implicit memory
measured determines the presence or absence of a deficit;
(2) perceptual implicit memory is preserved in schizophre-
nia independently of the intellectual functioning level of the
patients; and (3) conceptual priming is deteriorated even in
intellectually preserved patients. These findings suggest
that, for its complete evaluation, implicit memory should be
assessed with different tasks because it is not a unique
entity. Everyday memory is also impaired in association
with IQ. It only appears to be impaired in patients with
deteriorated intellectual function.
Overall, our results have important implications for
identifying suitable treatment and rehabilitation interven-
tions. These interventions should focus on the assessment of
individual cognitive functions and on the memory capacities
demanded of patients.
Acknowledgment
This study was supported by a grant from the Ministryof Education and Science of Spain (No. SEJ2006-07055/
PSIC).
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