จากความบกพร่องทางปัญญาในโรคจิตเภทการวิจัยที่สำคัญได้เน้นการทำงานหน่วยความจำที่กำหนดไว้มักจะเป็นความสามารถในการในกรณีที่ไม่มีการชี้นำทางประสาทสัมผัสของความบกพร่องทางปัญญาในโรคจิตเภทการวิจัยที่สำคัญได้เน้นการทำงานหน่วยความจำที่กำหนดไว้มักจะเป็นความสามารถในกรณีที่ไม่มีการชี้นำทางประสาทสัมผัสเพื่อ transiently รักษาและจัดการจำนวนจำกัด ของข้อมูลเพื่อเป็นแนวทางในการคิดหรือพฤติกรรม(Barch และสมิ ธ , 2008) Of the cognitive impairments in schizophrenia, substantial research
has focused on working memory, typically defined as the ability
in the
absence of sensory cues,
Of the cognitive impairments in schizophrenia, substantial research
has focused on working memory, typically defined as the ability, in the
absence of sensory cues, to transiently maintain and manipulate a
limited amount of information in order to guide thought or behavior
(Barch & Smith, 2008). Performance on working memory tasks
depends, at least in part, upon the neural circuitry of the dorsolateral
prefrontal cortex (DLPFC; Miller & Cohen, 2001), and in schizophrenia,
the DLPFC exhibits altered activation during working
memory tasks (Van Snellenberg et al., 2006; Deserno et al., 2012).
These impairments in working memory function and DLPFC activation
are present in both medicated and unmedicated subjects, in both
early and chronic phases of the illness, and in a manner that cannot be
attributed to non-specific factors such as lack of effort or interest
(Barch & Smith, 2008).
Cognitive deficits as the core feature of schizophrenia
The marked disruptions in managing interpersonal relationships and in
navigating the challenges of life in society that accompany the typical
emergence of psychosis during late adolescence or early adulthood
usually lead to the first clinical encounter and subsequent diagnosis of
schizophrenia. However, impairments in working memory and other
types of cognitive processes may be present for years prior to the
diagnosis of the illness (Lesh et al., 2011). Indeed, cognitive
impairments have been found throughout the life span of affected
individuals, including during childhood and adolescence as well as at
the initial onset of psychosis (Davidson et al., 1999; Cosway et al.,
2000). In addition, in individuals diagnosed with schizophrenia,
cognitive deficits occur with high frequency, are relatively stable over
time, and are independent of psychotic symptoms (Keefe & Fenton,
2007). Although, on average, individuals score 1.5–2 standard
deviations below normative means on tests of cognition (Keefe &
Fenton, 2007), some individuals diagnosed with schizophrenia (e.g.
those with prominent paranoid features) may have relatively preserved
cognitive function. Perhaps most importantly, the degree of cognitive
impairments, and not the severity of psychosis, is the best predictor of
long-term functional outcome for affected individuals (Green, 1996).
The unaffected relatives of individuals with schizophrenia also exhibit
similar, although milder, cognitive deficits (Egan et al., 2001),
suggesting that cognitive abnormalities reflect the genetic risk for
the illness. The combination of these findings has lead to the view that
cognitive deficits are the core abnormalities of the illness that set the
stage for the later emergence of psychosis.
The evidence that cognitive impairments are present, and in at least
some individuals are progressive, before the onset of psychosis
suggests that they may reflect an early and ongoing pathogenic
process. For example, as early as the 4th grade of school, individuals
who would later be diagnosed with schizophrenia scored, on average,
more poorly than their peers on standardized tests of scholastic
performance, and in many domains their performance relative to peers
worsened through the school years (Keefe & Fenton, 2007). Similarly,
mean childhood IQ scores were lower in individuals who grew up to
meet diagnostic criteria for schizophrenia relative to comparison
groups composed of either those who were later diagnosed with major
depression or those who did not develop a psychiatric illness
(Reichenberg et al., 2010). In particular, relative to both of these
comparison groups, individuals later diagnosed with schizophrenia
failed to show the normal degree of improvement in working memory
between ages 7 and 13 years (Reichenberg et al., 2010).
The findings that working memory impairments are a core feature of
schizophrenia, that they appear to be present and progressive during
childhood–adolescence, and that they are associated with abnormal lities
Alterations in DLPFC circuitry in schizophrenia
Individuals diagnosed with schizophrenia have smaller whole brain
volumes in the prodromal stage, at the first episode of psychosis, and
during the chronic phase of the illness (Lawrie & Abukmeil, 1998;
Steen et al., 2006; Levitt et al., 2010). In addition, young individuals
who, by virtue of having an affected first-degree relative carry an
elevated genetic risk for schizophrenia, have greater reductions over
time in the volumes of the prefrontal and temporal lobes and, in some
but not all studies, the decline in prefrontal volume was most marked
in those eventually diagnosed with schizophrenia (Pantelis et al.,
2003; Borgwardt et al., 2007; Sun et al., 2009; McIntosh et al., 2011;
Mechelli et al., 2011). Consistent with these findings, individuals with
childhood-onset schizophrenia show an increased rate of the decline in
prefrontal gray matter that normally occurs during adolescence (Giedd
& Rapoport, 2010).
Smaller prefrontal gray matter volumes in individuals diagnosed
with schizophrenia appear to be due to less cortical neuropil (i.e. small
dendritic shafts, dendritic spines, axons and axon terminals; Selemon
& Goldman-Rakic, 1999), and not to fewer cortical neurons (Akbarian
et al., 1995; Thune et al., 2001). The reduced neuropil in the DLPFC
appears to reflect fewer axon terminals, as suggested by findings in
post mortem studies of schizophrenia of lower levels of proteins
present in axon terminals (Glantz & Lewis, 1997), and of fewer
dendritic spines (Garey et al., 1998; Glantz & Lewis, 2000). These
alterations appear to be particularly pronounced in layer 3. For
example, in the DLPFC of subjects with schizophrenia, basilar
dendritic spine density was significantly lower on deep layer 3
pyramidal cells relative to both normal and psychiatrically ill
comparison subjects, but spine density was only modestly lower on
superficial layer 3 pyramidal neurons and unchanged on pyramidal
neurons in layers 5 and 6 (Glantz & Lewis, 2000; Kolluri et al., 2005).
Consistent with these findings, the mean somal volume of layer 3
pyramidal neurons was smaller in subjects with schizophrenia in the
DLPFC and in other cortical regions (Arnold et al., 1995; Rajkowska
et al., 1998; Pierri et al., 2001; Sweet et al., 2003), whereas the
volume of layer 5 pyramidal neurons was unchanged (Sweet et al.,
2004). Importantly, none of these findings appeared to be attributable
to medication use or length of illness (Lewis & Gonzalez-Burgos,
2008).
In addition to these alterations in excitatory pyramidal neurons,
multiple studies have reported alterations in markers of inhibitory caminobutyric
acid (GABA) neurotransmission. For example, lower
levels of the mRNA for the 67-kDa isoform of the GABA synthesizing
enzyme glutamic acid decarboxylase (GAD67) have
been consistently found in the DLPFC of subjects with schizophrenia
(Gonzalez-Burgos et al., 2010). This deficit in GAD67 mRNA
appears to be particularly pronounced in the subset of DLPFC GABA
neurons that express the calcium-binding protein parvalbumin (PV), as
about 50% of PV neurons lacked detectable levels of GAD67 mRNA
in individuals with schizophrenia (Hashimoto et al., 2003). Importantly,
the number of PV neurons in the DLPFC appears to be
unchanged (Lewis et al., 2005), although the expression of PV mRNA
per neuron is decreased (Hashimoto et al., 2003). The latter finding, in
addition to methodological confounds, appears to explain the reports in some studies of a lower density of cortical PV-immunoreactive
neurons in schizophrenia (Stan & Lewis, 2012).
PV neurons can be subdivided into two major classes based on the
principal target of their axon terminals. The axon terminals from the
basket cell class of PV neurons target the cell body and proximal
dendrites of pyramidal neurons. Both pre- and postsynaptic alterations
in PV basket cell–pyramidal cell connectivity in the DLPFC,
especially in layer 3, appear to be present in schizophrenia. First,
the density of PV-labeled axon terminals, presumably from basket
neurons, is reduced in DLPFC layer 3 in schizophrenia (Lewis et al.,
2012). Second, the level of GAD67 protein is markedly lower in these
terminals (Curley et al., 2011), suggesting that basket cells represent
the population of PV neurons with undetectable levels of GAD67
mRNA (Hashimoto et al., 2003). Third, mRNA expression of the
GABAA receptor a1 subunit, which is postsynaptic to PV basket cell
inputs, is preferentially lower in layer 3 in schizophrenia (Beneyto
et al., 2011), and this deficit is selective for pyramidal neurons and is
not present in GABA neurons (Glausier & Lewis, 2011).
The other major class of PV neurons, chandelier cells, gives rise to
axon terminals that form distinctive vertical arrays, termed cartridges,
which exclusively target the axon initial segments (AIS) of pyramidal
neurons. In the DLPFC of subjects with schizophrenia, the density of
cartridges immunoreactive for the GABA membrane transporter
(GAT1) is lower than comparison subjects (Woo et al., 1998),
whereas the density of pyramidal cell AIS immunoreactive for the
GABAA receptor a2 subunit, the dominant GABAA receptor a subunit
present in pyramidal cell AIS in layer 3, is markedly increased (Volk
et al., 2002). In addition, the density of AIS immunoreactive for
ankyrin-G, a protein that plays a key role in the structure and plasticity
of the AIS, is also lower in the DLPFC in schizophrenia (Cruz et al.,
2009). Importantly, both the chandelier and PV basket cell alterations
appear to be specific to the disease process of schizophrenia as they
are not observed in individuals with other psychiatric disorders or in
monkeys
การแปล กรุณารอสักครู่..