The manic and depressed states of bipolar disorder have a clear impact upon cognitive function (Clark and Sahakian, 2006). Cognitive deficits are also widely regarded to represent a core feature of schizophrenic illness (Green et al., 2004). Neuropsychological testing aims to characterize the domains of cognitive impairment and cognitive sparing in a disorder, with the use of standardized quantitative assessment (Lezak, 1995). In understanding the neuropsychology of psychiatric disorders, it is useful to distinguish state and trait markers of impairment (Nuechterlein et al., 1992; Clark and Goodwin, 2004). A state marker is manifested during acute episodes (such as depression) and recovers on symptom remission. A state-related deficit is likely to correlate with symptom ratings (e.g., depression ratings on the Hamilton scale). A trait marker, on the other hand, persists during periods of symptom remission and is therefore more likely to be associated with the underlying neuropathology of a disorder. A trait marker may be completely stable across phases of illness, or may be state-modulated, whereby the deficit persists in the absence of symptoms but is exacerbated during acute episodes (Nuechterlein et al., 1992).
According to the traditional Kraepelinian view of psychosis, bipolar disorder and schizophrenia represent categorically distinct disease entities, distinguished by the chronic, deteriorating course of schizophrenia contrasting with the episodic, fluctuating course of bipolar disorder (Kraepelin, 1899).
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