A newer alternative, the unitary model, draws from the tradition of Bleuler (1911/1950), who attempted to identify what he considered to be the single fundamental abnormality in schizophrenia. Bleuler defined the fundamental symptoms of schizophrenia as those caused by a ‘loosening of associations’, which were present in all patients, tended to occur only in schizophrenia and therefore were pathognomonic of the illness.
Thus, the unitary model stresses that there is a single unifying construct which explains the heterogeneity of the phenotype of schizophrenia. This model proposes that schizophrenia has one fundamental problem in a basic cognitive process.
One recent unitary model suggests that the fundamental deficit is a disruption of the fluid co-ordination of mental activity, called ‘cognitive dysmetria’ (Andreasen et al. 1996, 1998, 1999; Andreasen 1997, 1999). Synchrony refers to the normal fluid processing of information required during thought and speech, and ‘dysmetria of thought’ (Schmahmann 1991) or ‘cognitive dysmetria’ conveys its disturbance. This disruption would manifest itself in cognition, emotion and behaviour. Thus, the phenotype is defined by a unitary cognitive abnormality (cognitive dysmetria), which leads to the varied symptoms experienced by patients. This abnormality, not the symptoms, should be used to define the phenotype of the illness.
According to this unitary model, schizophrenia is a neurodevelopmental and cognitive disorder with an aetiology reflecting the interaction of genetics and environment and a pathophysiology consisting of the resultant abnormalities of brain development (which continues into early adulthood). Similar to the ‘multiple hit’ model of cancer, the unitary model suggests that the development of schizophrenia results from multiple aetiological factors leading to a shared pathophysiology and neurobiology in all people with schizophrenia. These complications in brain development from conception to early adulthood lead to disruptions in anatomic and functional connectivity, resulting in neural ‘misconnections’ expressed as ‘cognitive dysmetria’ and ultimately manifested as symptoms such as flattened affect, avolition, disorganized thought, hallucinations or delusions.
Several other theories have also attempted to explain the symptoms of schizophrenia via impairment in a single underlying cognitive process. These are the theories of willed action (Frith 1992), working memory (Goldman-Rakic 1994) and information processing and attention (Braff 1993).
Frith proposed that the general mechanism underlying the symptoms in schizophrenia is a disorder of consciousness or self-awareness, impairing one’s ability to think with ‘metarepresentations’ (higher order abstract concepts which are representations of mental states) (Frith 1992). The impairment in self-awareness is the basis for three features of schizophrenia.
1 A disorder of willed action which leads to an inability to generate spontaneous or willed acts, resulting in poverty of action, perseveration or inappropriate action.
2 A disorder in self-monitoring. An inability to monitor willed intentions may lead to delusions of control, auditory hallucinations or thought insertion.
3 A disorder in monitoring the intentions of others, which leads to delusions of reference or paranoid delusions.
Goldman-Rakic (1994) suggested that another cognitive operation, working memory, may be disrupted in schizophrenia, leading to behavioural disorganization and certain positive and negative symptoms. Working memory is defined as a memory system in which items are held ‘on-line’ while needed and then discarded after use. There is evidence that the prefrontal cortex is involved in the organization and processing of this transitory memory system. Dysfunction in one or more components of working memory could result in the diverse symptoms of schizophrenia. Alogia would result from impaired information retrieval. The inability to hold a concept in mind may lead to thought disorder. A failure to reference internal or external stimuli against established memories may lead to misrepresentations of causality, resulting in delusions and hallucinations.
Thus, the prefrontal cortex and its circuitry may be disrupted in schizophrenia, leading to impairments in working memory which surface as some of the positive symptoms and negative symptoms.
Braff (1993) hypothesized that patients with schizophrenia may have a fundamental deficit in information processing and attention. Patients are unsuccessful in allocating attentional resources to relevant tasks while inhibiting attention towards irrelevant stimuli. Subjectively, they may report trouble ‘focusing’ and feeling overwhelmed by external stimuli. On a wide variety of experimental paradigms to assess information processing and attention (such as prepulse inhibition, P50 gating, ocular motor function, continuous performance task and event-related potentials), patients with schizophrenia are less able to process information rapidly and efficiently, particularly in the context of distractions, high processing loads and multiple tasks.
R.L.M. Fuller, S.K. Schultz and N.C. Andreasen
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