General introduction 31 1 in the domains of attention, processing speed, (episodic) memory, executive functions, and verbal learning (Bortolato et al., 2015). Patients in a manic state have additional impairment of verbal learning, as patients with depression show more phonemic fluency impairment. Patients with bipolar disorder show on average between 0.6 and 0.9 standard diviations on neuropsychological test, lower than that in healthy controls, with letter fluency and cognitive flexibility figuring prominently (Bortolato et al., 2015). ‘Schizophrenia’: Schizophrenia can present itself as a combination of delusions, hallucinations, and extremely disordered thinking and behaviour. The symptoms are present for at least sixmonths (unless it is treated). Next to these ‘positive’ symptoms, there can be signs of ‘negative’ symptoms where the patient exhibits low activity and/or initiative and is not able to function normally. The American Psychiatric Association describes that deficits in declarative memory, workingmemory, language function, executive functions, and processing speed can occur in schizophrenia. Cognitive symptoms are a core symptom of schizophrenia (American Psychiatric Association, 2013), which are often present before the first episode and persist after remission (Quisenaerts, Morrens and Sabbe, 2013; Bortolato et al., 2015). Up to 70% of the patients will show cognitive impairment (O’Carrol, 2000). In addition to bipolar disorder, there is increasing evidence for the existence of cognitive within-group heterogeneity with clusters of severe impairment, mild-to-moderate impairment, and relatively intact cognitive functioning (van Rheenen 2017). Often, general cognitive disorders are already visible, especially working memory and attention, before there are psychotic characteristics, and cause the well-known decrease in social functioning (Reichenberg, 2010). They even interact with daily living more than positive or negative symptoms (Green, Kern and Heaton, 2004). This can and will interact with diagnostic certainty. To make it more complicated, as we mentioned earlier, this is also true for the early BPSD signs that can appear before cognitive symptoms in neurodegenerative diseases (Lyketsos et al., 2002). In schizophrenia, cognitive impairment is often between one or two standard deviations or more in multiple domains (Bortolato et al., 2015; Van Rheenen et al., 2019), but the most frequently noted deficits are in the domains of working memory and attention. There appears to be a modest association between positive symptoms and neuropsychological test outcomes (de Gracia Dominguez et al., 2009). However, a link exists between cognitive disorders and general (social) functioning (O’Carrol, 2000).
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