**2. Cognitive deficit in schizophrenia**

One of the main recently advances of the concept of schizophrenia has been the confirmation that this disorder is primarily associated to cognitive deficits, do not being a consequence to symptoms or drugs [21]. Nevertheless, the idea that cognitive domains played a fundamental role in this disorder was not so recent. Since the first descriptions of schizophrenia, which was known under the Dementia praecox label, the aspects related to what it is now understood as cognitive deficit were considered as central symptoms of the picture. Besides the essential idea of Bleuler was that the core of schizophrenia, its fundamental symptoms, was the fragmentation of the thought process and delusions and hallucinations were accessory symptoms, a consequence of the main process [22, 23]. It has also been shown that this cognitive deficit has not only been described in long-standing schizophrenic patients [24], but is also present in patients with a first psychotic episode [25–27], in remission [28], in patients without antipsychotic medication [21, 29] and even in studies in high-risk subjects [30] and in close relatives and healthy patients with schizophrenia [12].

It is estimated that among 61–78% of patients with schizophrenia manifest a significant level of cognitive deficit [1] reaching between 1 and 2 standard deviations below the control groups of the same age [6, 31]. These cognitive deficits seem independent of positive symptoms [32] and are maintained throughout the course of the disease.

Although some early investigations [33] showed that about 27% of schizophrenic patients, after neuropsychological evaluation, could not be considered deficit, in fact, it has been proven that they would continue to present neuropsychological deficit compared to healthy subjects, even matching IQ measurements [34]. This group of patients, without supposed deterioration, would show high levels of premorbid functioning, but once the disease was diagnosed, this would be considerably lower [35]. In addition, discordant monozygotic twin comparisons for schizophrenia suggest that almost all affected twins perform worse on cognitive tests than their unaffected twin [36]. In this sense, it can be safely talked about cognitive deficits in schizophrenic patients although, in any case, it is very likely that their cognitive performance is below than what was expected in the absence of disease. It is also interesting to mention that several authors have found a worse performance in tasks of work memory (especially in visual) and in learning verbal tests and free memories in unaffected monozygotic twin brothers and, even a lesser degree, in not affected dizygotic twin brothers against controls [37–40].

It is interesting to point out that many patients have a lack of awareness of cognitive dysfunction. Those who are better aware of their deficits are not associated with a lower use of treatment, nor with a lesser deterioration of executive function. But they do have better results in the rehabilitation of some cognitive domains, in adherence to treatment and in their functional capacity [41, 42].

The neurocognitive dysfunction affects the ability to perform activities of daily living, impairs the ability to solve social problems [15, 16] and has proved to be the better predictor of reincorporation to activities in social and community settings [43, 44] and, especially, in terms of work rehabilitation and maintenance works [45–48]. These data review the importance of cognitive domains as a reliable scale of clinical improvement [15, 49–52] and, as is to be expected, the degree of cognitive impairment implies a worse adjustment in the quality of life of these patients [52–54]. Some authors also consider cognitive functions as an integral part of the concept of resistance to treatment [55]. On the other hand, several studies emphasize the importance and efficacy of cognitive rehabilitation treatments in early stages of psychosis [56, 57].

As expected with these data, the cognitive alteration in schizophrenia is the current focus of attention for the research of therapeutic strategies, both pharmacological and psychological. Regarding the interventions on cognitive domains, the pharmacological treatment, although it has offered certain results, has not been very encouraging [58], however studies using cognitive stimulation (training in executive skills, memory and other cognitive processes) have achieved more hopeful data [59–62].
