Introduction
Cognitive dysfunctions as core features of schizophrenia are strongly associated with poor functional outcomes and about 80% of patients with schizophrenia struggle with various neurocognitive and socio-cognitive deficits. 
On the other hand, cognitive deficits are hardly responsive to treatments such as antipsychotic drugs. Therefore, finding new non-pharmacological treatments for cognitive impairments to improve patients’ functional outcomes based on extensive literature supporting the concept of brain plasticity and neurogenesis is underway. In this context, cognitive rehabilitation therapy (CRT) attempts to improve and or restore cognitive functioning using a range of approaches. 
Several quantitative reviews have suggested that CRT is effective in reducing cognitive deficits and improving the functional outcome of schizophrenia. Although many studies have been conducted on this topic, few investigated the effectiveness of cognitive remediation methods based on different cognitive levels of impairment in patients with chronic schizophrenia. It is essential to answer this question because, in the case of differences in efficiency, other forms of intervention tailored to patients’ needs should be considered.
Methods
In this clinical trial study with a two-way pre-test-posttest design, the participants were divided into two separate groups based on cognitive impairment levels (low impairment and moderate impairment). As well, all participants provided written informed consent before the study.
All participants were recruited and diagnosed with schizophrenia based on DSM criteria. Then, they were divided into two groups of low and moderate cognitive impairments by applying the Montreal cognitive assessment  (MoCA) and holding clinical interviews. In MoCA, scores of 18-26 and 10-17 are considered as low and moderate impairment, respectively. The cognitive tests, including CPT, Stroop test, and n-back, were administered to the participants before and after the treatment. The treatment program used in this study was the Scholberg and Mateer Program (2001): a pencil and paper cognitive and neuropsychological intervention to reduce or eliminate the cognitive deficits of patients by exercising and applying residual and healthy skills. This program focuses on memory and its elements, as well as attention and its dimensions and executive function. It was designed to run in the form of group or individual treatment in 16 sessions with determined instruction. These 16 sessions are conducted in small groups of 4-5 patients in 30- to 45-minute sessions, 3 times per week, and arranged in hierarchical order according to the complexity of the function. 
The repeated measures ANOVA test was performed to compare differences in means and evaluate the effect of the intervention on the pretest and posttest scores of each person. All analyses were done in SPSS v. 23.
Results
It was assumed that cognitive remediation would improve the working memory of patients with chronic schizophrenia. Findings of the repeated measures test showed that scores of participants increased significantly in the n-back test (Table 1).


The results showed an increase in the number of correct responses and a decrease in the unanswered ones, which suggests an improvement in working memory performance (α= 0.05). Moreover, since the repeated measures ANOVA is not significant, there is no difference between the improvement of the two groups in the field of working memory.
Results of the Stroop test and CPT to evaluate selective attention and sustained attention are presented in tables 2 and 3.


As shown in Table 2, the time effect is significant for errors of omission and correct response which means the number of errors is reduced, and the number of correct responses in the posttest is increased (α= 0.05). This result means that cognitive remediation could improve sustained attention. Also, the amount of F is significant (α=0.05) for both variables. So, the progression of the group with moderate cognitive impairment was higher than that of the low impairment, and cognitive remediation was more effective in the group with a moderate deficit in sustained attention.  
As shown in Table 3, there were no significant differences in the interference score and interference time between the pretest and the posttest, which means no improvement has occurred.


The amount of F is also not significant, indicating that the effectiveness of the intervention is comparable between the two groups in the field of selective attention.
Discussion
The results showed that cognitive rehabilitation could improve participants’ performance on the n-back test, which measures working memory. This finding is consistent with the findings of Vita et al., Shiran et al., and McGurk et al. Also, the research findings indicate that subjects’ performance in both groups has improved in the sustained attention area. The results of Mohammadi et al. and Wykes et al. also confirm this finding. Despite these changes, in the present study, there was no change in the results of the Stroop test, which assessed selective attention and cognitive flexibility. This finding is inconsistent with the research by Gary et al. and Stathopoulou and Lubar and is consistent with the findings of Suslow. In explaining the above findings, it should be noted that cognitive functions, which appear as separate factors, are highly interrelated with individual assessments. Hence, the dimensions of cognitive functions are intertwined. Moreover, most studies have confirmed the effectiveness of this type of intervention in promoting cognitive functions such as memory and attention. From what has been said, it can be concluded that different approaches to cognitive rehabilitation help patients with schizophrenia to perform better in cognitive tasks.
The comparison of changes between the two groups shows that the difference between the progress of the two groups in the cognitive function of selective attention and working memory was the same, and only in the cognitive function of sustained attention, did the group with more severe cognitive deficits shows more progress. These findings are consistent with the results of research by Bell et al. Regarding the further development of the group with moderate pathology in the task of sustained attention, the rehabilitation intervention used has a greater focus on this aspect of cognitive function. Furthermore, as mentioned before, in the group with moderate pathology, due to the difficulty of some tasks, some individual sessions were held for them, which can be effective in the effectiveness and final result of the work. However, due to the scarcity of comparative research, an accurate answer to this question requires further research.

Ethical Considerations
Compliance with ethical guidelines
This study has ethical approval from the University of Social Welfare and Rehabilitation Sciences (IR.USWR.REC.1394.323) and the ethical code of the Iranian Clinical Trials Registration Organization (IRCT2016060828336N1).

Funding
This article is taken from the master's thesis of Asal Fazli, Department of Psychology, Faculty of Psychology, University of Social Welfare and Rehabilitation Sciences, and has no financial support.

Authors contributions
Conceptualization: Behrouz Dolatshahi, Shima Shakiba and Asal Fazeli; Research and review: Asal Fazli; Editing and finalization: Asal Fazli; Supervision: Behrouz Dolatshahi and Shima Shakiba.

Conflicts of interest
The authors declared no conflict of interest.

Acknowledgements
The head of Tolo Rehabilitation Center, Dr. Hamid Taherkhani, is thanked and appreciated for his help and support. Also, the help and patience of all the patients who accepted to participate in this research is appreciated.


 
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