Cognitive dysfunction is a core feature of schizophrenia.[1, 2] The aspects of cognition that are specifically impaired in schizophrenia are verbal memory, working memory, motor function, attention, executive functions, and verbal fluency. Recently, studies have shown impaired mental imagery ability in the patients.[3-6] However, there is lack of evidence regarding the correlation between impairment in mental rotation and other cognitive deficits in patients with schizophrenia.
Mental rotation, the ability to imagine how a misoriented object would appear if rotated away from the presented orientation, is a widely studied area of spatial cognition. Mental rotation is usually studied by using Parson's hand rotation paradigm in which participants judge the laterality of pictures of left or right hands in different rotation angles. Typically, reaction times and error rates increase as a function of the rotation angle of the stimulus, suggesting that participants engage in a cognitive process of mental rotation.
A few studies have found compromised mental imagery ability in patients with schizophrenia.[3-6] A study by Danckert et al. showed poor correlation between imagined movement duration and target size in the Visually Guided Pointing Task (VGPT) in patients. The authors concluded that schizophrenic patients were unable to generate accurate internal images of their own movements. Maruff et al. found that only the patients with passivity symptoms were impaired in the VGPT task. de Vignemont et al. reported that while patients were slower and less accurate than controls on mental rotation tasks, they showed the same decrease in speed and performance with increasing angles of rotation as controls. They suggested that lower accuracy in patients results from a general impairment of attention and visuospatial working memory in patients, although they did not examine these correlations in their study. While this information suggests impaired mental rotation in schizophrenia, the lack of electrophysiological and neuropsychological data raises the question about the underlying neural and cognitive mechanisms. To our knowledge, no study has investigated the correlation between impairment in mental rotation and other cognitive deficits in schizophrenia, which is the main aim of the present study.
For mental rotation, basic processing of visual input relies on activation of the posterior parietal cortex, while frontal activation are required to integrate, manipulate, and employ visual information. Researches in cognitive processes of mental rotation have proposed the crucial role of the speed of spatial processing, executive function, and working memory.[7, 8]
Considering the wide range of cognitive deficits in patients with schizophrenia, as well as evidence of impaired mental rotation, further research is required to determine the neuropsychological correlates of this impairment. The length of cognitive assessment is a factor that affects patient cooperation. Therefore, to examine the effects of domains of cognitive functions that are impaired in patients with schizophrenia, we chose a brief battery, the Brief Assessment of Cognition in Schizophrenia (BACS). The BACS is specifically designed for assessment of cognitive function in schizophrenia and has several advantages, including brief administration (approximately 35 min) and scoring time, portability, and repeatability. Studies have shown acceptable reliability and concurrent validity of the BACS with a standard cognitive battery, which takes more than 2 h to administer.
The current study aimed to examine the correlation between cognitive performance, measured with the BACS, and performance on a mental imagery task, measured with the Hand Rotation Task (HRT). We hypothesized that impaired performance on task of mental hand rotation would be associated with impairment of other cognitive functions, particularly measures of working memory and executive functions.
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While evidence has shown that individuals with schizophrenia exhibit impairment on tasks of mental rotation, the cognitive processes that underlie have remained unclear. The primary aim of the current study was to examine the correlation between mental rotation ability and other domains of cognitive functions, assessed by the hand rotation task and the BACS, respectively. The main results can be presented into two parts. First, on hand rotation task: (i) schizophrenia patients displayed the same pattern of performance as the control group; and (ii) they performed significantly more slowly and less accurately compared to the control group. Second, impaired accuracy in mental rotation was associated with speed of information processing and executive dysfunction in the patient group.
Consistent with previous studies, our findings revealed that in general the pattern of performance of patients was comparable to the controls (same effects of angle of rotation), and their ability to mentally rotate stimuli was preserved. However, we should emphasize that there was an impairment in mental rotation, as the patients showed slower response time and higher error rate compared to controls, similar to findings reported by de Vignemont et al. These results suggest patients with schizophrenia have difficulties in accurately manipulating mental representation of hands.
The results showed the individual domains of cognitive function were moderately correlated with error rates of hand rotation task, with correlations ranging between 0.43 and 0.59. Moreover, the results of regression analysis revealed that cognitive domains of speed of information processing, reasoning and problem solving were the best predictor of accuracy rate on the hand rotation task in patients.
Our results indicated that processing speed is an important parameter for performing mental rotation tasks in patients, and its dysfunction could be considered as the fundamental underlying the observed mental rotation deficits. A recent meta-analysis demonstrated that patients with schizophrenia show significant deficits in speed of processing compared to other neurocognitive domains. Our finding supports previous studies and provides further evidence for the crucial role of deficient speed of processing on performance in cognitive tasks in patients with schizophrenia.
The present study showed that hand rotation accuracy was associated with worse performance on measure of executive function. Executive functioning involves multiple components, such as attention, inhibition, planning and monitoring. It is not easily clear to what extent any single specific task involves each of the different aspects of executive functioning. The Tower of London task, which was applied in the present study, is a widely used test to examine planning ability.[16, 17] In fact, the planning ability consists of several cognitive processes, including strategy information, coordination and holding information online, which are also important factors to perform a mental rotation task. This finding supports the proposal that suggests executive functioning deficits might contribute to many of the other cognitive deficits observed in schizophrenia.[19, 20]
It should be noted that the importance of these cognitive domains for mental rotation has been proposed in previous studies. Weber et al. found associations between hand rotation error rates and measures of executive functioning and working memory in individuals with HIV infection. Crucian et al. found the correlation between hand rotation accuracy and mental processing and psychomotor speed in patients with Parkinson's disease. As mentioned in the Introduction, de Vignemont et al. proposed the correlation between impaired mental imagery and visuospatial working memory in patients, which was not assessed in our study. Future studies are needed to examine the impairment of the spatial working memory and its relation to mental rotation in patients with schizophrenia. Moreover, future research should examine the association between mental rotation and its cognitive mechanisms, using positron emission tomography.
We did not find significant correlations between measures of hand rotation task and chlorpromazine equivalent dose, duration of disease, or PANSS scores, indicating that impairment on the hand rotation task is not simply related to the level of psychopathology and dose of medication.
A limitation of this study is that the numbers of patients and controls were relatively small. A possible confounding factor that might have affected performances of patients on cognitive tasks was the use of antipsychotic medications. However, no correlation was found between chlorpromazine equivalent dose and either measures of hand rotation or the BACS. Also, anticholinergic drugs have adverse effects on memory; therefore, future studies are required to examine the association between mental rotation and its cognitive mechanisms across a group of drug-naïve patients as well as in different subtypes of schizophrenia.
In conclusion, these results increase our knowledge regarding associations between neuropsychological and mental rotation deficits in patients with schizophrenia. The most robust finding is that impaired accuracy on hand rotation task is correlated with speed of information processing and executive dysfunctions in patients.