The aim of this study was to examine the possible neurobiological basis in schizophrenic patients with high suicide risk using n-back task fMRI data via DCM analysis. In the current study, patients with schizophrenia and healthy controls demonstrated opposite patterns of effective connectivity between MPFC and PCC. First, in these two cortical midline regions, the two patient groups (LSR and HSR) showed hyperactivity at 2-back task state compared with baseline. Second, the LSR group was unidirectional hyper-connectivity from PCC to MPFC. HSR and HC were bidirectional effective connectivity between PCC and MPFC. Compared with healthy controls, schizophrenic patients with high suicide risk showed hyper-connectivity from PCC to MPFC, and hypo-connectivity in the opposite direction.
The MPFC and PCC are directly involved in decision making, goal-directed behavior and emotional processing 
. The concept of motivational salience contains the decision making, which can explain and understand the evaluations and choices that humans make 
. In schizophrenia, the general hypothesis was that motivational salience is mis-assigned to unimportant, neutral or affectively ambiguous information 
. In this study, we found that reduced suppression occurred in both patient groups during 2-back task. Hyper-activation of the MPFC and PCC may constantly divide or misdirect the attention resources when patients attempt to perceive, think, or act on the external environment. When the brain does not properly process social information, it would become difficult to perform adaptive motivated behaviors in response to the information, in both present and future contexts. Normal brains need to form learned associations between social information cues in our environments and their associated emotional meanings, as well as perform adaptive emotional context 
. For the schizophrenic patients, it is possible that the emotional valence of social information and the subjective value of future outcomes are inappropriately estimated, finally leading to their biased decisions.
The previous study found that MPFC dysfunction was associated with risk for schizophrenia, whereas PCC dysfunction was associated either with the greater risk for the disease or the expression of the illness 
. The other study has demonstrated that the MPFC have direct fiber projections to the PCC 
. We speculate that the motivational and emotional information was integrated in MPFC, and then the information fed back to PCC to modulate the motivation and emotion at a relatively stable state and prompted the external expression of motivation and emotion. We found that the activity of MPFC was positively correlated with the scores of SSRS and the hypo-connectivity from MPFC to PCC only existed in HSR among two patients groups. We speculate that maybe the hypo-connectivity from MPFC to PCC is correlated with the high suicide risk in schizophrenia. In LSR group, the PCC could not receive information from the MPFC due to the absence of feedback, which may lead to the malfunction of motivational behavior and emotion expression. Compared with healthy controls, the hypo-connectivity from MPFC to PCC may result in a weakened regulation from MPFC to PCC in HSR group. Hyper-connectivity from PCC to MPFC was a prominent difference between patients with schizophrenia and healthy controls. More improperly processed social information from PCC was transmitted to MPFC, may lead to the confusion goal setting and planning, improper estimation of future outcomes, and assignment of self-reference to unrelated external events.
The previous study found that the prominent negative symptoms may prevent the emergence of suicide in patients with schizophrenia and the prominent positive symptoms (in particularly the suspiciousness and delusions) may increase the suicide risk 
. In this study, compared with the schizophrenic patients with low suicide risk, the schizophrenic patients with high suicide risk were characterized by prominent positive symptoms, lower levels of negative symptoms. Some studies suggests that the patients with schizophrenia spectrum disorders who have suicide behavior had a better cognitive function than those who have not 
. Interestingly, in the present study, we did not found a significant difference pertaining to the WAIS scores between HSR and LSR. One possibility is that we only chose the Information (I) subscale and the Digit Symbol (DS) subscale of the Wechsler Adult Intelligence Scale-Chinese Revised (WAIS-CR) to assess the intelligence, which possibly could not reflect the overall level of the intelligence. Future work devoted to assessing overall intelligence differences between HSR and LSR is warranted. It is widely believed that schizophrenia is characterized by longer reaction time and less accurate working memory performance. Whereas some studies did not show significant difference in reaction time between schizophrenic patients and healthy controls 
. In our study, compared with healthy controls, the schizophrenic patients showed a trend to have longer response time (). These inconsistent findings may be a consequence of insufficient statistic power owing to our relatively small sample, and larger samples will be required to evaluate them.
Our result is consistent with previous reports of functional integration impairment in schizophrenia and, in particular, adds to the evidence for a specific disturbance of the functional coupling between prefrontal and cingulate regions. Our data further extend the previous findings with respect to two main aspects. First, this is a pilot study on the possible neurobiological basis of schizophrenia patients with high suicide risk using n-back task fMRI data via DCM analysis. The previous studies investigated the coupling between medial prefrontal cortex and cingulate formation in terms of temporal correlations, that is “functional connectivity” 
, and could not indicate the direction of the impaired connection. By using dynamic casual modeling 
, we were able to show the altered connectivity between the cingulate formation and medial prefrontal cortex. Second, we investigated the interaction between cingulate and medial prefrontal cortex function in patients who had just developed schizophrenia as we did not use chronic patients. This minimized the risk that the findings were secondary to an effect of chronic illness or its treatments. Both functional and structural abnormalities in schizophrenia may change over the course of psychotic disorders 
and antipsychotic medication can modify activation during cognitive tasks 
In summary, using fMRI and dynamic casual modeling, we found that the hyper-connectivity from PCC to MPFC existed in the two patients groups, relative to healthy controls. This suggests that the abnormal effective connectivity is probably associated with risk for schizophrenia. Compared to healthy controls, the hypo-connectivity from MPFC to PCC only existed in HSR group. This abnormality may indicate a possibility of increased vulnerability to suicide attempt.