Background:
Neurological soft signs (NSSs) are impairments that indicate nonspecific cerebral dysfunction. NSSs are thought to be early premorbid traits of schizophrenia. Motor overflow, involuntary movement occurring during voluntary movement, is one such NSS found in schizophrenia.
Method:
Thirty-seven participants (19 with schizophrenia, 18 controls) were tested. Participants exerted 25% and 75% of their maximal force output while overflow was monitored in the passive hand. Three tran-scranial magnetic stimulation protocols were designed to investigate the cortical origin of motor overflow: 1) motor cortex was stimulated unilaterally at 140% RMT; MEPs were recorded bilaterally; 2) stimulation of ispilateral hemisphere at 140% RMT was performed during motor overflow. Resulting latencies between the cMEP onset and the iSP onset were compared; 3) facilitated MEPs produced (through stimulation of contralateral hemisphere) during voluntary contraction and facilitated MEPs produced during motor overflow were compared. All procedures were applied to both hemispheres.
Results:
Previous findings of increased motor overflow in schizophrenia compared with controls were confirmed (P > 0.05); neither group showed a significant difference between MEPs facilitated during voluntary movement and those facilitated during motor overflow (P > 0.05).
Conclusions:
Results suggest that in both groups, motor overflow results from an imbalance between the transcallosal processes occurring during voluntary movement, leading to bilaterally active corticospi-nal tracts. Specific deficits in cortical excitability are likely to be responsible for greater overflow seen in schizophrenia.
