Motor mechanisms in schizophrenic patients with tardive dyskinesia.

A specific aspect of the motor output of patients with tardive dyskinesia was investigated with an experimental procedure referenced in force control function (i.e., the maintenance of sustained force output). We hypothesized that response output (button press) at a given level would be significantly altered when preceded by an output of a higher force than when preceded by an output of a lower force. The difference could be interpreted as a behavioral measure of fatigability and/or threshold change, and its quantitative aspects could serve as a marker of motor impairment. Three groups of subjects participated in the study: schizophrenic patients with tardive dyskinesia (TD), schizophrenic patients without tardive dyskinesia, and normal control subjects. The test used was a target-matching task with force output ranging from 5 to 560 cN. Three dependent measures were used: output force, steadiness of force maintenance, and latency of target capture. Results indicated that precedent output at the previous trial significantly altered force, but even better group separation was attained with the measure of steadiness. The TD schizophrenic group's performance was least efficient, the control group's was most efficient, and the non-TD schizophrenic group's performance was intermediate. The principal conclusions reached were that the target-matching response reflects mechanisms related to both fatigability and recruitment of motor units involved in output control.