Square-wave jerks induced by pallidotomy in parkinsonian patients

Square-wave jerks (SWJs) are small, inappropriate saccades that intrude on steady fixation by taking the eye away from the target and then returning i t after approximately 200 msec. The pathophysiology of SWJs is unknown; the y have not been attributed to any specific lesion. We found that unilateral pallidotomy substantially increased the frequency of SWJs in three patient s with Parkinson's disease. This effect is likely due to imbalance in the f ixation system caused by asymmetric reactivation of prefrontal cortex via a scending thalamocortical projections. Alternatively, disruption of nigral p rojections to the superior colliculus might be responsible.