Unilateral Spatial Neglect without Hemiplegia: The Output-Mode Effect Revisited

Background: The occurrence of unilateral spatial neglect (USN) in non-hemiplegic right-hemisphere damaged patients is rare. Earlier studies of such patients revealed a significant advantage when typical neglect tests were performed by the patient's left hand as compared to the dominant right hand. The mechanism underlying this "output-mode effect" remains elusive. Methods: We analyzed the temporal dynamics of this effect using line-bisection task in 9 non-hemiplegic stroke patients with left-USN. Results: In 4 patients tested shortly after stroke onset (<= 6 weeks), the impact of hand laterality was variable (left-hand advantage in one patient; right-hand advantage in 2 patients; similar performance in both hands in one patient). Only later (> 6 weeks) a clear advantage of the left hand emerged in the majority of patients, similar to the earlier reports which were all based on late testing. Conclusions: We found variable dynamics in the expression of the output-mode effect in the first weeks following stroke onset, which may reflect changes of interhemispheric balance, related to recovery processes. We propose that therapeutic interventions aiming to manipulate the inter-hemispheric balance (e.g., by non-invasive brain stimulation) take into account the existence of such dynamics and their highly variate nature.