Directional bias of initial visual exploration. A symptom of neglect in Parkinson's disease

In the present study, side preferences in spontaneous visual exploration were assessed systematically in 27 patients with idiopathic Parkinson's disease (IPD) and 17 age-matched controls. Assessment of initial visual exploration asymmetry (IVE) was based on the exploration of texture arrays requiring attentive oculomotor scanning. As shown in a previous study, most healthy subjects exhibit a marked asymmetry of IVE with a strong left-sided bias when assessed by this paradigm, while most neglect patients initiate exploration in the right half of the arrays. Standard assessments for symptoms of neglect (line bisection, line cancellation and double simultaneous stimulation) were performed as reference tests. In the IVE task 65% of normal controls and 69% of patients with predominantly right-sided IPD started exploration in the left half of the arrays. By contrast only 14% of patients with predominantly left-sided disease showed a leftward IVE. The majority shows an ambiguous (21%) or rightward (64%) directional bias for initial exploration and thus a behaviour that corresponds to the IVE abnormalities found in neglect patients. No abnormalities were found in the standard neglect tests in any of the groups. The atypical IVE in patients with predominantly left-sided Parkinson's disease should be interpreted in the context of recent concepts of attention postulating that a bias in early spontaneous orientation directed to the ipsilesional hemifield reflects a mild and residual manifestation of hemineglect. Since this subtle orientational bias is less subject to compensation than more conspicuous clinical signs of neglect, sensitivity is higher in IVE testing than in conventional neglect assessments in chronic disorders with subclinical neglect. The present findings contribute a new aspect to the complex picture of cognitive and visuospatial abnormalities in Parkinson's disease. Furthermore our results extend previous knowledge on the mechanisms of neglect and the role of dopamine in the mediation of attention.