The Neglect-Syndrome has been characterised as a failure to report, respond or orient to novel or meaningful stimuli presented to the side opposite a brain lesion, when this failure cannot be attributed to either sensory or motor defects. This deficit is based on a disturbance of complex attentional control mechanisms, which reduces significantly the chance of rehabilitation and recovery. An effective therapy of the neglect-syndrome has not been established yet.
Animal studies did show a central role of dopaminergic fibres in the regulation of directional and non-directional attentional processes. Furthermore some case reports show significant improvement in patients with chronic neglect-syndrome after therapy with dopaminergic drugs. Paradoxically in parkinsonian patients who suffer from asymmetrical striatal dopamine depletion only few studies could demonstrate neglect-symptoms. One of the reasons for this contradiction may relate to conventional neuropsychological tests which were designed for the acute neglect-syndrome and may not be sensitive enough to produce evidence of attentional deficits in chronic neglect. In a previous study we were able to show subclinical neglect-symptoms in hemiparkinsonian patients using a specifically developed new paradigm [1].
In this paper we described the side preferences in spontaneous visual exploration 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 [2], 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. These 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.
Based on these findings our future approach aims to characterise the specific deficit seen in parkinsonian patients and to use this neglect-model for the evaluation of dopaminergic stimulation. These studies will involve de novo PD patients, patients with advanced PD and patients with functional neurostimulation of the globus pallidus and the subthalamic nucleus (both in ON- and OFF-conditions).
Additionally our studies will include the establishment of a refined test-battery, which may allow to differentiate between the attentional and intentional aspects of the neglect-syndrome. Thus, a Foveal-Neglect-Test (FNT) was designed to investigate the directional visual attention in the foveal visual field without oculomotor-contamination. In the second part we intend to extend these results to investigate the efficiency of dopaminergic drugs in the treatment of acute and chronic neglect after structural hemispheric lesions.