Brain Responses to Detection of Right or Left Somatic Targets are Symmetrical in Unilateral Parkinson's Disease: A Case Against the Concept of ‘Parkinsonian Neglect’

Signs of attentional dysfunction mimicking spatial neglect have been described both in humans with lateralised Parkinson's Disease (PD) and in animals with MPTP-related hemiparkinsonism. Such deficits have been attributed to dopamine loss in basal ganglia and cortical targets. However, in previous studies the existence of neglect was assumed from behavioural tests which needed a motor output, thus entailing interpretation ambiguities due to effects of directional hypokinesia. We recorded brain event-related potentials (ERPs) evoked by the presentation of target somatic stimuli to the affected and non-affected sides in 44 patients with unilateral or asymmetrical PD. The N2 and P3 ERP components were specifically analysed, since (a) they are triggered selectively by task-relevant, attended sensory stimuli; (b) their latency reflects stimulus evaluation time, independently from the execution of a motor response, and (c) they have proved to be abnormal in hemineglect syndromes due to focal brain lesions. Irrespective of the side (left or right) of motor symptom predominance there were no significant ERP differences to stimulation of the affected and non-affected limbs, nor was there any correlation between ERP latencies and the degree of dopaminerelated motor impairment. The P3 latency was abnormally delayed in 23% of the patients, but there was no trend for abnormalities to concentrate on the affected side. This study does not confirm the existence of a significant attentional impairment toward the affected limb in lateralised PD, and suggests that previous clinical evidence of ‘neglect’ behaviour in PD might be linked to directional hypokinesia, thus reflecting intentional, rather than attentional lateralised deficits.