Dissociation of Hemi-spatial and Hemi-motor Impairments in a Unilateral Primate Model of Parkinson's Disease

Monkeys with unilateral lesions of nigrostriatal dopamine projections were tested on a series of spatial tasks. One task, in which monkeys were required to use one or the other arm to retrieve food rewards from different positions, allowed separate assessment of the use of each arm in each hemi-space in order to distinguish hemi-spatial and hemi-motor impairments. The lesioned monkeys exhibited a persistent neglect of contralesional space when using either arm which could be dissociated from a motor impairment in the contralesional arm alone. Another task allowed free use of either arm across peri-personal space and demonstrated an ipsilesional bias in the monkeys' self-determined attention (orientation) to a task which they were trying to perform. It is argued that the tendency for monkeys with this lesion to rotate ipsilesionally is due to an ipsilesional deviation of the 'centre of interest' (determined by telencephalic circuitry) relative to 'straight ahead' (determined by brainstem circuitry). The dopamine projections may contribute to cortico-subcortical circuits which determine the spatial layout of mental representation, attention and intention. The results in this primate model of unilateral Parkinson's disease (PD) support the view that patients with left-sided Parkinsonian symptoms exhibit a unilateral deficit in spatial mental representation as well as their well-recognised motor symptoms. Patients with bilateral Parkinson's symptoms may exhibit bilateral deficits in mental representation.