Apparent unilateral visual neglect in MPTP-hemiparkinsonian monkeys is due to delayed initiation of motion

Monkeys made hemiparkinsonian by infusion of a solution of MPTP into one carotid artery appeared to ignore food presented from the contralateral side. Initial observations suggested neglect of visual stimuli presented as fruit treats by automated delivery system in the half-field contralateral to MPTP treatment. Further studies in which fruit treats were left in the 'neglected' visual field indicated that this apparent neglect, unlike neglect attending cortical lesions, was rather a marked delay in initiating movements (unilateral hypokinesia). These observations may explain apparent subcortical neglect and are consistent with the known role of nigrostriatal dopaminergic neurones in movement regulation. This is a useful animal model in which difficulties in initiation of movement (hypokinesia), a cardinal sympton of Parkinson's disease, can be studied separately from other deficits in motor performance.