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Post-mortem evidence has shown a depletion of dopamine in the mesocortical and mesolimbic pathways in brains of Parkinson patients. Since these dopaminergic pathways have been implicated in the control of attention in animals, selective attention to visual stimuli was studied in eight patients with early Parkinson's disease (Stage I or II as defined by the Hoehn and Yahr Scale) and eight normal controls of comparable age, sex and Full Scale Intelligence Quotient. Subjects with dementia, psychiatric disease and other neurological abnormalities were excluded. The Parkinson patients were more prone to interference in the presence of distractor items than the normal controls as shown on the focussing + distraction and switching + distraction of attention paradigms on the Distractor task. There findings are not accounted for by mood, intellectual status or memory and thus may be as a result of the loss of dopamine in the mesocortico-limbic projections.

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Fatigue is often associated with increased clumsiness. One possible explanation for this is that the proprioceptive signals from receptors in and around muscles change during muscle fatigue. Thirteen human subjects were tested for their ability to match the elbow angle of one arm with the contralateral arm, before and after a fatiguing contraction of one arm. Contractile fatigue was induced by a series of maximal voluntary contractions of the elbow flexors of the dominant arm. While fatigue of either the target arm or the matching arm usually changed the ability of individual subjects to match arm position, this effect varied markedly from one subject to another and no consistent pattern was discerned. In particular, there was no reciprocal change when the fatigued arm was the matching arm compared with when the nonfatigued arm was the matching arm. The absence of a consistent reciprocal effect indicates that the fatigue-related changes in the ability to match arm position are not solely due to changes in proprioceptive signals and that central fatigue processes are probably involved.