Dissociation between numerosity and duration processing in aging and early Parkinson's disease.

Numerosity and duration processing have been shown to be underlain by a single representational mechanism, namely an accumulator, and to rely on a common cerebral network located principally in areas around the right intraparietal sulcus. However, recent neuropsychological findings reveal a dissociation between numerosity and duration processing, which suggests the existence of partially distinct mechanisms. In this study, we tested the idea of partially common and distinct mechanisms by investigating, for the first time, both numerical and temporal processing abilities in non-demented Parkinson's disease (PD) patients known to suffer from duration impairment and in healthy elderly adults known to have impaired performance in duration tasks. The aim was to assess whether this impaired duration processing would extend to numerosity processing. The participants had to compare either the numerosity of flashed dot sequences or the duration of single dot displays. The results demonstrate an effect of aging on duration comparison, healthy elderly participants making significantly more errors than healthy young participants. Importantly, the performance of PD patients on the duration task was worse than that of the healthy young and elderly groups, whereas no difference was found for numerosity comparison. This dissociation supports the idea that partly independent systems underlie the processing of numerosity and duration.