Aging and Parkinson's disease as functional models of temporal order perception

Our study evaluates how aging and Parkinson's disease (PD) alter temporal order judgment (TOJ).

Two TOJ experiments were performed with young participants, healthy elderly participants, and PD patients. Two white squares were presented on opposite sides of a screen and participants responded which appeared first. In Experiment 1, it was assessed how accurately each group could judge temporal order at intervals from 0ms to 167ms. Detectability, the capacity of detecting which stimulus appeared first, was measured. In Experiment 2, participants were asked to orient their attention to the hemifield indicated by a cue. The PSS ("point of subjective simultaneity"), where the participant was equally likely to respond that either stimulus appeared first, was measured.

In Experiment 1, PD patients had smaller detectability than healthy elderly (p<0.05) and young participants (p<0.001), and healthy elderly participants had smaller detectability than young participants (p<0.001). In Experiment 2, PSS was 29ms for young participants, 121ms for healthy elderly participants, and 283ms for PD patients; differences were statistically significant for comparisons between PD patients and healthy elderly participants (p<0.001), PD patients and young participants (p<0.001), and healthy elderly and young participants (p<0.04).

TOJ is impaired by aging and PD. Our results suggest that dopamine loss increases latency and variability in visual decision making due to a lower signal-to-noise ratio in the visual pathways.