Perceptual rigidity in Parkinson's disease and normal aging
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Parkinson's disease (PD) is characterized by rigidity symptoms that extend motoric symptoms, including cognitive rigidity (e.g., reduced cognitive set-shifting) and "rigid" personality (e.g., reduced novelty-seeking). These non-motor symptoms have been associated with fronto-striatal dysfunction in this disorder. Disruption of fronto-parietal attentional networks in PD suggests rigidity may extend to perception. To examine perceptual rigidity, non-demented individuals with mild-moderate PD (16-27/experiment), matched normal control (NC; 15-25/experiment) and young control adults (YC; 17-22/experiment) were presented with bistable images. Study 1 examined perceptual flexibility in normal aging. The Necker cube, a bistable image that can be perceived as having the upper-left or lower-right face in front, was presented under passive-viewing and two volitional-control conditions: hold one percept in front, and switch between the two percepts. Under passive viewing, dominance durations (time spent on each percept) were shorter in YC than NC. Relative to YC, NC were less able to increase dominance durations in the hold condition but were comparable in the switch condition. Study 2 applied the Necker-cube experiment to PD and extended it to passive viewing during binocular rivalry. Inconsistently with our hypotheses, PD showed comparable dominance durations to NC in the passive viewing – Necker cube, while demonstrating shorter dominance durations (equivalent to faster perceptual alternation) during binocular rivalry. Relative to passive viewing, PD showed a trend toward less ability than NC to increase dominance durations in the Hold condition, and were significantly less able than NC to reduce dominance durations in the Switch condition, both results indicative of perceptual rigidity. Dominance durations on passive viewing correlated with personality (novelty-seeking) in PD, and not with cognitive rigidity in either group. Study 3 examined whether enhancement of low-level visual cues would reduce perceptual rigidity in PD. Low level cues helped both PD and NC to exert better control over the conditions they did not demonstrate difficulties in Study 1 and Study 2. These results suggest that perceptual rigidity occurs in mild-moderate PD. The provision of cues, however, does not compensate for this rigidity. Finally, these results further suggest an association in PD between novelty-seeking and the ability to explore the perceptually ambiguous world.