Influence of visual contrast and prominence of task-relevant stimuli on obstacle negotiation in fallers with and without Parkinson’s disease

L. Alcock; B. Galna; J.M. Hausdorff; S. Lord; L. Rochester

[Poster] BACKGROUND AND AIM: Successful obstacle negotiation is a complex motor task which relies on adequate vision to distinguish the obstacle from the ground. Reduced visual acuity and contrast sensitivity are common in ageing[1] and people with Parkinson's disease (PD)[2] which may impede safe negotiation of cluttered environments and contribute to increased falls risk. The aim of this study was to investigate the effect of visual contrast on obstacle negotiation in people who fall with and without PD. METHODS: Gait was measured in 9 people with PD (Mean[SD]age: 68.9[7.1]y, H&Y stage II-III) and 12 older adults (Age:75.5[7.6]y) with a history of recurrent falls. Participants negotiated a small (2x60x2cm), long (2x60x15cm), and high (15x60x2cm) obstacle under high and low contrast (hue (colour) manipulation) conditions. Gait characteristics (speed, step length, duration and width) were measured using an instrumented walkway (GAITRite). A general linear model evaluated the effects of obstacle type (small/long/high); step (penultimate (A2) and final (A1) approach steps, and the lead (Ld) and trail (Tr) crossing steps); and obstacle contrast (high/low) on gait in PD and older adult fallers. Ophthalmologic assessments were obtained binocularly. RESULTS: The visual acuity of older adult fallers was considerably worse than PD fallers (p=.066) but contrast sensitivity was similar (p=.703). PD fallers widened their step when crossing the small and long obstacles more than older adult fallers when obstacle contrast was high (11% and 14% increase, respectively). When contrast was low, PD fallers widened their step as a function of obstacle type (small-long-high, p=.038). Participants crossed the high obstacle (high contrast) more slowly and slowed down earlier in the approach for the high obstacle under low contrast conditions irrespective of group (p=.035, Figure 1). Increased step length was observed for the long obstacle (low contrast) condition, particularly for the lead step compared to the small and high obstacles (p=.003, Figure 1). CONCLUSIONS: Obstacle negotiation is altered when contrast is reduced. PD fallers widened their step when negotiating high obstacles under low contrast, suggesting that when task-relevant stimuli are less visually prominent additional motor alterations are required to compensate. The increased lead step length observed for the long obstacle (low contrast) was likely an adaptation in response to reduced obstacle edge definition and therefore both groups exaggerated their step to safely clear the obstacle. Improving the contrast of potential hazards in the home and community is important to reduce trip hazards particularly when object edge definition is reduced (minimal environmental contrast) and environmental hazards are less protrusive within the field of view (relative orientation of long vs. high obstacle). REFERENCES: [1] Lord (2001) JAGS 508-515 [2] Davidsdottir (2005) Vision Research 1285-1296

Influence of visual contrast and prominence of task-relevant stimuli on obstacle negotiation in fallers with and without Parkinson’s disease

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