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Guillaume L Giraudet, Marion Grun, Jocelyn Faubert; Effect of crowding on postural control. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):2923.
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© ARVO (1962-2015); The Authors (2016-present)
The aim of the current study was to determine the effect of foveal crowding on postural stability in young adults and elderly. Crowding describes the deleterious influence of neighboring objects (flankers) on the recognition of a central target.
Thirty-three subjects participated in the experiment: 20 young adults and 13 elderly. They stood, barefoot and arms crossed, in front of a projection screen. Optical motion sensors, located on a helmet, and 6 IR cameras (Optotrak system) were used to track and register head movements during trials. Each trial lasted 64s. The stimulus displayed on the screen was a black and white checkerboard with an empty grey area (5° of visual angle) in front of the subject’s eyes. This peripheral checkerboard could be either static or in motion. Four different tasks, proposed in different orders, were considered: i) subjects were instructed to look at a fixation point, located in the middle of the central empty area; ii) subjects had to find target letters within RSVP sequences of letters; iii) and iv) both last tasks were the same as the second one but with flankers in the periphery, far from (task 3 - low crowding) or close to (task 4 - high crowding) the central letters. Each trial was repeated 3 times. We calculated the Velocity Root Mean Square (VRMS) of head movements based on the signal provided by the optical sensors. VRMS represented a measure of postural instability induced by the various visual conditions and tasks.
Results showed that subjects were more stable when they had to fixate the central point (task1) than when they were instructed to find target letters within RSVP sequences (tasks 2, 3 and 4). However, the change of postural behavior as a function of the task was significantly different between both young adults and elderly groups. While the former maintained the same level of stability for the 4 tasks, the latter was significantly affected by the RSVP tasks and crowding.
Some previous studies showed that visual crowding increased with age, older people being more disrupted than young adults when the target to be recognized is surrounded by flankers (Scialfia et al, 2012 for review). Our experiment also showed that the effect of letter recognition and crowding on postural stability was higher for the elderly. The attentionnal load needed to perform letter recognition tasks, leaving fewer resources for the good control of postural stability, might explain this result.
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