Reliance on the visual frame of reference in ageing across different sensorimotor tasks : from perception to walking

Aging entails deficits in the mechanisms of sensory integration which may affect daily living tasks in old adults, ultimately leading to loss of autonomy and health risks, notably falls. Among the factors contributing to these risks, some may be associated with a degradation in sensory (re)weighting, leading to a greater reliance on visual cues and the associated frames of reference (FoR) (visual field dependence). Our aim was to study how preferential modes of spatial referencing influence sensorimotor control. Examining visual field dependence in the context of aging thus allows to better understand:•if age-related cognition and/or sensorimotor deficits are associated with increased reliance on the visual FoR;•whether this reliance indicates a preferred mode of spatial referencing or a consequence of age-related deficits;•how the above associations and mechanisms evolve by studying young, middle-aged and old adults.We first examined possible factors associated with greater reliance on the visual FoR with age (Chapter 2). We confirmed classic literature reports of increased visual field dependence in old age, and uncovered an association between greater visual field dependence and reduced i) reliance on the egocentric FoR, ii) parallel attentional visual processing ability, and iii) visual fixation stability.We subsequently examined the orientation and stabilisation behaviour of our participants during postural tasks and while walking under different conditions of linear ground optic flow. In Chapter 3, participants stood quietly or stepped in place (SIP – intermittent podal contacts with the ground surface) while confronted with 1- natural optic flow (no stimulus), 2- a static visual stimulation, 3- approaching and 4- receding optic flow. The results showed that the optic flow stimuli influenced SIP primarily as evidenced by anteroposterior drifting of the head, trunk and centre of pressure (COP). Old adults had larger amplitudes of drift compared to the younger participants, and drifted even under natural flow (natural drift) during SIP, indicating reduced egocentric self-motion perception. The most important directional optic flow effects were on the COP and were associated with i) increased reliance on the visual FOR, ii) reduced reliance on the egocentric FoR, and iii) greater natural drift.In Chapter 4 we investigated the influence of ground optic flow on the control of walking and head stabilisation. Reliance on the visual FoR in old adults was manifested under conditions of i) natural flow by a reduced head pitch orientation and ability to stabilise their head in space, which may indicate a strategy to maximise the salience of available visual cues and ii) imposed optic flow, by a re-orientation of the trunk in pitch and increase in stepping frequency. Our results also revealed a general improvement of head stabilisation under conditions of imposed visual stimulation towards a more frequent adoption of the head stabilisation in space strategy. This suggests that the artificial enhancement of optic flow provokes a postural adaptation in order to optimise sensory information processing when walking.Our findings extend current knowledge on the association between reliance on the visual FoR and sensorimotor control across adulthood and depending on the perceptivo-motor task. It is evident that this reliance is linked to a reduction in the exploitation of the egocentric FoR in terms of body orientation and self-motion perception, and that its manifestation depends on the task. Finally, our work provides insights for the design of training protocols aimed at frailer olds taking into account exacerbated reliance on the visual FoR.