The role of lower peripheral visual cues in the visuomotor coordination of locomotion and prehension
Introduction
Georgopoulus and Grillner [1] have proposed a speculative hypothesis that accurate upper limb movements evolved from the ability to precisely position lower limbs during locomotion. They hypothesized that the coordination of upper and lower limb movements need visuomotor guidance particularly when limb positioning during locomotion requires greater control such as in adaptive gait or walking on uneven terrains when the importance of visual cues grow. Although Georgopoulus and Grillner's paper is highly cited, only few subsequent studies have actually addressed the question of how walking and prehension are integrated and controlled. A common finding of these few studies is that reaching and grasping movements while walking are organized in hierarchical fashion, in a way that gait is adjusted based on the demands of the reaching and grasping task [2]. It has also been found that changes in gait imposed by the reaching task constraints may be task dependent [3], [4]. Although these studies brought valuable information to the investigation of coordination of prehension and locomotion, to date it is not clear which visual cues (if any) control and plan the coupling of upper and lower limbs. Previous studies have assigned to peripheral visual cues a relevant role in the online guiding of overground locomotion [5] and adaptive gait [6], [7], [8], [9], [10]. Peripheral visual cues have been classified as visual exproprioceptive since during movements they continuously update the relative spatial position of the feet/legs and terrain or obstacles [5], [6]. By occluding lower and/or peripheral visual field, foot placements before the obstacles and minimum toe clearance increased in order to enhance the margins of safety with obstacle and floor and decrease the likelihood of tripping [5], [6], [7], [8], [9], [10]. Although no prehension studies have actually occluded only the lower peripheral visual field, previous authors investigating visually guided hand movements found that grasping was more precise and pointing was faster and more accurate when the target appeared in the lower visual field [11], [12]. Hence the investigation of the coordination of prehension and locomotion may also benefit from the use of a lower visual occlusion since it may highlight what role lower visual cues (i.e., the visual availability of the moving hand and feet), play in the visual guidance of upper and lower limb movements.
The aim of the present study was to determine if and how lower peripheral visual cues contribute to the online control of the visuomotor coordination of upper and lower limbs. In the present study subjects walked up to the desk and reached out to grasp a full or a semi-empty glass of water with their dominant or non-dominant hand, under two visual conditions: lower visual field occlusion and full vision. The coordination of upper and lower limbs was investigated by the analysis of the foot placements before contacting the glass. Two glass conditions and two hand conditions were employed in order to understand how lower visual cues contribute to the coordination of prehension and locomotion under a variety of conditions characterized by different levels of difficulty. We expected that without lower visual cues provided by the upper and lower limbs approaching the desk, subjects would become more cautious particularly under full glass and non-dominant hand conditions. Hence, we predicted that foot placements would occur at higher distance from the desk/glass position and time to contact with the glass would be longer under lower occlusion compared to normal viewing conditions.
Section snippets
Participants
Twelve right handed and right footed participants took part in the present study: four males and eight females (mean ± 1 SD, age 26.1 ± 6.2 years). An informed consent form was signed by each subject. The tenets of the Declaration of Helsinki were upheld. The experiment gained approval from the local Research Ethics committee. Handedness and footedness were determined using the ‘Waterloo Handedness Questionnaire-Revised’ (WHQ-R) and the ‘Waterloo Footedness Questionnaire-Revised’ (WFQ-R) [13].
All
Results
Homogeneity of the data was assumed for all the dependent measures and all the main effects and interaction were considered.
Time to contact was longer under LO compared to FV, full glass compared to semi-empty glass and non-dominant hand compared to dominant hand (Table 1). The AP foot-targeting was higher under LO compared to FV (Table 1), but a significant two-way interaction of vision by hand was also found (p < 0.03). The interaction showed that under the dominant hand AP foot-targeting was
Discussion
The scope of this study was to determine whether lower visual cues provided the online control of the visuomotor coordination of prehension and locomotion. Under LO, subjects took a longer time to contact the glass and they placed their feet further away from the glass compared to FV. These findings suggest the existence of a motor control strategy to increase the margin of safety between body and desk/glass when visual exproprioceptive cues from the relative position of upper/lower limbs
Conflict of interest
No conflicts of interest.
Acknowledgments
The data reported in this paper was collected during Valentina Graci's doctoral studies completed under the supervision of Dr. John Buckley and Prof. David Elliott at the Vision and Mobility Laboratory of the Bradford School of Optometry and Vision Science, University of Bradford. Dr. Valentina Graci would like to thank Dr. Marina Bloj and Prof. Alan Wing for their input on this work and Dr Gretchen Salsich for her comments on the manuscript.
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