Sandra M. McKay

The use of peripheral vision to guide perturbation-evoked reach-to-grasp balance-recovery reactions.

For a reach-to-grasp reaction to prevent a fall, it must be executed very rapidly, but with sufficient accuracy to achieve a functional grip. Recent findings suggest that the CNS may avoid potential time delays associated with saccade-guided arm movements by instead relying on peripheral vision (PV). However, studies of volitional arm movements have shown that reaching is slower and/or less accurate when guided by PV, rather than central vision (CV). The present study investigated how the CNS resolves speed-accuracy trade-offs when forced to use PV to guide perturbation-evoked reach-to-grasp balance-recovery reactions. These reactions were evoked, in 12 healthy young adults, via sudden unpredictable antero-posterior platform translation (barriers deterred stepping reactions). In PV trials, subjects were required to look straight-ahead at a visual target while a small cylindrical handhold (length 25%> hand-width) moved intermittently and unpredictably along a transverse axis before stopping at a visual angle of 20°, 30°, or 40°. The perturbation was then delivered after a random delay. In CV trials, subjects fixated on the handhold throughout the trial. A concurrent visuo-cognitive task was performed in 50% of PV trials but had little impact on reach-to-grasp timing or accuracy. Forced reliance on PV did not significantly affect response initiation times, but did lead to longer movement times, longer time-after-peak-velocity and less direct trajectories (compared to CV trials) at the larger visual angles. Despite these effects, forced reliance on PV did not compromise ability to achieve a functional grasp and recover equilibrium, for the moderately large perturbations and healthy young adults tested in this initial study.

Reaching to recover balance in unpredictable circumstances: Is online visual control of the reach-to-grasp reaction necessary or sufficient?

Reaching to grasp an object for support is a common and functionally important response to sudden balance perturbation. The need to react very rapidly (to prevent falling) imposes temporal constraints on acquisition and processing of the visuospatial information (VSI) needed to guide the reaching movement. Previous results suggested that the CNS may deal with these constraints by using VSI stored in memory proactively, prior to perturbation onset; however, the extent to which online visual control is necessary or sufficient to guide these reactions has not been established. This study examined the speed, accuracy, and effectiveness of perturbation-evoked reach-to-grasp reactions when forced to rely entirely on either online- or stored-VSI by using liquid–crystal goggles to occlude vision either before or after perturbation onset. The reactions were evoked, in twelve healthy young adults, via sudden unpredictable antero-posterior platform translation (barriers deterred stepping reactions). Prior to perturbation onset, a small cylindrical handhold was positioned unpredictably (by a motor-driven device) at one of four locations in front of the subject. Results indicated that equilibrium could be recovered successfully by grasping the handhold using either online-VSI or stored-VSI to guide the arm reaction; however, both sources of VSI were required for optimal performance. Reach initiation and arm movement were slowed when dependent on online-VSI, whereas reach accuracy and grip formation were impaired when dependent on stored-VSI. Comparison with normal-VSI trials suggests that both sources of VSI are utilized when grasping a small handhold for support under normal visual conditions, with stored-VSI predominating during initiation/transport and online-VSI contributing primarily to final target acquisition/prehension.

Effect of Achilles tendon vibration on posture in children

This study investigated the effect of unilateral Achilles tendon vibration on postural response in children and young adults during standing. Thirty healthy subjects participated in this study including ten 6-year-old children (YC group), ten 10-year-old children (OC group), and ten young adults (YA group). Eight-second vibration was elicited in each trial from a small vibrator attached above the right Achilles tendon when participants stood barefoot on a force platform. Three 40-s trials were collected under both eyes-open and eyes-closed conditions. Center of pressure (COP) was calculated to examine postural response during the pre-vibration, vibration and post-vibration phases. Results show that both the YC and OC groups had a greater COP average velocity than the YA group in all three phases. Tendon vibration induced a directionally specific postural response in all three groups such that the onset of vibration induced a posterior and medial COP shift during the vibration phase, and the offset of vibration induced an anterior and lateral COP shift during the post-vibration phase. Timing of the maximal COP shift was comparable among three groups in both anterior-posterior (AP) and medial-lateral (ML) directions. However, only the OC group showed an adult-like magnitude of the maximal COP shift during the post-vibration phase in the AP direction. These results suggest that 6-year-old children may start showing an adult-like directionally specific response and temporal parameter to tendon vibration during standing; however, the development of an adult-like spatial postural response to tendon vibration may take more than 10 years.