The present study explored how motor cortical activity was influenced by visual perception of complex environments that either afforded or obstructed arm and leg reactions in young, healthy adults. Most importantly, we focused on compensatory balance reactions where the arms were required to regain stability following unexpected postural perturbation. Our first question was if motor cortical activity from the hand area automatically corresponds to the visual environment. Affordance-based priming of the motor system was assessed using single-pulse Transcranial Magnetic Stimulation (TMS) to determine if visual access to a wall-mounted support handle influenced corticospinal excitability. We evaluated if hand actions were automatically facilitated and/or suppressed by viewing an available handle within graspable range. Our second question was if the requirement for rapid movement to recover balance played a role in modulating any affordance effect in the hands. The goal was to disentangle motor demands related to postural threat from the impact of observation alone. For balance trials, a custom-built, lean and release apparatus was used to impose temporally unpredictable postural perturbations. In all balance trials, perturbations were of sufficient magnitude to evoke a compensatory change-in-support response; therefore, any recovery action needed to carefully take into account the affordances and constraints of the perceived environment to prevent a fall. Consistent with our first hypothesis, activity in an intrinsic hand muscle was increased when participants passively viewed a wall-mounted safety handle, in both seated and standing contexts. Contrary to our second hypothesis, this visual priming was absent when perturbations were imposed and the handle was needed to regain balance. Our results reveal that motor set is influenced by simply viewing objects that afford a grasp. We suggest that such preparation may provide an advantage when generating balance recovery actions that require quickly grasping a supportive handle. This priming effect likely competes with other task-dependent influences that regulate cortical motor output. Future studies should expand from limitations inherent with single-pulse TMS alone, to determine if vision of our surrounding world influences motor set in other contexts (e.g. intensified postural threat) and investigate if this priming corresponds to overt behavior.
Author ORCID Identifier
David A. E. Bolton https://orcid.org/ 0000-0003-2255-5472
David M. Cole https://orcid.org/ 0000-0001-9172-8598
Sarah Schwartz https://orcid.org/ 0000-0001-9980-7493
.csf, .sgcx, .sgs, .pdf
.csf files are full original data collected and processed using Signal software (Cambridge Electronic Design, Cambridge, UK). .sgcx files are signal configurations used to run data collection/acquire data .sgs files are scripts used to process data .cfs files can be read using the open source SigViewer app available for download at GitHub https://github.com/schloegl/sigviewer
Utah State University
Before the Fall: Anticipatory Brain Roles in Reactive Balance Control
a) Lean and release system: A custom-made lean-and-release cable system was used to impose temporally unpredictable forward perturbations. All testing was conducted with participants standing in a forward lean position. This forward lean position was maintained by means of a body harness attached to a cable, which was then secured to the wall behind the participant. The cable was fastened posteriorly at mid-thoracic level to the body harness. At the start of each trial, participants were placed with their feet approximately hip width apart. The experimenter had the ability to suddenly release the cable tension thereby perturbing the participant forward. Throughout testing participants were instructed to remain relaxed and react only if the cable released. Gaze fixation was standardized across participants to maintain a consistent handle presence in the peripheral visual field. The handle was placed ~ 30º to the right of central vision. Moreover, body position was set to ensure that the handle was clearly within a graspable range. The experimenter instructed participants to lean as far forward as the cable allowed while keeping both feet in contact with the floor. This position required anterior rotation about the ankle, as the rest of the body remained aligned. The exact forward lean position for each participant was determined as the minimal lean angle where a change-of-support reaction (i.e. forward step) was necessary to recover balance upon cable release.
b) Affordances and constraints: A support handle was positioned on the wall to the right and slightly forward of participants while they leaned into the cable. For half of the trials the handle was freely available and visible. In the event of perturbed balance, this handle acted as a stable support surface to target a compensatory reach-to-grasp. On the trials where the support handle was available, a block was also present directly in front of the participant’s legs to obstruct potential stepping reactions. For trials where the support handle was not available to grasp, a black tarp covered the handle to block it from view. The handle remained mounted at the same location; however, it was physically blocked to prevent direct visual access and to prevent any supportive grasp. For trials without a support handle, no leg block was present. In this situation, a freely available step path afforded a rapid change of leg support to regain balance in the event of perturbed balance.
c) Control of vision: Visual access to a complex (i.e. choice-demanding) environment was limited to a time window immediately before postural perturbation. Access to vision was manipulated in this study by use of liquid crystal goggles. These goggles can be programed to open at precise time points, allowing a means for controlling the onset of visual stimuli in the environment. While closed, these goggles allowed an illuminated view without access to the visual scene therefore participants were unaware of the upcoming response setting. During this visual occlusion period, the configuration of obstacles and handholds were changed for each trial.
d) TMS protocol: In this study, single-pulse TMS was delivered over the hand motor cortical representation while participants stood in a leaning position. TMS pulses occurred in a manner time-locked to the opening of the liquid crystal goggles for all experimental conditions. The purpose was to investigate the influence on motor preparation immediately upon receiving visual access to the environment. It is critical to note that TMS was delivered soon after visual access, but prior to any movement (in trials where movement was required). Magnetic stimuli were delivered to the left primary motor cortex (M1) at the optimal position to obtain a motor evoked potential (MEP) in a representative grasp muscle of the contralateral hand. Specifically, TMS pulses were delivered over the optimal site, the hotspot, to elicit an MEP for the right FDI. Once the hotspot was located, a test stimulus intensity was determined, which was a stimulus intensity where the average MEPs were approximately 1–1.5 millivolts peak-to-peak. The TMS coil remained fixed on the hotspot for all trials and the coil position was reset following any head motion associated with a corrective balance response. Note that test stimulus intensity was determined while subjects were in a standing, forward-lean position (but no cable release) to control for any postural state influence on CSE.
See Guidance Notes for complete description of files, scripts, and data configurations.
See README for description of experimental design.
Cognition and Perception | Cognitive Psychology
This work is licensed under a Creative Commons Attribution 4.0 License.
Bolton, D. A. E., Cole, D. M., Butler, B., Mansour, M., Rydalch, G., & MacDannald, D. (2019). Motor Preparation for Compensatory Reach-to-Grasp Responses When Viewing a Wall-Mounted Safety Handle. Utah State University. https://doi.org/10.26078/7RHG-9J26
Additional FilesREADME.txt (14 kB)
Guidance notes.pdf (167 kB)
Laboratory log.pdf (231 kB)
original cfs files 001_020.zip (1982136 kB)
original cfs files 021_040.zip (1983185 kB)
original cfs files 041_065.zip (2511600 kB)
Signal configurations.zip (10 kB)
Signal analysis scripts.zip (6 kB)