Motor affordance for grasping a safety handle

Abstract

Mere observation of objects in our surroundings can potentiate movement, a fact reflected by visually-primed activation of motor cortical networks. This mechanism holds potential value for reactive balance control where recovery actions of the arms or legs must be targeted to a new support base to avoid a fall. The present study was conducted to test if viewing a wall-mounted safety handle - the type of handle commonly used to regain balance - results in activation of motor cortical networks. We hypothesized that the hand area of the primary motor cortex would be facilitated shortly after visual access to a safety handle versus when no handle was visible. Here, we used transcranial magnetic stimulation (TMS) to measure corticospinal excitability in hand muscles directly following access to vision while participants performed a seated reach-grasp task. Vision was controlled using liquid crystal lenses and TMS pulses were time-locked to occur shortly after the goggles opened but prior to any cue for movement. Between trials the response environment was unpredictably altered to present either a handle or no handle (i.e. covered). Our results demonstrated a rapid motor facilitation in muscles of the right hand when participants viewed a handle versus trials where this handle was covered. This effect was selective both in terms of the muscles activated and the timing at which it emerged. The First Dorsal Interosseus and Opponens Pollicus muscles (synergists in closing the hand) were facilitated 120 ms after viewing the handle. Interestingly, this effect was absent at earlier (80 ms) and later (160 ms) points. Conversely, Abductor Digiti Minimi, which moves the little finger out from the rest of the hand, tended to diminish when viewing the handle. These findings suggest a rapid engagement of muscles suitable for grasping a handle based on vision. This is consistent with the concept of affordances where vision automatically translates viewed objects into appropriate motor terms. The fact that this affordance effect was present for a wall-mounted safety handle commonly used to regain balance has implications for automatically priming recovery actions with upper limbs suited to our surroundings, even before postural perturbation is detected.

Priming of grasping muscles when viewing a safety handle is diminished with age

“…The previous analysis in our lab focused exclusively on the young adults and revealed reduced ADM activation when viewing the handle [9]. This contrasts the increase in FDI and OP, both muscles which act to close the hand.…”

“…In our laboratory, we recently demonstrated a rapid and selective change in hand muscle activity when young adults viewed a wall-mounted safety handle (i.e. a handle commonly used to aid stability) [9]. These results suggest a rapid engagement of muscles suitable for grasping a safety handle based on vision, and extend upon past affordance work by using a visual stimulus explicitly associated with maintaining stability.…”

“…Based on our findings in the young adults [9], FDI-OP average was facilitated when TMS was delivered specifically at 120 ms after viewing the handle, whereas ADM was reduced across time when the handle was present. Therefore, the planned comparisons here focus on testing if these effects are also evident in older adults.…”

“…Furthermore, participants were screened prior to testing to assess the suitability for TMS using guidelines developed by a consortium of experts [12]. Note that the YA data are taken from McDannald et al [9].…”

Forecast or Fall: Prediction's Importance to Postural Control

“…Furthermore, human studies have demonstrated a measurable link between simple object observation and motor cortical activation even with no requirement to move ( 77 ). Remarkably, the rare clinical condition of alien hand syndrome (which sometimes results from a stroke in the frontal lobes) also offers support for this idea ( 92 ). These patients lack inhibitory oversight and instead are irresistibly compelled to interact with surrounding objects.…”