The Moving Platform Aftereffect: Limited Generalization of a Locomotor Adaptation

Abstract

We have recently described a postural after-effect of walking onto a stationary platform previously experienced as moving, which occurs despite full knowledge that the platform will no longer move. This experiment involves an initial baseline period when the platform is kept stationary (BEFORE condition), followed by a brief adaptation period when subjects learn to walk onto the platform moving at 1.2 m/s (MOVING condition). Subjects are clearly warned that the platform will no longer move and asked to walk onto it again (AFTER condition). Despite the warning, they walk toward the platform with a velocity greater than that observed during the BEFORE condition, and a large forward sway of the trunk is observed once they have landed on the platform. This aftereffect, which disappears within three trials, represents dissociation of knowledge and action. In the current set of experiments, to gain further insight into this phenomenon, we have manipulated three variables, the context, location, and method of the walking task, between the MOVING and AFTER conditions, to determine how far the adaptation will generalize. It was found that when the gait initiation cue was changed from beeps to a flashing light, or vice versa, there was no difference in the magnitude of the aftereffect, either in terms of walking velocity or forward sway of the trunk. Changing the leg with which gait was initiated, however, reduced sway magnitude by approximately 50%. When subjects changed from forward walking to backward walking, the aftereffect was abolished. Similarly, walking in a location other than the mobile platform did not produce any aftereffect. However, in these latter two experiments, the aftereffect reappeared when subjects reverted to the walking pattern used during the MOVING condition. Hence, these results show that a change in abstract context had no influence, whereas any deviation from the way and location in which the moving platform task was originally performed profoundly reduced the size of the aftereffect. Although the moving platform aftereffect is an example of inappropriate generalization by the motor system across time, these results show that this generalization is highly limited to the method and location in which the original adaptation took place.

Attention modulates adaptive motor learning in the ‘broken escalator’ paradigm

“…Once learnt, certain motor strategies are executed automatically (implicitly) (Voss et al 2008), hastening the response (Mazzoni and Krakauer 2006) and freeing attention resources for other activities (Malone and Bastian 2010). Contextual cues presumably dictate whether the previously learnt motor program has to be released (the sled or escalator may move) or not (a solid platform or stairs will not move) (Reynolds and Bronstein 2004;Fukui et al 2009), probably through an internal probabilistic risk assessment (Green et al 2010). Breaching this contextual threshold releases the LAE even whilst performing a secondary cognitive task.…”

“…Stepping onto a broken (stationary) escalator may cause a stumble and an odd sensation (Fukui et al 2009), termed the 'locomotor aftereffect' (LAE) (Reynolds and Bronstein 2003;Reynolds and Bronstein 2004;Bronstein et al 2009) that results from prior adaptation to a moving escalator. The LAE occurs despite prior knowledge that the escalator is broken and will not move (Reynolds and Bronstein 2003;Reynolds and Bronstein 2004;Bronstein et al 2009).…”

“…The LAE occurs despite prior knowledge that the escalator is broken and will not move (Reynolds and Bronstein 2003;Reynolds and Bronstein 2004;Bronstein et al 2009). Indeed, transcranial direct current stimulation (tDCS) applied over the motor cortex before the adaptation task has been shown to enhance the LAE (Kaski et al 2012), suggesting that the aftereffect relies upon cortical processing.…”

Generalization of Gait Adaptation for Fall Prevention: From Moveable Platform to Slippery Floor

“…In contrast, little interlimb transfer has been observed for the adaptation of walking acquired with a split-belt treadmill training (Choi and Bastian 2007). Along the same lines, the "observed aftereffect" of stepping onto a moving sled with the training limb has persisted in only a few subjects when they stepped on the sled with the untrained limb (Reynolds and Bronstein 2003a).…”

“…Previous findings on such issues are inconsistent. For example, Reynolds and Bronstein (2003a) have demonstrated persistence of the postural "aftereffect" of the acquired adaptation (increased trunk anterior lean) only under the "context of adaptation," i.e., the "sled condition." Transfer of obstacle avoidance skills acquired while walking on a level surface has been observed when the surface inclination was changed to downhill walking (Lam and Dietz 2004).…”