Estimating the Relevance of World Disturbances to Explain Savings, Interference and Long-Term Motor Adaptation Effects

Abstract

Recent studies suggest that motor adaptation is the result of multiple, perhaps linear processes each with distinct time scales. While these models are consistent with some motor phenomena, they can neither explain the relatively fast re-adaptation after a long washout period, nor savings on a subsequent day. Here we examined if these effects can be explained if we assume that the CNS stores and retrieves movement parameters based on their possible relevance. We formalize this idea with a model that infers not only the sources of potential motor errors, but also their relevance to the current motor circumstances. In our model adaptation is the process of re-estimating parameters that represent the body and the world. The likelihood of a world parameter being relevant is then based on the mismatch between an observed movement and that predicted when not compensating for the estimated world disturbance. As such, adapting to large motor errors in a laboratory setting should alert subjects that disturbances are being imposed on them, even after motor performance has returned to baseline. Estimates of this external disturbance should be relevant both now and in future laboratory settings. Estimated properties of our bodies on the other hand should always be relevant. Our model demonstrates savings, interference, spontaneous rebound and differences between adaptation to sudden and gradual disturbances. We suggest that many issues concerning savings and interference can be understood when adaptation is conditioned on the relevance of parameters.

Savings upon Re-Aiming in Visuomotor Adaptation

“…In conclusion, it is becoming clear that motor learning results from a panoply of learning processes that allow flexible behavior in varied environments (Wolpert et al, 2011;Taylor et al, 2014;Huberdeau et al, 2015). The current results highlight that within-session savings for a visuomotor adaptation task is accounted for by action selection (Huang et al, 2011;Haith et al, 2015;Huberdeau et al, 2015;McDougle et al, 2015;Orban de Xivry and Lefévre, 2015), arising from the recall of an aiming strategy.…”

“…Counter to these predictions, we observed savings in groups exposed to large rotations after a long washout where the slow process should have returned to baseline (based on parameters from Zarahn et al, 2008). Berniker and Körding (2011) also proposed a two-process model, one in which small and large errors are treated differently. Critically, the Berniker model cannot account for savings observed when an aiming strategy is evoked by a cue rather than re-exposure to an error.…”

“…A third class builds on the context idea but assumes that the learning process is modified by the context. For example, the large error encountered when a perturbation is reintroduced may entail the recruitment of additional error-based processes (Berniker and Körding, 2011) or lead to a change in the gain on the learning rate based on the direction and magnitude of past errors (Herzfeld et al, 2014).…”

Savings in locomotor adaptation explained by changes in learning parameters following initial adaptation

“…Different variations of the SSM have been recently suggested to explain adaptation and savings during force field (Donchin et al 2003;Smith et al 2006), object rotation (Ingram et al 2011), and visuomotor (Lee and Schweighofer 2009;Zarahn et al 2008) perturbations. Most of these models assume linear time invariant (LTI) properties of the parameters (Donchin et al 2003;Ingram et al 2011;Lee and Schweighofer 2009;Smith et al 2006) while the rest model assumes varying parameters that change with experience (Berniker and Kording 2011;Zarahn et al 2008). All of these error-based models suggest that trial-by-trial adaptation occurs by updating the appropriate internal models (i.e., states) to reflect the behavior of the perturbation.…”

“…Recent results suggest that long-term retention (savings) is affected by the slow learning process (Joiner and Smith 2008) and that the slow process may be sensitive to reward whereas the fast process is not (Huang et al 2011). Furthermore, Berniker and Kording (2011) recently suggested that the fast and slow processes represent assignment of the source of the error to internal and external perturbations, respectively. All these perspectives suggest that savings may be the outcome of a slow learning and slow decaying process.…”