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.

Figure 1. The likelihood of relevance.

A) Before adapting, estimates for body and world disturbances are zero. The hand's path, along with the estimated hand and cursor location, fall along a straight path to the target. The large observed errors (red and blue arrows) of the perturbed visual display indicate the likelihood of a world disturbance being relevant is high. B) During adaptation, the hand's path is adjusted to compensate for the estimated body and world disturbances. Even though errors between the estimated cursor location and the observed cursor location have been reduced (blue arrow), the large error between the estimated hand location and the observed feedback (red arrow) continues to indicate the likelihood for the world parameter relevance is still high.

Figure 2. Short-term savings after washout.

A) Angular reach errors during the first presentation of a visuomotor disturbance, washout (while grasping robot) and subsequent presentation of the same disturbance B) Inferred body and world rotation parameters during adaptation and the corresponding probability of relevance. C) Angular reach errors from first and second presentation of visuomotor disturbance overlaid.

Figure 3. Long-term savings and interference.

A) Inferred body and world rotations and the corresponding probability of relevance during the first presentation of a visuomotor disturbance, washout (after experiment has ended) and subsequent presentation of the same disturbance on a second day. B) Angular reach errors from the first and second presentation of the visuomotor disturbance overlaid. C) Experimental findings after the same adaptation (reproduced from [19]). D) Inferred body and world rotations and probability of relevance during a visuomotor disturbance, an oppositely oriented disturbance, washout (after experiment has ended) and subsequent presentation of the original disturbance on a second day. E) Angular reach errors from the first and second presentation of disturbance overlaid. F) Experimental findings after same adaptation (reproduced from [19]).

Figure 4. Savings after a gradually introduced disturbance.

A) Inferred body and world rotations and probability of relevance while a disturbance is gradually introduced, washout (after experiment has ended) and presentation of full disturbance on a second day. B) Angular reach errors on first and second day after adapting to a gradually (grey lines), or suddenly (black lines) introduced disturbance. C) Experimental findings of same adaptations (reproduced from [23]).

Figure 5. Error clamps and spontaneous rebound.

A) Inferred body and world rotation parameters and probability of relevance during adaptation to a visuomotor disturbance and subsequent error clamp. In the error clamp, feedback indicates a lack of errors regardless of movements. B) Experimental data of normalized reaching forces during adaptation to a force disturbance and subsequent error clamp (reproduced from [24]). C) Inferred body and world rotations and the probability of relevance during presentation of a visuomotor disturbance, visuomotor disturbance of opposite orientation and subsequent error clamp. D) Experimental data of normalized reaching forces during a force disturbance, opposite disturbance and subsequent error clamp (reproduced from [5]).