The effect of target jumps on implicit adaptation of reaching
Date
2020-05
Authors
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Publisher
University of Delaware
Abstract
Sensorimotor adaptation is an implicit motor learning process that corrects and
compensates for motor errors, contributing to the successful execution of everyday
movements. Previous research into the control of reaching movements suggests that
the implicit motor adaptation system is sensitive to two types of error signals - sensory
prediction error (SPE), defined as the difference between where visual feedback of
hand position is expected to appear and where it actually appears, and target error
(TE), defined as the difference between the cursor position and final target position
(Leow et al., 2018). Traditional adaptation experiments confound these two error
signals, with the SPE and TE typically being the same. This prompted the introduction
of shifting targets during reaching movements, a potential means of isolating TE and
its effects. We have previously assumed that this method is able to induce a TE by
having the target jump to a new location approximately one centimeter into the reach.
However, it is unclear whether the motor system processes the target jump as a target
error or if the jump introduces uncertainty regarding the SPE, as it is no longer
anchored to the original target location. We refer to this latter perspective as a “lowlevel
processing error”. Here, we observed the effects of target jumps during three
different conditions designed to assess adaptive responses to three combinations of
SPE and TE conditions, “amplified”, “consistent”, and “opposite” in reference to
whether the target jump amplified the TE, remained consistent throughout the trial, or
elicited a TE in opposition to the SPE. This experiment aimed to determine whether
target jumps elicit a TE, independent of SPE, or if they result in non-specific
attenuation of adaptation due to low-level processing errors. Support for the former
hypothesis would result in a graded amount of adaptation, with the most observed
during amplified trials and the least during opposite trials. In contrast, if target jumps
result in low-level processing errors, then adaptation should be attenuated a similar
amount during amplified and opposite trials, as the absolute target jump magnitudes
were equivalent for these two trial types. Our results revealed a trend towards greater
amounts of adaptation during the consistent condition, which had no target jump,
compared with less adaptation in the amplified and opposite conditions. These results
argue against the interpretation that target jumps elicit a “pure” TE, and suggest that
they may have non-specific effects on adaptation from SPE, potentially due to a lowlevel
processing error.
Description
Keywords
exercise science, target jumps, reaching