Abstract
Adaptation to deterministic force perturbations during reaching movements was extensively studied in the last few decades. Here, we use this methodology to explore the ability of the brain to adapt to a delayed velocity-dependent force field. Two groups of subjects preformed a standard reaching experiment under a velocity dependent force field. The force was either immediately proportional to the current velocity (Control) or lagged it by 50 ms (Test). The results demonstrate clear adaptation to the delayed force perturbations. Deviations from a straight line during catch trials were shifted in time compared to post-adaptation to a non-delayed velocity dependent field (Control), indicating expectation to the delayed force field. Adaptation to force fields is considered to be a process in which the motor system predicts the forces to be expected based on the state that a limb will assume in response to motor commands. This study demonstrates for the first time that the temporal window of this prediction needs not to be fixed. This is relevant to the ability of the adaptive mechanisms to compensate for variability in the transmission of information across the sensory-motor system.
Highlights
Fast reaching movements are ballistic, voluntary movements of the arm from a starting point to a given target [1,2]
After prolonged exposure to deterministic perturbation forces, which depend on the state of motion of the hand – i.e., on its position and velocity – the internal model adapts to fit the combination of the arm dynamics and the applied force field [4,9]
The presence of adaptation to the delayed force field is seen clearly by analyzing the short movements of the training sessions: there is a process of learning during sessions 3–5, which is highlighted by after-effect in catch-trials at the end of these sessions (Figure 2)
Summary
Fast reaching movements are ballistic, voluntary movements of the arm from a starting point to a given target [1,2]. After prolonged exposure to deterministic perturbation forces, which depend on the state of motion of the hand – i.e., on its position and velocity – the internal model adapts to fit the combination of the arm dynamics and the applied force field [4,9]. At this point, an unexpected removal of the perturbation (called a ‘‘catch-trial’’) results in an erroneous movement, which generally resembles the mirror image of the initial deviation, caused by the perturbing force. This typical response to a catch trial is known as after-effect of adaptation
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