Understanding the use of haptic assistance to facilitate motor learning is a critical
issue, especially in the context of tasks requiring control of motor variability.
However, the question of how haptic assistance should be designed in tasks with redundancy,
where multiple solutions are available, is currently unknown. Here we examined the
effect of haptic assistance that either allowed or restricted the use of redundant
solutions on the learning of a bimanual steering task. 60 college-aged participants
practiced steering a single cursor placed in between their hands along a smooth W-shaped
track of a certain width as quickly as possible. Haptic assistance was either applied
at (i) the 'task' level using a force channel that only constrained the cursor to
the track, allowing for the use of different hand trajectories, or (ii) the 'individual
effector' level using a force channel that constrained each hand to a specific trajectory.
In addition, we also examined the effect of simply 'fading' assistance in a linear
fashion- i.e., decreasing force gains with practice to reduce dependence on haptic
assistance. Results showed all groups improved with practice - however, groups with
haptic assistance at the individual effector level performed worse than those at the
task level. Besides, we did not find sufficient evidence for the benefits of linearly
fading assistance in our task. Overall, the results suggest that haptic assistance
is not effective for motor learning when it restricts the use of redundant solutions.