The presence of motor redundancy means that movement variability can be split into
a 'task-space' component that affects task performance, and a 'null space' component
which has no effect on task performance. While the control of task-space variability
during learning is essential, because it is directly linked to performance, how the
nervous system controls null space variability during learning has not been well understood.
One factor that has been hypothesized to govern the change in null space variability
with learning is task difficulty, but this has not been directly tested. Here, we
examined how task difficulty influences the change in null space variability with
learning. Healthy, college-aged participants (N = 36) performed a bimanual steering
task, where they steered a cursor through a smooth W-shaped track of a certain width
as quickly as possible while attempting to keep the cursor within the track. Task
difficulty was altered by changing the track width and participants were split into
one of the three groups based on the track width that they practiced on-wide, narrow,
or progressive (where the width of the track progressively changed from wide to narrow
over practice). The redundancy in this task arose from the fact that the position
of the cursor was defined as the average position of the two hands. Results showed
that movement time depended on task difficulty, but all groups were able to decrease
their movement time with practice. Learning was associated with a reduction in null
space variability in all groups, but critically, there was no effect of task difficulty.
Further analyses showed that while the task-space variability showed an expected speed-accuracy
tradeoff with movement time, the null space variability showed a qualitatively different
pattern. These results suggest differential control of task and null space variability
in response to changes in task difficulty with learning, and may reflect a strong
preference to minimize overall movement variability during learning.