  The study of action selection in humans can present challenges of task design
since our actions are usually defined by many degrees of freedom and therefore
occupy a large action-space. While saccadic eye-movement offers a more
constrained paradigm for investigating action selection, the study of
reach-and-grasp in upper limbs has often been defined by more complex
scenarios, not easily interpretable in terms of such selection. Here we present
a novel motor behaviour task which addresses this by limiting the action space
to a single degree of freedom in which subjects have to track (using a stylus)
a vertical coloured target line displayed on a tablet computer, whilst ignoring
a similarly oriented distractor line in a different colour. We ran this task
with 55 subjects and showed that, in agreement with previous studies, the
presence of the distractor generally increases the movement latency and
directional error rate. Further, we used two distractor conditions according to
whether the distractor's location changes asynchronously or synchronously with
the location of the target. We found that the asynchronous distractor yielded
poorer performance than its synchronous counterpart, with significantly higher
movement latencies and higher error rates. We interpret these results in an
action selection framework with two actions (move left or right) and competing
'action requests' offered by the target and distractor. As such, the results
provide insights into action selection performance in humans and supply data
for directly constraining future computational models therein.
