Abstract
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.
Highlights
Selecting an appropriate target for action in an unpredictable environment is crucial to survival for all animals [1, 2]
In the synchronous distractor condition, latencies were returned from 79.1% of trials; in the asynchronous condition from 78% of trials
The psychophysical results showed that latencies in either distractor condition were significantly longer than latencies in the no-distractor condition
Summary
Selecting an appropriate target for action in an unpredictable environment is crucial to survival for all animals [1, 2]. In each of these steptracking experiments the subject would move a manipulandum lever to position a cursor image so that it matched the location of a target image This task forms a simplified framework by which task selection, as expressed in limb movements and distinct from (or together with [39]) saccadic eye movement, can be studied. The task is designed to minimise the cognitive complexity of the target selection and reach-tasks which it is intended to probe It uses vertical lines which move in one dimension only (horizontally), meaning that subjects could plan their movements internally in a one dimensional space, even though the end point moves in two dimensions and the arm movement is carried out in multiple rotational dimensions. This new result is distinct from previous action selection investigations because the results do not have potential confounding factors associated with higher level cognitive loading and explicit 3D visual/movement transformations, illustrating the value of the line task
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