Target-distractor synchrony affects performance in a novel motor task for studying action selection

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.

The full list of reasons for omission is given below, with the descriptive code for the reason in bold typeface and the number of occurrences (out of 17617 events) in italics.
Target position change less than minimum jump size: If the target line's position changed less than a threshold of 20 pixels, it was omitted from analysis. This was used to allow the output of the script to be compared with an alternative method for measuring latency and error rates, described later. 410 occurrences.
Stable position later than event onset: If the stylus did not achieve a stable pre-event position, then it was not safe to measure a latency for the movement for the event. This usually occurred when two target events followed each other in quick succession and the subject had not had time to reach the first target position before it moved for a second time. 120 occurrences.

Too fast (targ) and Too fast (distractor):
Any event with a movement latency below 100 ms (the value of the parameter A.fastest_brain_decision in the lt_analyse_latency.m script) was omitted. Previous research suggests that participants cannot make movements faster than this [1,2]. The cause of such apparently short latencies in the raw data was usually the lack of a stable pre-event stylus position; the subject was not able to settle at the previous target position. 130 occurrences (target), 20 occurrences (distractor).

No movement detected
No movement was detected for a target event, and therefore no latency could be determined. This may have occurred if the target position change was very small. 21 occurrences.

Stable stylus period too short
If the stable stylus position period was less than 200 ms for targets (A.min_stableposition_period or 150 ms for distractors (A.min_stableposition_period_dist) then the event was omitted, which is to say that a latency was measured only if the stylus remained in a stable position for at least the above periods of time. It was the second most common reason for omitting an event from latency and error measurements. 731 occurrences (target), 1743 occurrences (distractor).

Drift too great during stable stylus period
If the stylus position at the end of the stable stylus period had moved more than 15 px (A.max_move_thresh) with respect to the stylus position at the start of the stable stylus position period, then the event was omitted with this reason. 21 occurrences (target), 18 occurrences (distractor).

Drift too great during stable period (avg)
If the average speed of the stylus during the stable stylus period was greater than a threshold of 0.01 px/ms (0.011 cm/s) for a target event (A.max_avg_drift_speed) or 0.02 px/ms (0.023 cm/s) for a target event (A.max_avg_drift_speed_dist) then the event was omitted. Because the mean time between target events was set to 1.2 s, a fast rate, this was the most common reason for omitting movements from analysis. 1820 occurrences (target), 957 occurrences (distractor).

Movement occurs beyond next target
This omit reason was recorded for a distractor event when any detected motion occurred after the next target event. It means that the subject did not make an error movement, and therefore no latency could be measured. However, these distractor events were included when computing the proportion of distractor events which caused an error movement. 1756 occurrences (distractor).

Subject was distracted by closely previous distractor
These last four reasons for omission concern the case where there was an asynchronous distractor and a distractor event and target event occur temporally close together. In this particular case, either the target or the distractor may have caused the movement; both lines were displaced in the same direction with respect to the stable stylus position. The distractor event occurred before the target event. Because the recorded motion was away from the target, the target was omitted and the event was recorded as a distraction in the earlier distractor event. 4 occurrences (target).

Incorrect move was recorded in previous distractor event
In this case, the target and distractor lines were located in opposite directions from the initial stylus position; again, the distractor line occurred first. The stylus initially followed the distractor and so an incorrect movement was recorded in the distractor event, and the target event was omitted with this reason code. 4 occurrences (target).