Inhibition in movement plan competition: reach trajectories curve away from remembered and task-irrelevant present but not from task-irrelevant past visual stimuli

A violation of Fitts' Law occurs when a target range is presented before and during movement

According to Fitts' Law, the time to reach a target (movement time, MT) increases with distance. A violation of Fitts' Law occurs when target positions are outlined before and during movement, as MTs are not different when reaching to the farthest and penultimate targets. One hypothesis posits that performers cognitively process the edges of a target array before the center, allowing for corrective movements to be completed more quickly when moving to edge targets compared to middle targets. The objective of this study was to test this hypothesis by displaying a target range rather than outlines of individual targets in an effort to identify the effects of array edges. Using a touch-screen laptop, participants (N = 24) were asked to reach to one of three targets which would appear within a presented range. Separately, targets were also presented without a range to determine if the display protocol could evoke Fitts' Law. Movements were assessed with the touch screen and optical position measurement. A main effect was found for relative position within a range (touch: F2,44 = 15.4, p < 0.001, η2p = 0.412; position: F2,40 = 15.6, p < 0.001, η2p = 0.439). As hypothesised, MT to the farthest target in a range was not significantly different than MT to the middle target (touch: p = 0.638, position: p = 0.449). No violation was found when a target range was not presented (touch: p = 0.003, position: p = 0.001). Thus, a target range reproduces the Fitts' Law violation previously documented with individually outlined targets, which supports and extends the discussed hypothesis.

A Violation of Fitts' Law is Maintained in Ecologically Valid Settings

A 'violation' of Fitts' Law, or Fitts' Equation, occurs when each potential target location is outlined before and during a reaching movement. Past studies have measured the violation in highly controlled laboratory environments, limiting the generalizability of findings. The purpose of the study was to replicate the violation of Fitts' Equation in the homes of participants using a novel portable apparatus during the COVID-19 pandemic. Movements were measured independently with an accelerometer and touch screen, which allowed for kinematic, temporal, and spatial outcomes to be measured in remote environments. The violation of Fitts' Equation was found with the touch and acceleration measurements and was thus seen in ecologically valid environments. The apparatus used may be used as a model for future field research.

Reaching movements are attracted by stimuli that signal reward

When presented with a set of possible reach targets, the movement trajectory can reveal aspects of the underlying competition for action selection. Current goals and physical salience can affect the trajectory of reaching movements to be attracted towards a distractor. Some studies demonstrated that stimuli associated with reward can also cause an attraction when reaching towards the reward stimulus was previously rewarded and the reward stimulus was physically salient. Here we demonstrate that a non-salient stimulus that signals the availability of reward attracts reaching movements even when moving towards it was never necessary nor rewarded. Moreover, the attraction by reward is particularly evident with short-latency movements. We conclude that neither physical salience nor reinforcing the movement towards a stimulus is necessary for reward to gain priority in the selection for action.

The attractiveness of salient distractors to reaching movements is task dependent

Previous studies in visual attention and oculomotor research showed that a physically salient distractor does not always capture attention or the eyes. Under certain top-down task sets, a salient distractor can be actively suppressed, avoiding capture. Even though previous studies showed that reaching movements are also influenced by salient distractors, it is unclear if and how a mechanism of active suppression of distractors would affect reaching movements. Active suppression might also explain why some studies find reaching movements to curve towards a distractor, while others find reaching movements to curve away. In this study, we varied the top-down task set in two separate experiments by manipulating the certainty about the target location. Participants had to reach for a diamond present among three circles. In Experiments 1 and 3, participants had to search for the reach targets; hence, the target's location certainty was low. In Experiments 2 and 3, the target's location was cued before the reach; hence, the target's location certainty was high. We found that reaches curved towards the physically salient, color singleton, distractor in the search-to-reach task (Experiments 1 and 3), but not in the cued reach task (Experiments 2 and 3). Thus, the saliency of the distractor only attracted reaching movements when the certainty of the target's location was low. Our findings suggest that the attractiveness of physically salient distractors to reaching movements depends on the top-down task set. The results can be explained by the effect of active attentional suppression on the competition between movement plans.