Motor decisions are not black and white: selecting actions in the "gray zone"

Abstract

In many situations, multiple actions are possible to achieve a goal. How do people select a particular action among equally possible alternatives? In six experiments, we determined whether action selection is consistent and biased toward one decision by observing participants' decisions to go over or under a horizontal bar set at varying heights. We assessed the height at which participants transitioned from going over to under the bar within a "gray zone"-the range of bar heights at which going over and under were both possible. In Experiment 1, participants' transition points were consistently located near the upper boundary of the gray zone, indicating a bias to go over rather than under the bar. Moreover, transitional behaviors were clustered tightly into a small region, indicating that decisions were highly consistent. Subsequent experiments examined potential influences on action selection. In Experiment 2, participants wore ankle weights to increase the cost of going over the bar. In Experiment 3, they were tested on a padded surface that made crawling under the bar more comfortable. In Experiment 4, we introduced a secondary task that required participants to crawl immediately after navigating the bar. None of these manipulations altered participants' decisions relative to Experiment 1. In Experiment 5, participants started in a crawling position, which led to significantly lower transition points. In Experiment 6, we tested 5- to 6-year-old children as in Experiment 1 to determine the effects of social pressure on action selection. Children displayed lower transition points, larger transition regions, and reduced ability to go over the bar compared to adults. Across experiments, results indicate that adults have a strong and robust bias for upright locomotion.

Keywords: Action selection; Affordance perception; Locomotion; Motor decisions; Obstacles.

Figure 1

Walkway apparatus, procedure in the “Over/Under” task, and various methods for going over and under a horizontal bar impeding the path of locomotion. (A) Participants started each trial on a raised walkway, with their back to the opening. The trial began when the experimenter prompted the participant to turn around and approach the opening. Participants decided whether to go over or under each bar height. Coders classified locomotor methods for going over as (B) stepping, (C) tiptoeing (going over a height higher than the measured leg length without hopping), and (D) hopping (both feet off floor for at least one frame). Coders classified methods for going under as (E) crawling (both hands and both knees, or torso or buttocks touched the floor), (F) midway (any combination of hands or knees on the floor that did not fulfill requirements for crawling), and (G) bipedal (hands, knees, buttock, belly, or back did not touch the floor). Crawling occurred on hands and knees or on the belly. Bipedal strategies including walking, ducking, and squat-walking.

Figure 2

Summaries of average transition regions and transition points for Experiments 1–6 represented as (A) a percentage of participants’ standing height and (B) a percentage of their gray zone (the range of bar heights where participants could go both over and under the bar). The cartoon figure in (A) represents participants’ average standing height. The size of the gray bars in (A) shows that the adults in Experiments 1–5 had gray zones that spanned from mid-calf to hip height. Compared with adults, the children in Experiment 6 had smaller gray zones with lower boundaries closer to the knee and upper boundaries closer to mid-thigh. In both (A) and (B), the horizontal red lines represent the average transition point (the estimated height where participants transitioned from going over to under the bar), the vertical black lines represent the average extent of the transition region (the range of bar heights where participants chose to go both over and under), and the black horizontal lines represent the boundaries of the gray zone. Only Experiments 5 and 6 produced significantly lower transition points than Experiment 1, both ps < .04.

Figure 3

Individual participants’ transition points and transition regions as a percentage of their gray zone (the range of bar heights where participants could go both over and under the bar) in Experiments 1–6. The horizontal red lines represent participants’ transition points (the estimated height where participants transitioned from going over to under the bar), and the vertical black lines represent the transition regions (the range of bar heights where participants chose to go both over and under). Adults (Experiments 1–5) were highly consistent in their decisions (as indicated by their small transition regions) and were consistently biased to go over the bar (as indicated by all transition points above 66% of their gray zone). Children (Experiment 6) were highly inconsistent in their decisions (as indicated by their large transition regions) and they did not show a consistent bias to go over the bar (transition points spanned the entire range of the gray zone).