How children and adults learn to intercept moving gaps

Highlights

• We used a virtual environment to examine how cyclists learn to intercept moving gaps.

• We asked if children and adults benefit from variability of practice for this task.

• Child cyclists show more overcorrections in speed and cross less safely than adults.

• When no speed adjustment is needed, child cyclists do not learn as quickly as adults.

• When speed adjustment is needed, variability of practice affects children’s learning.

Abstract

We used an immersive virtual environment to examine how children and adults learn to intercept moving gaps and whether children and adults benefit from variability of practice. Children (10- and 12-year-olds) and adults attempted to bicycle between two moving vehicle-size blocks without stopping. In Experiment 1, block motions were timed such that if participants maintained a constant speed, they would intercept the gap between the blocks. By the last set of intersections, adults learned to maintain a constant speed throughout the approach to the intersection, 12-year-olds exhibited less variability in time-to-spare when they intercepted the blocks, and 10-year-olds exhibited no significant change across intersection sets. In Experiment 2, block motions during the first eight intersections were timed such that participants needed to either speed up or slow down on all intersections or needed to speed up on half and slow down on half of the intersections. On the last four intersections, all age groups encountered a novel block timing in which no adjustment in speed was necessary to intercept the blocks. The adults performed well regardless of whether they experienced consistent or variable block timings. The 10-year-olds in the variable condition performed better on slow-down trials than their peers in the slow-down condition but performed worse on speed-up trials than their peers in the speed-up condition. Discussion focuses on possible developmental changes in reliance on perceptually available and remembered information in complex perception–action tasks.

Introduction

A critical component of refining any perceptual–motor skill is learning how to bring motor actions more tightly in line with perceptual information. Skill in perception–action tuning is essential for performing temporally sensitive tasks and becomes critical when the tasks in question have potentially severe consequences for failure. One such high-stakes task is crossing through busy traffic as a pedestrian or bicyclist. A sizable body of literature has shown that children and young adolescents are not as proficient as adults at road crossing (Lee et al., 1984, Plumert et al., 2004, te Velde et al., 2005, Young and Lee, 1987). This performance deficit is borne out in pedestrian and bicyclist injury data, where children are overrepresented relative to adults (National Highway Traffic Safety Administration, 2009a, National Highway Traffic Safety Administration, 2009b). A number of studies have indicated that children’s problems with road crossing may be due to immature perceptual–motor skills and in particular children’s ability to coordinate their movement with the movement of other objects in the environment (Chihak et al., 2010, te Velde et al., 2005). However, we know little about how perceptual–motor tuning in movement synchronization tasks improves as children gain experience with performing such tasks.

Contemporary views of perceptual–motor development suggest that short-term learning experiences produce long-term developmental changes in the perception–action system (Newell et al., 2001, Thelen and Smith, 1994). It seems likely, then, that exploring differences in how adults and children learn to perform tasks over a relatively short interval will provide insight into the mechanisms by which short-term changes in perception–action tuning links to long-term development. The objectives of the current study were to explore how short-term learning in a gap interception task varies between child and adult bicyclists and to determine whether children and adults benefit from variability of practice while performing this task.

Road crossing requires the successful completion of two perceptual–motor tasks. Roadway users must first identify a gap in traffic that affords safe crossing. This involves user assessments of how long they will take to cross the lane of traffic and how long vehicles will take to reach their crossing path (i.e., time-to-arrival). A gap affords crossing if the individual’s (projected) crossing time is less than the temporal size of the gap (Lee et al., 1984). Once an acceptable gap has been identified, users must then coordinate their movement through the gap with the movement of traffic in order to avoid a collision with a vehicle. This involves cutting in closely behind the lead vehicle in the gap while crossing as quickly as possible. To effectively cut in behind the lead car with minimal clearance, roadway users need to exert prospective control over movement by anticipating exactly when to begin moving.

Previous studies by Plumert et al. (2004) and Plumert, Kearney, Cremer, Recker, and Strutt (2011) explored how children (10- and 12-year-olds) and adults cross roadways by having them ride a bicycle through a virtual environment consisting of a straight residential street with multiple intersections. Participants faced cross traffic from their left-hand side and waited for gaps that they judged were adequate for crossing. The results clearly showed that, relative to adults, children’s gap choices and road-crossing behavior were less finely tuned. Children and adults chose to cross through the same size gaps, yet children ended up with less time-to-spare when they cleared the path of the car. For 10-year-olds in particular, this resulted in a very narrow safety margin. Further analyses revealed that children delayed their entry into the roadway relative to the lead vehicle in the gap (as compared with adults). This resulted in less time-to-spare when children cleared the path of the approaching car. These differences in how children and adults time movement suggest that immature perceptual–motor skills may play a role in putting children at greater risk for car–bicycle collisions.

A follow-up study by Chihak et al. (2010) modified this road-crossing task to examine movement synchronization in children and adults by providing a single target gap through which riders were to cross without stopping. The target gap in the cross traffic was timed to arrive at the intersection such that riders would need to either speed up or slow down in order to successfully intercept the gap. Although children and adults appeared to be using similar strategies for intercepting a moving gap without stopping, children were much more variable on their approach to the intersection and made significant speed overcorrections as they approached. Interestingly, the incidence and magnitude of children’s speed overcorrections were significantly greater on trials where the rider was required to slow down than to speed up in order to intercept the gap. More specifically, children slowed down far more than was necessary as they approached the intersection, which then required them to accelerate sharply in order to intercept the gap. The net effect of children’s imprecise movement timing was a significantly smaller safety margin than was seen in adults. In all, child bicyclists appeared to be less skilled at synchronizing their movement with the movement of other objects.

Children’s difficulty with coordinating self and object movement is not limited to road crossing and has been observed across a number of interceptive tasks. For example, te Velde, van der Kamp, and Savelsbergh (2008) found that younger children (5- to 7-year-olds) were less successful than older children (10- to 12-year-olds) and adults when performing a small-scale interception task that involved moving a doll across a small-scale “roadway” in between two approaching model cars. In another study, Chohan, Verheul, Van Kampen, Wind, and Savelsbergh (2008) had 5- to 7-year-olds, 10- to 12-year-olds, and adults walk to intercept a moving target in the real world. As in the Chihak et al. (2010) study, children’s performance suggested that although they were trying to use a strategy similar to that of adults when intercepting the target, their performance was significantly more variable due to less precise movement timing.

Numerous studies have shown that adults exhibit a tightening of their interception skills with practice, even over relatively short time periods (Buekers et al., 1999, Camachon et al., 2007, Montagne et al., 2003). For example, using a highly constrained interception task in a virtual environment, Montagne et al. (2003) found that adult performance became less variable and more structured over the course of the experimental session. Other work has shown that even 8-month-old infants are able to manually intercept (i.e., “catch”) moving objects provided that the objects are moving along a stable arc at a relatively slow speed (von Hofsten, 1983). However, little work has been devoted to investigating how children become more proficient at performing interceptive tasks involving translation of the actor through the environment. In particular, what changes in timing skills are occurring as children become better at performing a movement synchronization task, and are these changes different from what is observed in adults?

Some indication of how children might be learning to coordinate movement can be drawn from previous work looking at short-term changes in children’s road-crossing performance. Plumert et al. (2011) explored how movement timing changed in both child (10- and 12-year-olds) and adult cyclists while crossing 12 intersections during the course of a single experimental session. The 10-year-olds in particular benefited from experience with the road-crossing task in that they showed improved timing of their movement relative to the lead car in the gap across the trials (i.e., cutting in closer behind the lead car when they entered the intersection). As a result, they also improved their safety margins during the later trials. These types of gains in movement timing over short-term timescales may help to produce the developmental changes seen over longer term timescales.

One question left unanswered is how variability of practice affects children’s perceptual–motor learning in interception tasks. Previous work indicates that when individuals perform a task over a range of conditions (e.g., throwing balls of varying weights), their overall performance for that entire class of movements (ball throwing) will increase more rapidly than if they perform the task the same number of times under consistent conditions (e.g., throwing balls of the same weight). This phenomenon has been documented over a broad range of perceptual and motor tasks such as ball throwing, airplane guidance, and even discrimination of speech sounds (Rost and McMurray, 2009, Schmidt, 1975, Shea and Wulf, 2005). For example, recent work by Huet et al. (2011) manipulated elements of the visual scene that could be used to guide a landing task in a flight simulator. In the variable practice condition, adult participants encountered different values of these visual variables on different trials. Participants in the constant practice condition experienced the same number of trials, but the visual variables were held constant. Adults who received variable practice performed better in the absence of feedback and were better at adapting to a novel landing scenario than participants who had experienced constant practice. Although considerable work has been done exploring the benefits of variable practice for children when performing simple motor tasks (e.g., Clifton, 1985, Smoll and DenOtter, 1976, Wulf, 1991), the effects of variable practice when coordinating a complex movement synchronization task such as road crossing have yet to be explored.

The objective of the current investigation was to identify differences in how children and adults learn to perform a gap interception task in a single experimental session. Children (10- and 12-year-olds) and adults rode a bicycling simulator through a series of intersections in an immersive, interactive virtual environment. Participants’ task at each intersection was to pass without stopping through a gap between two fast-moving objects (i.e., rectangular blocks the size of a typical car that moved at 35 miles per hour [mph]). In Experiment 1, we were interested in seeing whether repeated performance on a task in which no speed adjustment was required would produce differential learning effects for adults and children over the course of the experimental session. Therefore, the blocks at all of the intersections were triggered to begin moving such that if the rider made no adjustments in speed, he or she would pass directly through the middle of the target gap. We were particularly interested in whether children would exhibit overcorrections in their approach (i.e., slowing down far more than is necessary) and whether the variability and magnitude of overcorrections in children changed over the course of the session.

In Experiment 2, we wanted to determine how practice with consistent versus variable trial types affected the rate at which children and adults improved their performance on the interception task and adapted to a novel block timing. To address this question, the block timings were adjusted such that on the first eight trials participants needed to adjust their speed in order to successfully intercept the gap. Participants were required to either speed up on all of these trials, slow down on all of these trials, or speed up on some trials and slow down on other trials. During the last four intersections, participants in all conditions experienced a block timing they had not previously encountered in which no speed adjustment was necessary (the same timing as in the first experiment). We expected that children in the variable trials condition would demonstrate more rapid perception–action tuning during the first eight trials and would be quicker to adapt to the novel block timing during the last four trials than those who had seen only speed-up or slow-down trials.