RESEARCH NOTE The affordance of barrier crossing in young children exhibits dynamic, not geometric, similarity Winona Snapp-Childs Æ Geoffrey P. Bingham Received: 21 November 2008 / Accepted: 3 July 2009 / Published online: 22 July 2009 Ó Springer-Verlag 2009 Abstract Previous research has shown that adults per- ceive affordances like the passability of apertures, climb- ability or crossability of steps and graspability of objects. In this study, the affordance for stepping over or onto barriers was examined in young children. This was done by placing three distinct barriers (a foam obstacle, a gap, and a single step up), which were scaled to body size, in the walking paths of 4- and 6-year olds and adults, and observing how they crossed the barriers. Age-related dif- ferences in the scaling of these actions corresponded to levels of movement variability, indicating that children as young as 4 years old are sensitive to their own constraints and scale their actions accordingly. These results indicate that affordances are not directly related to leg geometry, but rather entail the dynamics of the developing percep- tion–action system. Keywords Motor development Á Walking Á Barrier crossing Á Affordance Introduction Imagine walking down a sidewalk and encountering a tree branch, puddle or crack in the sidewalk obstructing your path. As a proficient walker, you are sensitive to what the obstructions allow (is the puddle narrow enough that it can be stepped over or does it have to be jumped over) and you can use this information to make the appropriate changes to your gait, to step or jump over the barrier and continue on your way. We perform such tasks with relative ease, because we have extensive experience walking in cluttered environments. It is through such experiences that one develops the ability to detect the relevant properties of the environment, then plan, modulate and scale one’s move- ments to accommodate environmental constraints or demands (Adolph et al. 2003; Bojczyk and Corbetta 2004). That is, one becomes sensitive to affordances or what the environment allows (Gibson 1979). Studies performed with adults, using tasks such as walking through apertures (Warren and Whang 1987), up stairs (Mark 1987; Warren 1984), over gaps (Burton 1992) and over obstacles (Patla et al. 1996), illustrate how well tuned the perception–action system is to affordances. Warren (1984), for example, demonstrated that the perceived limit of stair climbability was invariant between tall and short individuals when considered as a ratio between stair riser height and leg length; critical stair height was 0.88 (stair height/leg length). Moreover, Warren demonstrated that optimal stair height as determined by both energetics and visual preference tests corresponds to about one-quarter of the leg length. Warren postulated that these ratios were invariant because the proportional geometry of adults’ legs was invariant despite overall dif- ferences in stature, meaning that the length of each leg segment is a constant proportion of the total leg length. Pufall and Dunbar (1992) investigated Warren’s (1984) findings in a study of children 6-, 8- and 10 years old. In one part of the study, children selected the highest step that was climbable; that is, the highest step onto which they could step. Pufall and Dunbar predicted that the ratio between the critical step height and leg length would be the same for 6–10-year-old children and adults, because the proportional geometry of children’s legs is the same as W. Snapp-Childs (&) Á G. P. Bingham Department of Psychological and Brain Sciences, Indiana University, 1101 East Tenth Street (A326), Bloomington, IN 47405-7007, USA e-mail: wsnappch@indiana.edu 123 Exp Brain Res (2009) 198:527–533 DOI 10.1007/s00221-009-1944-9