Do Exploratory Arm Movements Contribute to Reach-Ability Judgments?
Authors: Widlus Benjamin P. , Jones Keith S. 
Published: October 2017

Gibson (1979/1986) argued that exploratory movements generate information about agents’ action-capabilities within a given environment, that is, about the agent-environment system’s affordances. To date, the scant literature on exploratory movements has revealed two important findings. First, restricting exploratory movements degrades the accuracy of affordance judgments (Mark et al., 1990; Yu, Bardy, & Stoffregen, 2011). Second, exploratory movements can be very subtle (Stoffregen, Yang, & Bardy, 2005; Yu, Bardy, & Stoffregen, 2011).

However, many questions regarding exploratory movements have yet to be answered. For example, what exploratory movements are necessary to perceive a given affordance, and how do exploratory movements differ from related movements? Our long-term goal is to address such gaps in the literature. We decided to begin by examining what exploratory movements must be executed in order to perceive whether the actor can reach an object.

Reaching exploratory movements likely have two key components: 1) head movements and 2) shoulder movements. The former can generate information about the absolute distance between the actor and the to-be-reached object (Bingham & Stassen, 1994), and have been confirmed to be necessary to produce accurate reaching judgments (Mantel, Stoffregen, Campbell & Bardy, 2015). The latter generates information about the actor’s arm length (Anderson & Turvey, 1998; Shibata, Gyoba, & Takeshima, 2012;), but their necessity to the reach-ability judgment has yet to be studied.

The current experiment used a restriction paradigm to determine whether exploratory arm movements are necessary to make accurate reaching judgments. Participants (n = 32) judged their maximum reaching ability either while holding their arms behind their backs with their dominant hand grasping their non-dominant wrist (the Restricted condition), or while their arms swung naturally at their sides (the Unrestricted condition). Judgments were made actively, by walking forward or backward, in order to allow participants to generate the exploratory movements they would normally create (with the exception of arm movements in the Restricted condition) when moving toward an object with the intention to perform a reach (Mantel, Bardy, & Stoffregen, 2010).

The study utilized a within-subjects design, with starting condition counterbalanced. For each condition, participants completed 1 practice trial followed by 9 experimental trials. Starting distances (from object) and angles were drawn equally and randomly from ranges of 1 – 24”, 25 – 48”, 49 – 72”, and 0 - 29o, 30 - 59o, 60 - 89o, respectively. Distances and angles were not repeated to prevent memorization.

In line with previous affordance perception research, the dependent variable, Accuracy, was computed in terms of percentage of absolute error (|[judged maximum reach / actual maximum reach] -1| *100) (Oudejans, Michaels, Bakker, & Dolné, 1996). Accuracy was significantly greater when arm movements were unrestricted as compared to restricted, supporting the theory that exploratory arm movements are a component of reach-ability judgments. Reaching judgments in neither condition were perfectly accurate, which may have been due to the reaching judgment being the focal task (Heft, 1993).

The present results have practical implications for operational situations in which actors’ arm movements might be restricted. For example, U.S. police and military personnel sometimes wear body armor that covers their shoulders, mounts ballistic plates to their upper arms, or some combination thereof. To the extent that such body armor restricts arm movements, then our results suggest that their reach-ability judgments would be degraded.