Abstract
Affordances—possibilities for action—are constrained by the fit between the physical properties of the actor and the environment. For example, fitting the body through a narrow opening is constrained by the dimensions and orientation of both the aperture and actors' body. Vision allows actors to compare their body dimensions to the size of the aperture, and enables them to plan prospectively how best to orient the appropriate body parts relative to the shape of the opening. Adaptive motor decisions depend on the ability to detect affordances accurately.
In three experiments, we examined the correspondence between motor decisions and actual affordances for reaching through apertures. Participants fit their hands through a range of diamond-shaped apertures (0 cm – 40 cm) to retrieve small targets. Using an adaptive psychophysical procedure, we estimated affordance thresholds for each participant (smallest aperture they could fit their hand through 50% of the time). We examined participants' motor decisions normalized to each participant's affordance threshold.
Experiment 1 showed that participants accurately scaled motor decisions for fitting their dominant hand through apertures to their hand size, and that decisions and thresholds were reliable over two blocked protocols. Experiment 2 examined the effects of habitual practice; participants made equally accurate motor decisions when reaching with their more practiced dominant hand and less practiced non-dominant hand. Experiment 3 showed that participants recalibrated their motor decisions to take changing body dimensions into account; motor decisions normalized to affordance thresholds while wearing a hand-enlarging glove were similar to motor decisions normalized to affordance thresholds without the glove.
In all 3 experiments affordance thresholds were highly correlated with participants' hand width. Additionally, participants produced a range of information gathering behaviors ranging from solely visual exploration to manual exploration of the apertures.