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John Franchak, Karen Adolph; Perceiving changing affordances for action: Pregnant women walking through doorways. Journal of Vision 2007;7(9):116. doi: 10.1167/7.9.116.
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Affordances for action depend on the characteristics of the environment relative to people's body dimensions and physical abilities. Thus, adaptive motor control requires that people perceive variations in both the environment and their own bodies, and scale their actions accordingly. The current study addressed this issue in a situation where affordances for action undergo dramatic change: We observed pregnant women longitudinally as they decided whether they could fit their bodies through wide and narrow doorways.
Women were tested on a monthly basis, from their third month of pregnancy to one month post-partum. Facing an adjustable doorway from 3 m, women decided whether they could walk through without becoming wedged. The size of the doorway aperture varied from 0 to 74 cm in .5 cm increments. Women provided verbal reports as they approached the doorway, and either attempted passage or refused by turning away. An adaptive psychophysical procedure was used to estimate affordance thresholds (the doorway width that allowed passage on 50% of attempts) and decision thresholds (the doorway width that participants attempted passage on 50% of trials) for each participant at each session.
As expected, affordance thresholds increased over months of pregnancy and decreased sharply following childbirth. Affordance thresholds were highly correlated with body dimensions (abdomen, breast, and hip circumferences). Overall, participants successfully tracked changes in their body size - changes in decision thresholds paralleled changes in affordance thresholds, indicating that pregnant women scale their motor decisions to the relationship between the dimensions of their rapidly changing bodies and small variations in doorway width. On average, however, women overestimated their ability to fit through apertures, by attempting apertures slightly smaller than their affordance thresholds. In addition, participants' visual exploration was determined using a head-mounted eye-tracker.
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