Functional Specificity of the Affordance of Reaching

The information that specifies whether an object is within reach is a complex pattern that depends on body-scaled parameters measured from an egocentric reference point. The pattern is a function of relevant body proportions (eye height, shoulder height [SH], arm length) with respect to the spatial location of the target object. In addition to not knowing how these factors map onto perception, it is also not known whether the egocentric viewpoint is centered at the eye or the shoulder. In three experiments, we systematically tested whether observers can perceive eye height and SH (Experiment 1), whether they can point accurately in the direction of a target object (Experiment 2), and whether they can point accurately to judge if the target object is within reach (Experiment 3). Experiment 1 demonstrated that participants are more accurate at judging their own eye height than SH. Experiment 2 revealed that participants can more accurately point to a target object's location when measured from the shoulder as a reference point than when measured from the eye. In Experiment 3, we showed that a higher-order variable that includes arm length, body height, and angle of declination to the target successfully predicted affordance judgments, regardless of a reference point. We consider this as evidence that the invariant is functionally specific, not tied to any one particular anatomical body part.