Estimating reachability: Whole body engagement or postural stability?

People overestimate the maximum distance at which objects can be contacted when utilizing just one degree of freedom. This estimation bias may reflect either an inability to mentally immobilize the body's remaining degrees of freedom (whole body engagement), or a person's concern about postural stability. In Experiment 1, 10 participants either stood upright or lay supine to eliminate additional degrees of freedom and balance concerns. Their task was to estimate when they could reach an object by merely extending their arm. For objects at one's side, estimated reachability bias was reduced when lying supine. This result favors the whole body engagement hypothesis. For reaching above one's head, however, an increased estimation bias suggested that postural stability also affects estimates. In Experiment 2, estimates of reachability for 12 participants were not selectively reduced while standing on one leg instead of both legs. This result could be interpreted to be inconsistent with the postural stability hypothesis. The lack of an interaction between supporting leg and object location, however, argued against the whole body engagement hypothesis. Finally, approaching objects always appeared reachable at larger distances than objects departing from the observer. These results cast doubt on single-factor accounts of reachability estimates. (C) 2000 Elsevier Science B.V. All rights reserved. PsycINFO classification: 2221; 2330.