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 stab ility. 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 whe n lying supine. This result favors the whole body engagement hypothesis. Fo r reaching above one's head, however, an increased estimation bias suggeste d that postural stability also affects estimates. In Experiment 2, estimate s of reachability for 12 participants were not selectively reduced while st anding on one leg instead of both legs. This result could be interpreted to be inconsistent with the postural stability hypothesis. The lack of an int eraction between supporting leg and object location, however, argued agains t the whole body engagement hypothesis. Finally, approaching objects always appeared reachable at larger distances than objects departing from the obs erver. These results cast doubt on single-factor accounts of reachability e stimates. (C) 2000 Elsevier Science B.V. All rights reserved. PsycINFO clas sification: 2221; 2330.