Perceiving Object Width by Grasping

Abstract

Ordinarily when an object is grasped between the thumb and finger, the linear extent of the object and finger span covary and match in size. Six experiments examined perceived size by grasping when this contingency was broken. Perceived size was indicated by producing a span with the left hand to match the width of a presented object. Under ordinary circumstances of grasping with the bare fingers, haptically perceived width did not differ from visually perceived width (Experi- ment 1) and was a linear function of object width when grasping did not induce muscle strain (Experiment 2). When object width and finger span covaried but did not match (because 1 cm rubber attachments were affixed to the thumb and forefinger), perceived width did not differ from grasping with the bare fingers (Experiment 3). When match and covariation were both eliminated with the use of attachments that varied inversely with the width of the objects (producing a constant 8 cm finger span), object width could still be discriminated (Experiment 4). Width was perceived equally well with rocking, tapping, and sliding styles of exploration (Experiment 5), and round, flat, and tapered tips (Experiment 6) were all successful in revealing the extent of grasped objects. The style of exploration and the type of tip used affected the width perceived. The results were discussed in terms of the nature of information made available by grasping, providing the basis of discriminations of linear extents. It was concluded that the exterospecific information about the extent of an object grasped is different from the propriospecific information about the span of the grasp.