Principles of Part–Whole Selective Perception by Dynamic Touch Extend to the Torso

ABSTRACT

The haptic subsystem of dynamic touch expresses a novel form of part–whole selective perception. When wielding a nonvisible rod grasped at some intermediate point along its length, an individual can attend to and report the length of a part of the rod (e.g., the segment forward of the hand) or the length of the whole rod. Both perceptions relate to the rod's mass moments about the point of grasp but in systematically different ways. Previous demonstrations of this part–whole selectivity have been in respect to rods grasped by hand or attached to a foot. The authors demonstrated the part–whole selectivity for nonvisible rods attached to the shoulder girdle and wielded primarily by movements of the trunk with benchmark performance provided by the same rods grasped and wielded by hand. Their results suggest that part–whole selectivity is a haptic capability general to the body.

Keywords:

• dynamic touch
• haptic perception
• selective perception
• tensegrity
• whole-body dexterity

ACKNOWLEDGMENTS

The preparation of this manuscript was supported by grants from the National Science Foundation (BCS-0925373) and the Provost's Office at the University of Connecticut.

Notes

1. The implied independence of whole and partial length perceptions has been confirmed (Cooper, Carello, & Turvey, 2000).

2. In the standard root mean square (RMS), deviations of perceived from actual length are summed prior to taking the root. The benefit of MRS is that it scales the error as a dimensionless Weber fraction (and functions, therefore, like the reliability measure). The standard RMS is not invariant over different units of measurement and, in consequence, is more difficult to interpret. RMS and MRS are linearly correlated.

3. Imagine wielding a rod by hand with the rod's long axis vertical, on the average. Then torque about the rotation axis could be interpreted as the sum of two torques—one associated with a regular spherical pendulum (the rod segment below the grasp), and one associated with an inverted spherical pendulum (the rod segment above the grasp). Attending to one of the torques (e.g., that associated with the regular pendulum) would be equivalent to inverting the mechanical principle that torques can be summed without affecting the outcome.