Lessons for Dynamic Touch From a Case of Stroke-Induced Motor Impairment

Abstract

LW, an individual with a stroke-related motor impairment, was asked to perceive the lengths of rods of different mass distributions by dynamic touch. His impairment dictated that wielding was primarily about the shoulder rather than the wrist. Although perceived rod lengths were in the range of actual rod lengths, scaling to the objects' mass moments was atypical for both the affected and unaffected limbs. A group of healthy young adults asked to mimic his wielding style yielded the same atypical scaling. The typical scaling was restored when LW's wielding was fixed about a mechanical axis. Discussion considered what frame of reference is suitable for revealing an object's mass moments relevant to a given task. In particular, it appears that individuals can exploit alternative forms of interaction with environmental objects that leave invariant the parameters specifying to-be-perceived properties. Perception by dynamic touch is not a function of particular neuromuscular patterns but of information made available to the haptic system during limb-object interactions.

Notes

¹The mean scalings of the power functions characterizing the relationship between mean L_P and static moment (0.91 and 0.90 for affected and unaffected limbs, respectively) were also greater than would be predicted from dimensional analyses. According to such analyses, the length of a rod having cylindrical symmetry is related to static moment by a power function with a ½ scaling. Hence, LW did not seem to have exploited such a variable even though it was also systematically related to his judgments. This possibility, however, was considered in light of the results of previous studies suggesting that dynamic touch under conditions of holding with minimal movement is tied mainly to the tissue deformation consequences of the rods' static moment (Kingma, van de Langenberg, & Beek, 2004).