Preserved grip selection planning in chronic unilateral upper extremity amputees

Abstract

Upper limb amputees receive no proprioceptive or visual sensory feedback about their absent hand. In this study, we asked whether chronic amputees nevertheless retain the ability to accurately plan gripping movements. Fourteen patients and matched controls performed two grip selection tasks: overt grip selection (OGS), in which they used their intact hand to grasp an object that appeared in different orientations using the most natural (under- or overhand) precision grip, and prospective grip selection (PGS), in which they selected the most natural grip for either hand without moving. We evaluated planning accuracy by comparing concordance between grip preferences expressed in PGS vs. OGS for the intact hand and PGS vs. the inverse of OGS responses for the affected hand. Overall, amputees showed no deficits in the accuracy of grip selection planning based on either hand and a consistent preference for less awkward hand postures. We found no evidence for a speed-accuracy tradeoff. Furthermore, selection accuracy did not depend on phantom mobility, phantom limb pain, time since amputation, or the residual limb's shoulder posture. Our findings demonstrate that unilateral upper limb amputees retain the ability to plan movements based on the biomechanics of their affected hand even many years after limb loss. This unimpaired representation may stem from persistent higher-level activity-independent internal representations or may be sustained by sensory feedback from the intact hand.

Fig. 1

Experimental components. a Stimulus object at 4 sample orientations for each hand. b Possible hand postures. Adapted from Jacobs et al. (2010). c Illustration of “neutral” posture, both arms down. d “Crossed” postures, with one arm (here, right) crossed over chest

Fig. 2

Trial time course. a Overt grip selection (OGS) task. b Prospective grip selection (PGS) task. c Awkwardness rating test

Fig. 3

Awkwardness ratings and OGS choice likelihood, for each thumb placement, group mean ± SEM. All OGS data from intact hand, mirror reversed to produce affected (left) hand data; see text for details. Congruence between high choice likelihood and low awkwardness demonstrates selection of non-awkward grips. a Left-hand. b Right-hand

Fig. 4

Relationship between awkwardness ratings and PGS choice likelihood, for affected hand in both amputees and controls. See Fig. 3 for format details

Fig. 5

Effect of posture on task performance, group mean ± SEM. Results collapsed across hand and orientation and group, to show posture effect. * Significant difference (P < 0.05) by Tukey’s HSD. a OGS onset time (OT). b OGS movement time (MT). c PGS response time (RT). d PGS accuracy

Fig. 6

Effect of amputation on PGS selection accuracy, group mean ± SEM. No effect of group (P = 0.569), hand (P = 0.277), or interaction (P = 0.985)

Fig. 7

Orientation effects on measures of task speed, group mean ± SEM. a Effect of orientation on RT with intact hand. b Effect of orientation on RT with affected hand. c Effect of orientation on MT with intact hand. OT not shown, due to lack of orientation effect

Fig. 8

Correlation (Kendall τ) between PGS accuracy (participant mean) and amputee characteristics. a Phantom limb pain, 0–1. b Vividness of phantom mobility, 0–1. c Years since amputation