Perception of Arm Position in Three-Dimensional Space

Abstract

Proprioception refers to the senses of body position, movement, force and effort. Previous studies have demonstrated workspace and direction-dependent differences in arm proprioceptive sensitivity within the horizontal plane. In addition, studies of reaching in the vertical plane have shown that proprioception plays a key role in anticipating arm configuration dependent effects of gravity. This suggests that proprioceptive sensitivity could vary with the direction of arm displacement relative to the gravitational vector, as well as with arm configuration. To test these hypotheses, and to characterize proprioception more generally, we assessed the direction-dependence and arm postural-dependence of proprioceptive sensitivity in 3D space using a novel robotic paradigm. A subject’s right arm was coupled to a 7-df robot through a trough that stabilized the wrist and forearm, allowing for changes in configuration largely at the elbow and shoulder. Sensitivity was evaluated using a “same-different” task, where the subject’s hand was moved 1–4 cm away from an initial “test” position to a 2nd “judgment” position. The proportion of trials where subjects responded “different” when the positions were different (“hit rate”), and where they responded “different” when the positions were the same, (“false alarm rate”), were used to calculate d’, a measure of sensitivity derived from signal detection theory (SDT). Initially, a single initial arm posture was used and displacements were performed in six directions: upward, downward, forward, backward, leftward and rightward of the test position. In a follow-up experiment, data were obtained for four directions and two initial arm postures. As expected, sensitivity (d’) increased monotonically with distance for all six directions. Sensitivity also varied between directions, particularly at position differences of 2 and 3 cm. Overall, sensitivity reached near maximal values in this task at 2 cm for the leftward/rightward directions, 3 cm for upward/forward and 4 cm for the downward/backward directions. In addition, when data were grouped together for opposing directions, sensitivity showed a dependence upon arm posture. These data suggest arm proprioceptive sensitivity is both anisotropic in 3D space and configuration-dependent, which has important implications for sensorimotor control of the arm and human-robot interactions.

Effects of Transcutaneous Electric Nerve Stimulation on Upper Extremity Proprioceptive Function

“…Figure 4 shows the average hit rates and false alarm rates (+/-SEM) for each stimulation group and distance. As observed in previous studies without stimulation [12], hit rates increased monotonically with distance and false alarm rates decreased in all stimulation groups. However, there were no observable effects of stimulation frequency on either hit rates or false alarm rates.…”

“…For larger distances (3-4 cm) bias decreased precipitously, approaching zero at 4 cm. This trend was expected as it indicates that as the proprioceptive signal strengthened, subjects' responses became less biased [12]. Regarding effects of stimulation, although there was some tendency for bias to be lower for the 3kHz stimulation group across all distances, this trend was not confirmed by statistical analysis.…”

“…The observation that 30 Hz TENS was only effective at improving proprioception at discrimination distances of 2 cm may indicate that this neuromodulation approach is beneficial only when signals are detectable but weak, and not otherwise. Our previous work [12] has shown that at 1 cm, proprioceptive sensitivity (d') generally fluctuates around zero, indicating approximately chance performance. This is likely due to the fact that at this discrimination distance the proprioceptive signal is too weak to be detected, thus subjects simply guess at whether the positions are different or not.…”

“…Procedures for assessment of proprioceptive sensitivity have been described in detail elsewhere [12]. In brief, subjects' right arms were coupled to a 7-DoF anthropomorphic robot arm (LWR4+, KUKA Inc.) through an arm trough that restricted wrist motion but allowed movement of the elbow and shoulder.…”

A cross-species neural integration of gravity for motor optimisation

“…Amongst others, one of these solutions is to learn and store internal models to disambiguate sensory information and to predict forthcoming movement dynamics. On earth, a pervasive component affecting perception and motion is gravity (Bringoux et al, 2012; Dakin and Rosenberg, 2018; Elmore et al, 2014; Gaveau et al, 2011, 2016; Jorges and Lopez-Moliner, 2017; Klein et al, 2018; Lacquaniti et al, 2013; McIntyre et al, 2001; Van Pelt et al, 2005; Rosenberg and Angelaki, 2014; Saradjian et al, 2014; La Scaleia et al, 2019; Senot et al, 2005; Tajadura-Jimenez et al, 2018; De Vrijer et al, 2008). Studies in both humans and non-human primates have provided strong evidence that the brain evolved an internal representation of gravity.…”