Date Published: January 17, 2019
Publisher: Public Library of Science
Author(s): Lewis A. Ingram, Annie A. Butler, Simon C. Gandevia, Lee D. Walsh, François Tremblay.
Previous studies revealed that healthy individuals consistently misjudge the size and shape of their hidden hand during a localisation task. Specifically, they overestimate the width of their hand and underestimate the length of their fingers. This would also imply that the same individuals misjudge the actual location of at least some parts of their hand during the task. Therefore, the primary aim of the current study was to determine whether healthy individuals could accurately locate the actual position of their hand when hidden from view, and whether accuracy depends on the type of localisation task used, the orientation of the hidden hand, and whether the left or right hand is tested. Sixteen healthy right-handed participants performed a hand localisation task that involved both pointing to and verbally indicating the perceived position of landmarks on their hidden hand. Hand position was consistently misjudged as closer to the wrist (proximal bias) and, to a lesser extent, away from the thumb (ulnar bias). The magnitude of these biases depended on the localisation task (pointing vs. verbal), the orientation of the hand (straight vs. rotated), and the hand tested (left vs. right). Furthermore, the proximal location bias increased in size as the duration of the experiment increased, while the magnitude of ulnar bias remained stable through the experiment. Finally, the resultant maps of perceived hand location appear to replicate the previously reported overestimation of hand width and underestimation of finger length. Once again, the magnitude of these distortions is dependent on the task, orientation, and hand tested. These findings underscore the need to control and standardise each component of the hand localisation task in future studies.
Proprioception provides information about the body’s position in space, a fundamental requirement for motor control. While muscle spindles, joint capsules, and the skin provide proprioceptive afferent signals specifying the degree to which each joint is flexed or extended (e.g. ref [1–4]), knowing the spatial configuration of the body is insufficient to determine its position in space. Information about size and shape of the body’s parts is necessary to localise its position (e.g. [5–7]), yet this is not provided by proprioceptive and tactile inputs. This has led researchers to propose the presence of a centrally stored representation of the body’s metric properties referred to as the body model  or a body form representation .
All of the pointing trials and all of the verbal trials from a single participant performed with their right hand positioned straight are shown in Fig 2A and Fig 2B respectively. During the pointing trials, the participant consistently misjudged their hand as closer to their wrist (proximal error) and away from their thumb (ulnar shift). During the verbal trials, the magnitude of proximal error appears greater while the degree of ulnar mislocation appears smaller. A shortening of finger length and a widening of space between knuckles are also evident in the figure during the pointing trials, while only an apparent shortening of finger length appears during the verbal trials.
The results of the current study reveal that healthy participants consistently misjudge the location of their hand as closer towards their wrist (proximal bias) compared to its actual location. Although the magnitude of this proximal bias was dependent on the type of localisation task performed and the orientation of the tested hand, some participants mislocated their hand by over 6 cm. Furthermore, this bias in the proximal direction was shown to increase by approximately 2–3 cm over time (proximal proprioceptive drift). In contrast, a smaller and less consistent bias was observed along the radio-ulnar axis of the hand. Likewise, the size of these mislocations changed with the task performed, the orientation of the hand, as well as the hand tested (dominant vs. non-dominant). There was no change in perceived hand location in the radial or ulnar direction over time. The distortions in hand shape and size observed in the resulting maps of perceived hand location (Fig 3) replicate previous findings [9–14,16–19,36–37]. These findings further advance our understanding of human proprioception and build the foundation towards a potential measurement of proprioception in the clinical setting.
The current study reveals that healthy participants consistently misjudge the location of their hidden hand as closer towards their wrist (proximal bias) and, to a lesser extent, away from their thumb (ulnar bias). Furthermore, the perceived location of the hand drifts closer towards the wrist (proximal proprioceptive drift) over time, while remaining stable along the radio-ulnar axis of the hand. Lastly, calculated finger length and hand width–derived from perceived landmark location judgements–reveal shorter fingers and a wider hand compared to the actual hand’s dimensions.