The hidden hand is perceived closer to midline

How we perceive the width of grasped objects: Insights into the central processes that govern proprioceptive judgements

Low-level proprioceptive judgements involve a single frame of reference, whereas high-level proprioceptive judgements are made across different frames of reference. The present study systematically compared low-level (grasp → $\rightarrow$ grasp) and high-level (vision → $\rightarrow$ grasp, grasp → $\rightarrow$ vision) proprioceptive tasks, and quantified the consistency of grasp → $\rightarrow$ vision and possible reciprocal nature of related high-level proprioceptive tasks. Experiment 1 (n = 30) compared performance across vision → $\rightarrow$ grasp, a grasp → $\rightarrow$ vision and a grasp → $\rightarrow$ grasp tasks. Experiment 2 (n = 30) compared performance on the grasp → $\rightarrow$ vision task between hands and over time. Participants were accurate (mean absolute error 0.27 cm [0.20 to 0.34]; mean [95% CI]) and precise (R 2 $R^2$ = 0.95 [0.93 to 0.96]) for grasp → $\rightarrow$ grasp judgements, with a strong correlation between outcomes (r = -0.85 [-0.93 to -0.70]). Accuracy and precision decreased in the two high-level tasks (R 2 $R^2$ = 0.86 and 0.89; mean absolute error = 1.34 and 1.41 cm), with most participants overestimating perceived width for the vision → $\rightarrow$ grasp task and underestimating it for grasp → $\rightarrow$ vision task. There was minimal correlation between accuracy and precision for these two tasks. Converging evidence indicated performance was largely reciprocal (inverse) between the vision → $\rightarrow$ grasp and grasp → $\rightarrow$ vision tasks. Performance on the grasp → $\rightarrow$ vision task was consistent between dominant and non-dominant hands, and across repeated sessions a day or week apart. Overall, there are fundamental differences between low- and high-level proprioceptive judgements that reflect fundamental differences in the cortical processes that underpin these perceptions. Moreover, the central transformations that govern high-level proprioceptive judgements of grasp are personalised, stable and reciprocal for reciprocal tasks. KEY POINTS: Low-level proprioceptive judgements involve a single frame of reference (e.g. indicating the width of a grasped object by selecting from a series of objects of different width), whereas high-level proprioceptive judgements are made across different frames of reference (e.g. indicating the width of a grasped object by selecting from a series of visible lines of different length). We highlight fundamental differences in the precision and accuracy of low- and high-level proprioceptive judgements. We provide converging evidence that the neural transformations between frames of reference that govern high-level proprioceptive judgements of grasp are personalised, stable and reciprocal for reciprocal tasks. This stability is likely key to precise judgements and accurate predictions in high-level proprioception.

Enhanced efficiency in visually guided online motor control for actions redirected towards the body midline

Reaching objects in a dynamic environment requires fast online corrections that compensate for sudden object shifts or postural changes. Previous studies revealed the key role of visually monitoring the hand-to-target distance throughout action execution. In the current study, we investigate how sensorimotor asymmetries associated with space perception, brain lateralization and biomechanical constraints, affect the efficiency of online corrections. Participants performed reaching actions in virtual reality, where the virtual hand was progressively displaced from the real hand to trigger online corrections, for which it was possible to control the total amount of the redirection and the region of space in which the action unfolded. The efficiency of online corrections and the degree of awareness of the ensuing motor corrections were taken as assessment variables. Results revealed more efficient visuo-motor corrections for actions redirected towards, rather than away from the body midline. The effect is independent on the reaching hand and the hemispace of action, making explanations associated with laterality effects and biomechanical constraints improbable. The result cannot either be accounted for by the visual processing advantage in the straight-ahead region. An explanation may be found in the finer sensorimotor representations characterizing the frontal space proximal to body, where a preference for visual processing has been documented, and where high-value functional actions, like fine manipulative skills, typically take place. This article is part of a discussion meeting issue 'New approaches to 3D vision'.

Proprioception: a new look at an old concept

Proprioception, which can be defined as the awareness of the mechanical and spatial state of the body and its musculoskeletal parts, is critical to motor actions and contributes to our sense of body ownership. To date, clinical proprioceptive tests have focused on a person's ability to detect, discriminate or match limb positions or movements, and reveal that the strength of the relationship between deficits in proprioception and physical function varies widely. Unfortunately, these tests fail to assess higher-level proprioceptive abilities. In this Perspective, we propose that to understand fully the link between proprioception and function, we need to look beyond traditional clinical tests of proprioception. Specifically, we present a novel framework for human proprioception assessment that is divided into two categories: low-level and high-level proprioceptive judgments. Low-level judgments are those made in a single frame of reference and are the types of judgments made in traditional proprioceptive tests (i.e. detect, discriminate or match). High-level proprioceptive abilities involve proprioceptive judgments made in a different frame of reference. For example, when a person indicates where their hand is located in space. This framework acknowledges that proprioception is complex and multifaceted, and that tests of proprioception should not be viewed as interchangeable, but rather as complimentary. Crucially, it provides structure to the way researchers and clinicians can approach proprioception and its assessment. We hope this Perspective serves as the catalyst for discussion and new lines of investigation.

Reaching movements are automatically redirected to nearby options during target split

Motor behavior often occurs in environments with multiple goal options that can vary during the ongoing action. We explored this situation by requiring subjects to select between different target options during an ongoing reach. During split trials the original target was replaced with a left and a right flanking target, and participants had to select between them. This contrasted with the standard jump trials, where the original target would be replaced with a single flanking target, left or right. When participants were instructed to follow their natural tendency, they all tended to select the split target nearest the original. The near-target preference was more prominent with increased spatial disparity between the options and when participants could preview the potential options. Moreover, explicit instruction to obtain the "far" target during split trials resulted many errors compared with a "near" instruction, ~50% vs. ~15%. Online reaction times to target change were delayed in split trials compared with jump trials, ~200 ms vs. ~150 ms, but also highly automatic. Trials in which the instructed far target was correctly obtained were delayed by a further ~50 ms, unlike those in which the near target was incorrectly obtained. We also observed nonspecific responses from arm muscles at the jump trial latency during split trials. Taken together, our results indicate that online selection of reach targets is automatically linked to the spatial distribution of the options, though at greater delays than redirecting to a single target.NEW & NOTEWORTHY This work demonstrates that target selection during an ongoing reach is automatically linked to the option nearest a voided target. Online reaction times for two options are longer than redirection to a single option. Attempts to override the near-target tendency result in a high number of errors at the normal delay and further delays when the attempt is successful.

Judgements of hand location and hand spacing show minimal proprioceptive drift

With a visual memory of where our hands are, their perceived location drifts. We investigated whether the perceived location of one hand or the spacing between two hands drifts in the absence of visual memories or cues. In 30 participants (17 females, mean age 27 years, range 20-45 years), perceived location of the right index finger was assessed when it was 10 cm to the right or left of the midline. Perceived spacing between the index fingers was assessed when they were spaced 20 cm apart, centred on the midline. Testing included two conditions, one with ten measures at 30 s intervals and another where a 3 min delay was introduced after the fifth measure. Participants responded by selecting a point on a ruler or a line from a series of lines of different lengths. Overall, participants mislocalised their hands closer to the midline. However, there was little to no drift in perceived index finger location when measures were taken at regular intervals (ipsilateral slope: 0.073 cm/measure [[Formula: see text] to 0.160], mean [99% CI]; contralateral slope: 0.045 cm/measure [[Formula: see text] to 0.120]), or across a 3 min delay (ipsilateral: ([Formula: see text] cm [[Formula: see text] to 0.17]; contralateral: [Formula: see text] cm [[Formula: see text] to 0.24]). There was a slight drift in perceived spacing when measures were taken at regular intervals (slope: [Formula: see text] cm/measure [[Formula: see text] to [Formula: see text]]), but none across a 3 min delay (0.08 cm [[Formula: see text] to 1.24]). Thus, proprioceptive-based perceptions of where our hands are located or how they are spaced drift minimally or not at all, indicating these perceptions are stable.

Stability of perception of the hand's aperture in a grasp

KEY POINTS

How we judge the location of our body parts can be affected by a range of factors that change how our brain interprets proprioceptive signals. We examined the effect of several such factors on how we perceive an object's width and the spacing between our thumb and fingers when grasping. Grasp-related perceptions were slightly wider when using all digits, in line with our tendency to grasp larger objects with the entire hand. Surprisingly, these perceptions were not affected by the frames of reference for judgements (object width versus grasp aperture), whether the object was grasped actively or passively, or the strength of the grasp. These results show that the brain maintains a largely stable representation of the hand when grasping stationary objects. This stability may underpin our dexterity when grasping a vast array of objects.

ABSTRACT

Various factors can alter how the brain interprets proprioceptive signals, leading to errors in how we perceive our body and execute motor tasks. This study determined the effect of critical factors on hand-based perceptions. In Experiment 1, 20 participants grasped without lifting an unseen 6.5 cm-wide object with two grasp configurations: thumb and all fingers, and thumb and index finger. Participants reported perceived grasp aperture (body reference frame) or perceived object width (external reference frame) using visual charts. In Experiment 2, 20 participants grasped the object with three grasp intensities (1, 5 and 15% maximal grasp force) actively or passively and reported perceived grasp aperture. A follow-up experiment addressed whether results from Experiment 2 were influenced by the external force applied during passive grasp. Overall, there was a mean difference of 0.38 cm (95% confidence interval (CI), 0.12 to 0.63) between the two grasp configurations (all digits compared to thumb and index finger). Perceived object width compared to perceived grasp aperture differed by only -0.04 cm (95% CI, -0.30 to 0.21). There was no real effect of grasp intensity on perceived grasp aperture (-0.01 cm; 95% CI, -0.03 to 0.01) or grasp type (active versus passive; 0.18 cm; 95% CI, -0.19 to 0.55). Overall, grasp-related perceptions are slightly wider when using all digits, in line with our tendency to grasp larger objects with the entire hand. The other factors - frame of reference, grasp intensity and grasp type - had no meaningful effect on these perceptions. These results provide evidence that the brain maintains a largely stable representation of the hand.