Planning a movement with even a simple tool such as a pointing stick is non-trivial: the brain must recognize the tool’s size, shape, and dynamics; locate the target in visual and body coordinates; integrate this with the stored body representation; and compute the limb-angle changes needed to touch the target. Humans are nevertheless dexterous with tools, generally because they can incorporate tools as extensions of the body. Tool embodiment is evident as tool-induced lengthening of perceived limb lengths and as extensions of the neural representation of peri-personal space (PPS), but this has been examined predominantly after extended tool use lasting tens of minutes.
People can immediately switch from pointing with a pointing stick to pointing with a much shorter pen without practice, suggesting immediate incorporation processes. Such immediate adaptations of body representations had been suggested previously but never proved; similarly, studies reporting PPS changes without specific tool training never examined the effect of those changes on motor behaviour with tools. This study aimed to isolate the immediate tool incorporation processes that determine motor planning with a tool. It used a two-choice forced discrimination task to examine immediate changes in perceived free-arm reach space during tool use, and a novel tool-held reaching task to examine the cause of those changes and how they affect movement planning. A haptic manipulandum allowed precise measurement of tool-held movements and equalization of dynamics across tool and no-tool conditions. The results show that tools induce immediate changes in body representation: they immediately shorten perceived arm length, and this shortening critically determines movement planning with the tool. Successful use of a tool with these initial planning processes is arguably a key prerequisite for the long-term tool incorporation processes regularly observed in earlier studies.
Key Points
Tool use requires recognizing tool size, shape, and dynamics; locating the target in visual and body coordinates; integrating with the stored body representation; and computing limb-angle changes.
Prior work on tool embodiment focused on protracted tool use, showing lengthening of perceived limb lengths and extension of peri-personal space.
This study isolated immediate tool incorporation processes using two tasks: a key-in-hole (KIH) two-choice forced discrimination task and a novel tool-held reaching (THR) task.
The KIH task estimated perceived free-arm reach space during tool use; the THR task tested how tool-induced changes affect movement planning.
A haptic manipulandum measured tool-held movements precisely and equalized dynamics between tool and no-tool conditions.
In KIH, each subject made 60 no-tool trials and 180 tool trials divided equally among three keys over three sessions, with tool and no-tool trials mixed randomly in each session.
Keyholes were presented within ±6 cm of the reach boundary; the reach boundary was calibrated before each session with 45 no-tool-only trials. There were three test sessions, each preceded by a calibration session.
KIH results: no differences between the three keys in decision boundaries, F(2,18)=1.36, P=0.28, or sensitivities, F(2,18)=0.86, P=0.44. No-tool reachable space remained similar to calibration, T(9)=0.218, P=0.83.
With a tool present, the reachable space shrank across tools, T(9)=3.59, P=0.0058; subjects judged closer keyholes as not reachable. Sensitivity of reach perception also decreased, T(9)=3.13, P=0.012.
In the control experiment, with visual feedback of only hand position, subjects consistently reached the target with the tool and the radial deviation r was absent, T(9)=1.17, P=0.27. A target overshoot remained, T(9)=3.71, P=0.0048, but was smaller than in THR for the same tool length, T(17)=2.66, P<0.017.
The absence of deviations when hand-position feedback is present shows THR deviations were due to misperception of hand/body position, not visual misperception of tool length.
The effects are opposite to those of repeated tool use: immediate shortening of perceived arm length versus slow elongation of body representations and PPS.
The proposed hypothesis is that two parallel processes with different time constants operate during tool use: a fast shortening process in the presence of a tool and a slow elongation process from extended tool use.
The immediate shortening may be a deliberate CNS strategy to plan an approximate deviation away from the target, avoid target occlusion, and then rely on visual feedback to bring the tool to the target at the end of the reach.
To the authors’ knowledge, this is the first direct evidence that tool-induced changes in body representation have a functional role in motor control with a tool.
Concepts
Tool embodiment / tool incorporation: The ability to treat a tool as an extension of the body, allowing dexterous action even with newly picked-up tools.
Body representation / body schema: The stored internal representation of body metrics. Tool presence can immediately shorten perceived arm length, whereas extended tool use can elongate body representations.
Peri-personal space (PPS): The neural representation of space around the body. Repeated tool use extends PPS; the current study concerns immediate changes distinct from those protracted effects.
Key-in-hole (KIH) task: A two-choice forced discrimination task in which subjects judged whether a keyhole outline was reachable with the currently held key. The judgement implicitly required estimating whether the hand could reach the base of the keyhole, giving an estimate of the immediate effect of the tool on perceived arm length.
Tool-held reaching (THR) task: A novel reaching task used to examine the cause of tool-induced perceptual changes and how they affect movement planning. THR subjects had no feedback of either hand position or tool during reaching.
Haptic manipulandum: A device used to measure tool-held movements precisely and to equalize dynamics across tool and no-tool conditions.
BRC model: Body-representation-change model, in which perceived shortening of limb lengths explains movement errors with tools.
VPC model: Visual-perception-change model, in which a simple deformation of visual perception explains tool-related errors. A more complicated VPC model considering simultaneous elongation along the movement and warping along the tool length could also explain the results.
Motor noise and deviation r: In THR, an efficient strategy is to deviate the movement away from the target (r) and rely on visual feedback at the end. The deviation correlated with an individual’s motor noise measured in no-tool trials (R=0.37, P<0.01, Supplementary Fig. 3).
Details
KIH experiment. Subjects made a short reaching movement holding a haptic manipulandum, followed by a reachability judgement task. They received visual feedback of hand position, tool, and target throughout a reach trial. They reached either with their hand cursor (no-tool trial) or with the tip of a virtual tool—the key—presented on a screen with its base over their right hand (tool trial). Immediately after every reach, the outline of their current key (the keyhole) appeared at a random location on the table. Subjects judged whether the keyhole was reachable with the key as currently held, pressing “yes” or “no” with the free left hand. The keyhole was distinct from the reach target. Subjects held their right hand at the reach target during the judgement and never actually reached to the keyhole. They were explicitly told the keyholes always matched the size and angle of the held key, so the judgement implicitly required estimating whether their hand could reach the base of the keyhole. This provided a way to estimate the immediate effects of the tool on perception of arm length during key use. The arm reaching in the KIH task served only as a contextual cue for tool use and helped magnify the tool incorporation effect.
KIH design and numbers. Each subject made 60 no-tool trials and 180 tool trials, divided equally between three keys, over three sessions. Tool and no-tool trials were mixed and presented randomly in each session, with three keys—tools of different orientation—presented randomly within tool trials. Each subject received one of two tool sets. Keyholes were presented within ±6 cm of the reach boundary, the origin of the abscissa in the psychometric plot. The reach boundary was calibrated before each session using a calibration session in which subjects made 45 no-tool-only trials. There were three test sessions, each preceded by a calibration session. Judgement data from randomly presented tool and no-tool trials generated a psychometric plot for each case.
KIH results. The psychometric plot of keyhole reachability judgement was averaged over 10 subjects and three test sessions. Tool-trial data were combined because no differences were observed between the three keys in decision boundaries, F(2,18)=1.36, P=0.28, repeated measures one-way ANOVA, or sensitivities, F(2,18)=0.86, P=0.44, repeated measures one-way ANOVA. The reachable space in no-tool trials remained similar to the preceding calibration session; the red trace passed through the origin of the abscissa, two-tailed t-test, T(9)=0.218, P=0.83. With a tool present, the reachable space shown by the decision boundary of the hand shrank across the tools, T(9)=3.59, P=0.0058, two-tailed t-test on individual differences between tool and no-tool trials, such that subjects judged closer keyholes to be not reachable. Sensitivity of reach perception also decreased in the presence of tools, T(9)=3.13, P=0.012, two-tailed t-test on individual differences between the slopes of tool and no-tool trials.
THR control experiment. A separate group of 10 subjects received visual feedback of only their hand position, not the tool, during the reaching movement. Subjects in the THR task had no feedback of either hand position or tool. With visual feedback of hand position, subjects could consistently reach the target with the tool—the pink plot in Fig. 6b—and the radial deviation r was absent, two-tailed t-test of difference from zero, T(9)=1.17, P=0.27. A target overshoot remained, T(9)=3.71, P=0.0048, two-tailed t-test, but was significantly smaller than in the THR experiment for the same tool length, two-sample two-tailed t-test between r values from Control and THR, T(17)=2.66, P<0.017. The absence of deviations when hand-position feedback is present shows the THR deviations were not due to visual misperception of tool length but to misperception of the hand/body position.
Discussion. Prior primate work on tool embodiment repeatedly showed changes in peri-personal space and body representation induced by repeated tool use, but these protracted-use effects do not explain the ability to immediately use a tool just picked up. This study shows tool incorporation includes immediate perception changes critical to that ability. The simple two-choice discrimination task (KIH), similar to Bourgeois et al., showed that tools can induce an immediate change in the perceived reachable space of the tool-free arm. The movement task (THR) showed the changes are well explained by changes in body representation in the presence of a tool. Both experiments used tools of different size and orientation and mixed tool and no-tool trials, ensuring the effects were immediate and distinct from changes due to repeated use of the same tool.
The effects are opposite to previous studies: perceived arm length shortens immediately with a tool, whereas repeated tool use elongates body representations and peri-personal space. The hypothesis is that two parallel processes of different time constants operate during tool use: a fast shortening process in the presence of a tool and a slow elongation process from extended tool use. This is supported by the immediate shortening—Figs. 1d, 2b, 3c—plus a slow increase in perceived reach boundary across the KIH calibration sessions (Fig. 4), agreeing with previously reported extensions.
Rendering diagram…
A proposed reason for two parallel adaptations is that with repeated tool use, the association between control commands to the arm while holding the tool and the sensory signals received with the tool can lead the central nervous system to develop new sensory–motor associations, arguably the cause of the extensions previously observed. This long-term association probably reduces the computational time and cost of motor planning with tools and determines skill with the tool, but it takes at least a few trials and is not useful for immediately using a new tool after a tool change. Using a new tool, as in the THR task, requires immediate estimation of tool kinematics, movement planning corresponding to it, and execution aided by visual feedback. Visual feedback, most active at the end of the reach, is probably the key factor determining successful reach with a new tool. For efficient visual feedback, the crucial requirement is that the hand and tool movement does not occlude the view of the reach target. Since tool-held reaching can be affected by visual perception errors, motor planning noise, and motor execution noise, an efficient strategy is to plan the movement away—deviate—from the target (r in Fig. 5c), ensure the target is not occluded, then rely on visual feedback to bring the tool to the target at the end of the reach.
The shortening of limb representation is suggested to be a deliberate procedure adopted by the human CNS to achieve an approximate deviation (r in Fig. 5c), where the deviation is calculated using the estimate of one’s own motor noise but without requiring accurate visual calculations of tool characteristics, especially orientation. Partial support for the deliberate nature of the shortening comes from a significant correlation, R=0.37, P<0.01, Supplementary Fig. 3, between an individual’s motor noise measured in no-tool trials and the associated deviation r in tool trials in the THR experiment. To the authors’ knowledge, this is the first direct evidence that tool-induced changes in body representation have a functional role in motor control with a tool.
The results are well explained by perceived shortening of limb lengths (BRC model) and not by a simple deformation of visual perception (VPC model). A more complicated VPC model considering simultaneous elongation along the movement and warping along the tool length of visual perception in the presence of tools could also explain them. Two aspects suggest body representation changes as the major source of movement errors with tools: (1) the elegant explanation of seemingly complex target miss patterns by a simple body representation change model (Fig. 6); and (2) the dependence of the deviation on an individual’s internal motor-noise estimate.
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