Abstract
Prospective judgments about one’s capability to perform an action are assumed to involve mental simulation of the action. Previous studies of motor imagery suggest this simulation is supported by a large fronto-parietal network including the motor system. Experiment 1 used fMRI to assess the contribution of this fronto-parietal network to judgments about one’s capacity to grasp objects of different sizes between index and thumb. The neural network underlying prospective graspability judgments overlapped the fronto-parietal network involved in explicit motor imagery of grasping. However, shared areas were located in the right hemisphere, outside the motor cortex, and were also activated during perceptual length judgments, suggesting a contribution to object size estimate rather than motor simulation. Experiment 2 used TMS over the motor cortex to probe transient excitability changes undetected with fMRI. Results show that graspability judgments elicited a selective increase of excitability in the thumb and index muscles, which was maximal before the object display and intermediate during the judgment. Together, these findings suggest that prospective action judgments do not rely on the motor system to simulate the action per se but to refresh the memory of one’s maximal grip aperture and facilitate its comparison with object size in right fronto-parietal areas.
Highlights
Prospective judgments about one’s capability to perform an action are assumed to involve mental simulation of the action
The fronto-parietal activations observed during graspability judgments might indicate that they are achieved through the direct comparison of magnitude estimates, computed from grip aperture and object size, without implying any form of motor simulation as assumed earlier[1]
An functional magnetic resonance imaging (fMRI) experiment assessed the possible neural overlap of prospective judgments and explicit imagery of grasping, and tested its specificity relative to the neural correlates of perceptual length judgments
Summary
Prospective judgments about one’s capability to perform an action are assumed to involve mental simulation of the action. In a seminal paper[2], Johnson went one step further by assuming that this comparison is preceded by the mental simulation of the reach and grasp movement This simulation would recruit the same processes as those involved in action planning and execution, namely encoding the visual properties of the object (e.g., the size and orientation), activating the sensorimotor representation of the effectors (e.g., the hand), and computing the visuo-motor transformations required to achieve the action given a set of biomechanical rules[2,3,4]. Making prospective judgments about an object-directed action and mentally simulating the action both require the computation of magnitude estimates, such as grip aperture or reaching distance, but existing studies have left out the possibility that the fronto-parietal areas activated during prospective action judgments and motor imagery might reflect magnitude processing. Under the comparison-without-simulation hypothesis, no difference in MEP amplitude is expected during graspability and length judgments since the comparison of object size with grip aperture is analogue to the comparison of two lengths
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