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BackgroundWhen lifting objects whose center of mass (CoM) are not centered below the handle one must compensate for arising external torques already at lift-off to avoid object tilt. Previous studies showed that finger force scaling during object lifting may be impaired at both hands following stroke. However, torque control in object manipulation has not yet been studied in patients with stroke.In this study, thirteen patients with chronic stage left hemispheric stroke (SL), nine patients with right hemispheric stroke (SR) and hand-matched controls had to grasp and lift an object with the fingertips of their ipsilesional hand at a handle while preventing object tilt. Object CoM and therewith the external torque was varied by either relocating a covert weight or the handle. The compensatory torque at lift-off (Tcom) is the sum of the torque resulting from 1) grip force being produced at different vertical finger positions (∆CoP x GF) and 2) different vertical load forces on both sides of the handle (∆Fy x w/2).ResultsWhen having to rely on sensorimotor memories, ∆CoP x GF was elevated when the object CoM was on the ipislesional-, but decreased when CoM was on the contralesional side in SL, whereas ∆Fy x w/2 was biased in the opposite direction, resulting in normal Tcom. SR patients applied a smaller ∆CoP x GF when the CoM was on the contralesional side. Torques were not altered when geometric cues were available. The sensorimotor spatial bias of ∆CoP x GF was more pronounced and translated to a bias of overall Tcom in SL patients with signs of apraxia (n = 3). ConclusionOur findings provide evidence for an object-centered spatial bias of manual sensorimotor torque control with the ipsilesional hand following stroke. While both intact finger force-to-position coordination and visuomotor control may compensate for the spatial sensorimotor bias in most stroke patients, it may result in a hampered manual torque control in patients with apraxia.

When we grasp and lift novel objects, we rely on visual cues and sensorimotor memories to predictively scale our finger forces and exert compensatory torques according to object properties. Recently, it was shown that object appearance, previous force scaling errors, and previous torque compensation errors strongly impact our percept. However, the influence of visual geometric cues on the perception of object torques and weights in a grasp to lift task is poorly understood. Moreover, little is known about how visual cues, prior expectations, sensory feedback, and sensorimotor memories are integrated for anticipatory torque control and object perception. Here, 12 young and 12 elderly participants repeatedly grasped and lifted an object while trying to prevent object tilt. Before each trial, we randomly repositioned both the object handle, providing a geometric cue on the upcoming torque, as well as a hidden weight, adding an unforeseeable torque variation. Before lifting, subjects indicated their torque expectations, as well as reporting their experience of torque and weight after each lift. Mixed-effect multiple regression models showed that visual shape cues governed anticipatory torque compensation, whereas sensorimotor memories played less of a role. In contrast, the external torque and committed compensation errors at lift-off mainly determined how object torques and weight were perceived. The modest effect of handle position differed for torque and weight perception. Explicit torque expectations were also correlated with anticipatory torque compensation and torque perception. Our main findings generalized across both age groups. Our results suggest distinct weighting of inputs for action and perception according to reliability. Electronic supplementary material The online version of this article (10.1007/s00221-019-05711-y) contains supplementary material, which is available to authorized users.