Grasp context-dependent sensorimotor cortical interactions
Previous work on grasping and manipulation has used a grasping context that predominantly elicits memory-based control of digit forces by constraining where the object should be grasped. For this constrained grasping context, primary motor cortex (M1) is involved in storage and retrieval of digit forces used in previous manipulations. In contrast, when choice of digit contact points is allowed (unconstrained grasping), behavioral studies revealed that forces are adjusted, on a trial-to-trial basis, as a function of digit position.1,2 This suggests a role of online feedback of digit position for force control. However, despite the ubiquitous nature of unconstrained hand-object interactions in activities of daily living, the underlying neural mechanisms are unknown. To determine the extent to which feedback and memory interact in each type of grasp, we used TMS to induce ‘virtual lesions’ of primary motor or sensory cortex via continuous theta burst stimulation (cTBS). We found the role of primary motor (M1) and somatosensory (S1) cortices to be sensitive to grasping context. In constrained grasping, M1, but not S1, is involved in storing and retrieving learned digit forces and position. In contrast, in unconstrained grasping M1 and S1 are involved in modulating digit forces to position (Fig. 1).
Figure 1. Effect of cTBS on digit load force, grip force, and position across all experimental conditions. (A) From top to bottom, traces denote time course of the difference between thumb and index finger load force, grip force averaged across thumb and index finger, and the vertical distance between thumb and index finger center of pressure (d y ) from contact (“0”) to object lift onset. Data are averages of the last 5 trials prior to cTBS (Pre5) and first trial following cTBS (Post1). d Y data are plotted from the time at which they can be accurately estimated using force and torque sensors (Fu et al. 2010) . Data from each experimental group are shown across columns. Shaded plots denote Tcom variables that were significantly affected by cTBS. (B) Data from Pre5 and each post-cTBS trial are shown for each Tcom variable and experimental group. ** denotes P < 0.0125. Data are averages (± SE) of all subjects.
Our findings suggest that the relative contribution of memory and online feedback modulates sensorimotor cortical interactions for dexterous manipulation (Fig. 2).3
Figure 2. Cortical sensorimotor mechanisms for neural control of dexterous manipulation. Prior to object contact, interactions between M1, sensory, as well as premotor and parietal cortical areas, lead to hand shaping and positioning the digits at remembered locations used in previous manipulations. Somatosensory and visual inputs contribute to guiding the hand towards the planned contact points on the object. Following contact, the roles of M1 and S1 for the control of dexterous manipulation differ according to whether contact points are constrained or unconstrained.
For more information on these projects contact: Marco Santello
References
- Fu Q, Zhang W, Santello M (2010). Anticipatory planning and control of grasp positions and forces for dexterous two-digit manipulation. Journal of Neuroscience 30:9117-9126.
- Fu Q, Hasan Z, Santello M (2011). Transfer of learned manipulation following changes in degrees of freedom. Journal of Neuroscience 31:13576-13584.
- Parikh P, Fine JM, Santello M (2020). Dexterous object manipulation requires context-dependent sensorimotor cortical interactions in humans. Cerebral Cortex 30: 3087-3101.