The effects of acute cortical somatosensory deafferentation on grip force control

Abstract

Grip force control involves mechanisms to adjust to unpredictable and predictable changes in loads during manual manipulation. Somatosensory feedback is critical not just to reactive, feedback control but also to updating the internal representations needed for proactive, feedforward control. The role of primary somatosensory cortex (S1) in these control strategies is not well established. Here we investigated grip force control in a rare case of acute central deafferentation following resection of S1. The subject had complete loss of somatosensation in the right arm without any deficit in muscle strength or reflexes. In the first task, the subject was asked to maintain a constant grip force with and without visual feedback. The subject was able to attain the target force with visual feedback but not maintain that force for more than a few seconds after visual feedback was removed. In the second task, the subject was asked to grip and move an instrumented object. The induced acceleration-dependent loads were countered by adjustments in grip force. Both amplitude and timing of the grip force modulation were not affected by deafferentation. The dissociation of these effects demonstrates the differential contribution of S1 to the mechanisms of grip force control.

Keywords: Deafferentation; Internal model; Lesion; Motor control; Somatosensory.

Figure 1

Operative findings and surgical resection of somatosensory cortex. A, Pre-resection findings. Cystic lesion abutting the posterior extent of somatosensory cortex (*). B, Following resection of cystic lesion and seizure onset zone within somatosensory cortex. The intact hand motor cortex is illustrated (sterile number tickets) in relation to the fundus of the skeletonized central sulcus (arrow heads). Subcortical white matter mapping during awake resection verified integrity of descending corticospinal fibers anterior to the resection cavity.

Figure 2

Results of the static grip force experiment conducted in each of three post-resection sessions. Guided by visual feedback during the first 2 s of the trial, the subject reliably attained the target force of 25 +/− 2.5 N in both hands. In the subsequent 5-s hold period without visual feedback (gray region), the grip force was held steady in the healthy hand but drifted from the target in the deafferented hand. Time intervals defining the attained and maintained force performance metrics are indicated.

Figure 3

Results of the dynamic grip force experiment conducted 126 days after resection. A, Steady-state (gravitational) and transient (inertial) load forces were experienced as the subject rapidly moved an instrumented object upwards over ~40cm. Load and grip force trajectories for each trial were aligned on the peak load force. The mean force across trials (black lines) and 95% confidence intervals on the mean (gray regions) are shown. B, A transient increase in grip force that was in phase with the peak load force was observed in both the healthy and deafferented hand. C-F, Four performance metrics comparing grip force-load force coupling. There were no significant performance differences between the left and right hand.