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Alexa Riehle (Institut de Neurosciences de Timone)

Spatio-temporal maps of neuronal activity in monkey motor cortex during a delayed reach-to-grasp task

Reaching to and grasping an object involves shaping the hand and fingers in relation to the object’s physical properties. It requires the coordinated activation of a variety of arm and hand muscles. Reaching involves the proximal muscles of arm and shoulder to transport the hand toward the target object, whereas grasping involves the distal muscles of the hand for its shaping and force control. Earlier studies suggest that the control of reaching, on the one side, and of hand shaping and grasp force, on the other, involve partially segregated motor cortical networks. However, it is still unclear how information originating from these networks is processed and integrated in space and time. We addressed this issue by analyzing massively parallel neuronal activities recorded during a delayed reach-to-grasp task, by using 100 electrodes Utah arrays chronically implanted in monkey motor cortex. These high-density electrode arrays allow one to record simultaneously both the spiking activity of large populations of single neurons (up to 160 neurons) and local field potentials (LFPs ; one on each electrode). We studied the spatio-temporal structure of the various brain signals recorded during our reach-to-grasp task, such as (i) the movement-related potentials (MRP) of the LFP around movement onset, (ii) the temporal profile of single neuron spiking activity obtained on each electrode, (iii) the somatosensory receptive fields of single neurons, (iv) the propagation of spiking activity due to local intracortical micro-stimulation, (v) the "traveling waves" of LFP oscillations in the beta range, and (vi) the synchrony patterns among single neurons ("cell assemblies"). We observe a large spatial overlap of LFP and spiking activities as a function of the proximo-distal representations of the upper limb. Our data show that a precise spatio-temporal pattern of activation, mainly oriented along the medio-lateral axis in motor cortex, is involved in the control of reach-to-grasp movements. The combination of massively parallel recordings and intracortical microstimulation using high density multielectrode arrays provides novel insights into the functional organization of motor cortex. It is a useful tool to investigate the topological organization of intrinsic motor cortical functional connectivity involved in complex movements.