Programming and updaing of reaching movements

Abstract: Neuronal activity in the motor cortex (area 4) was recorded during reaching movements in cats, trained to perform target-reaching in response to a tone burst. Out of 350 task related neurones 12% generated short latency (20?110 ms) sensory responses to the cue triggering the reaching movement. In the same cells, the response pattern showed a second movement related component following the sensory response. It is suggested that the sensory responses identify the signal as relevant for initiation of the movement. During preparation for reaching, there was a progressive recruitment of motor cortical neurones during which activity related to the sensory cue was transformed into discharges initiating the movement. The conditional signal generated identical sensory responses in both hemispheres. Ipsilateral to the moving limb, 8% of the neurones showed a short latency (40 ms) inhibitory response to the sensory cue. It is suggested that the conditional signal initiates elaboration of motor commands in both hemispheres, but further processing is actively inhibited in the ipsilateral motor cortex. A second question was whether a correction of ongoing reaching in response to a change in target location, is controlled by the same mechanisms as reaching towards stationary targets. A test with reaching to three tubes was used with the correct tube indicated by illumination. In some trials the illumination was shifted to another tube during the ongoing movement resulting in a short?latency corrective movement, which occurred after 50-60 ms (minimum) during switching directed laterally. The latency of switching directed medially was 10-30 ms longer. Selective spinal cord lesions ventrally in C2 prolonged the latency of switching directed medially by 20?30 ms, but had much less effect on switching to directed laterally, which accordingly depends on dorsal pathways, whereas switching directed medially may depend on a reticulospinal pathway. The activity of motor cortical neurones was investigated during trajectory updating. Neurones with responses time?locked to the onset of the illumination shift were found, as well as cells with responses locked to the onset of the corrective movement. The activity of individual cells depended on the direction of the correction. It is suggested that the former group of cells encodes the direction of the target shift, whereas the latter relates to the direction of the corrective movement. None of the cells responded with a latency short enough to contribute to the initial phase of the corrective movement. It is postulated that subcortical centres may command fast trajectory updating whereas the motor cortex controls the further movement to the new target.