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Neurons that were antidromically invaded from either pyramidal tract (PT) stimulation or red nucleus (RN) stimulation were studied in the anterior portion of the supplementary motor cortex of anaesthetized rhesus monkeys. The conduction velocity of 58 PT neurons ranged from 11 to 63 m/sec while those of the 29 RN neurons ranged from 10 to 38 m/sec. None of the neurons were found to be antidromically invaded from both stimulation sites. Thus, the corticorubral projection from the SMA does not appear to consist primarily of PT axon collaterals. There appeared to be a trend towards spatial segregation of the PT and RN neurons in the monkey cortex, the RN neurons tended to be located nearer the pial surface than the PT neurons, which tend to have somata nearer the subcortical white matter.

An instrument has been developed for recording intracellulary from cortical neurons in an exposed pulsating brain. The electrode moves with the brain pulsations maintaining the electrode within the impaled cell. Stable recordings have been obtained for up to 20 min.

An instrument has been developed for positioning microelectrodes near neurons in the exposed pulsating cerebral cortex of man or animals. The electrode moves with the cortex and maintains a fixed relationship with the desired neuron. Stable extracellular single cell recordings have been obtained for 17.5 min from an exposed human brain that pulsated 1.5 mm at the surface.

Single unit recordings were obtained from 103 precentral cortical cells whose activity was related to an alternate wrist flexion-extension task in monkeys. Although the task was carried out under different loads only a weak relationship between cortical cell firing rates and static force levels was observed for force in one direction. A large change in firing rate occurred, however, when direction of force shifted as a result of a change in the predominant activity between extensor and flexor muscles. The firing patterns of the observed cortical cells suggest that the motor cortex is involved in specifying the muscles to be activated for a given movement and not the level of force to be produced by these muscles. During the dynamic phase of the movement little change in cortical cell firing pattern was observed for large changes in the rate of change of force. Motor cortex cell firing patterns appear to be unrelated to the large values of rate of change of force seen in this experiment.

Recordings have been obtained simultaneously from several, individually selected neurons in the motor cortex of unanesthetized monkey as the animal performed simple arm movements. With the use of comparatively simple quantitative procedures, the activity of small sets of cells was found to be adequate for rather accurate real-time prediction of the time course of various response measurements. In addition, the results suggest that hypotheses concerning the response variables "controlled" by cortical motor systems may well depend upon whether or not the temporal relations between simultaneously active neurons are taken into account.