In order to investigate higher level representations involving ensembles of neurons, it is necessary to record and isolate many units at the same time. This laboratory was one of the first to demonstrate the ability to record simultaneously from multiple neurons in primate motor cortex using a chronically implanted electrode array (Figure 1A). Up to 21 units from MI (Figure 1B) were simultaneously recorded while monkey performed hand movements in various directions (the center-out task is described in Research Design and Methods) in order to determine whether they possessed directional tuning properties. Georgopoulos and his colleagues have shown that neurons in primary motor cortex (MI) possess very broad directional tuning, many of which can be fit with a cosine function(Georgopoulos, Kalaska, Caminiti & Massey, 1982). By recording from 15-17 cells on 4 separate recording sessions, we found that 81% of the neurons had directional tuning that fit a cosine function (r2>0.5; p<0.05) whose peak defines the preferred direction of the neuron (Figure 1C). These results provide strong evidence that we are able to record groups of typical MI neurons from the arm area of primary motor cortex in a behaving monkey.

We have also begun to record from neurons in the supplementary motor cortex (SMA) while a monkey performs the same center-out task. Unlike MI neurons, the few cells we have recorded appear not to be strongly directionally selective. By aligning the data on movement onset, it is evident that firing rate builds gradually for over 2 second from the beginning of the instructed delay period until about 200 ms before movement onset at which point it drops rather sharply (Figure 1D). This preparatory activity is consistent with other electrophysiological studies using awake behaving monkeys ((Tanji, Taniguchi & Saga, 1980)).

Examples of neural activity recorded on different electrodes in chronically implanted array:

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