Many voluntary movements involve coordination between the limbs. However, there have been very few attempts to study the neuronal mechanisms that mediate this coordination. Here we have studied the activity of cortical neurons while monkeys performed tasks that required coordination between the two arms. We found that most neurons in the primary motor cortex (MI) show activity specific to bimanual movements (bimanual-related activity), which is strikingly different from the activity of the same neurons during unimanual movements. Moreover, units in the supplementary motor area (SMA; the area of cortex most often associated with bimanual coordination) showed no more bimanual-related activity than units in MI. Our results challenge the classic view that MI controls the contralateral (opposite) side of the body and that SMA is responsible for the coordination of the arms. Rather, our data suggest that both cortical areas share the control of bilateral coordination.

Previous studies have shown that activity of neuronal populations in the primary motor cortex (MI), processed by the population vector method, faithfully predicts upcoming movements. In our previous studies we found that single neurons responded differently during movements of one arm vs. combined movements of the two arms. It was, therefore, not clear whether the population vector approach could produce reliable movement predictions also for bimanual movements. This study tests this question by comparing the predictive quality of population vectors for unimanual and bimanual arm movements. We designed a bimanual motor task that requires coordinated movements of the two arms, in which each arm may move in eight directions, and recorded single unit activity in the MI of two rhesus (Macaca mulatta) monkeys during the performance of unimanual and bimanual arm movements. We analysed the activity of 212 MI cells from both hemispheres and found that, despite bimanual related activity, the directional tuning and preferred directions of most cells were preserved in unimanual and bimanual movements. We demonstrate that population vectors, constructed from the activity of MI cells, predict accurately the direction of movement both for unimanual and for bimanual movements even when the two arms move simultaneously in different directions.

Single units were recorded from the primary motor (MI) and supplementary motor (SMA) areas of Rhesus monkeys performing one-arm (unimanual) and two-arm (bimanual) proximal reaching tasks. During execution of the bimanual movements, the task related activity of about one-half the neurons in each area (MI: 129/232, SMA: 107/206) differed from the activity during similar displacements of one arm while the other was stationary. The bulk of this "bimanual-related" activity could not be explained by any linear combination of activities during unimanual reaching or by differences in kinematics or recorded EMG activity. The bimanual-related activity was relatively insensitive to trial-to-trial variations in muscular activity or arm kinematics. For example, trials where bimanual arm movements differed the most from their unimanual controls did not correspond to the ones where the largest bimanual neural effects were observed. Cortical localization established by using a mixture of surface landmarks, electromyographic recordings, microstimulation, and sensory testing suggests that the recorded neurons were not limited to areas specifically involved with postural muscles. By rejecting this range of alternative explanations, we conclude that neural activity in MI as well as SMA can reflect specialized cortical processing associated with bimanual movements.

It is well established that the discharge of neurons in primate motor cortex is tuned to the movement direction of the contralateral arm. Interestingly, it has been found that these neurons exhibit a directional tuning to the ipsilateral arm as well and that the preferred directions to both arms tend to be similar. A recent study showed that motor cortex cells are also directionally selective to bimanual movements, but the relationship between the bimanual and unimanual representations remains unclear. To address this issue, we analyzed the responses of motor cortical neurons recorded from two macaque monkeys during unimanual and bimanual reaching movements. We decomposed the bimanual movement representation into contralateral and ipsilateral directionally tuned components. Our major finding is that the movement of the contralateral arm modifies the tuning of the cells to the ipsilateral arm such that: (1) the offset and modulation depth of the tuning are suppressed; and (2) the preferred directions are randomly shifted. Both these effects eliminate the correlation between the contralateral and ipsilateral representations during bimanual movements. We suggest that the modification of the ipsilateral arm representation is caused by the recruitment of local inhibition that conveys callosal inputs during bimanual movements. This hypothesis is supported by the analysis of a model of two motor cortical networks, coupled with sparse random interhemispheric projections that reproduce the main features observed in the data. Finally, we show that the modification of the ipsilateral arm representation reduces the interference between the movements of both arms.

We recorded local field potentials (LFP) in primary (MI) and supplementary (SMA) motor areas of rhesus monkey cortex in order to compare movement-evoked potentials (mEP) in bimanual and unimanual movements with single-unit activity recorded concurrently. The mEP was often different during bimanual and unimanual movements (a "bimanual-related" effect), but, unlike the single units, the size of the mEP in both MI and SMA was always greater during bimanual movements than during unimanual movements. This increase primarily reflected an increase in the late positive peak of the mEP, a result that may reflect greater overall cortical activation during bimanual movements. In addition, analysis of the mEP revealed differences between MI and SMA not seen in the single-unit activity. mEP in MI had greater contralateral preference than in SMA. Also, SMA mEP was more correlated to the single-unit activity than in MI. This greater correlation was also more apparent in the late peaks of the mEP than in the early peaks and may reflect a greater influence of recurrent activation in SMA than in MI. Our results further reinforce the idea that unimanual and bimanual movements are represented differently both in MI and in SMA and also show that a complex relationship between spikes of individual neurons and LFP may reflect the different input-output relations of different cortical areas.

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