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Neuronal activity in the deep layers of the macaque (Macaca mulatta) superior colliculus (SC) and the underlying reticular formation is correlated with the initiation and execution of arm movements (Werner, 1993). Although the correlation of this activity with EMGs of proximal arm muscles is as strong as in motor cortex (Werner et al., 1997a; Stuphorn et al., 1999), little is known about the influence of electrical microstimulation in the SC on the initiation and trajectories of arm movements. The experiments on three macaque monkeys I am presenting here clearly show that arm movements can be elicited by electrical microstimulation in the deep layers of the lateral SC and underlying reticular formation. A large repertoire of arm movements categorized into three movement types were elicited and compared before and after training. Therefore, arm movements induced by electrical stimulation in the monkey SC represent a further component of the functional repertoire of the SC using its impact on motoneurons in the spinal cord perhaps directly in the initiation, execution, and amendment of arm and hand movements.

Evidence is accumulating that neurons in primary motor cortex (M1) respond during action observation, a property first shown for mirror neurons in monkey premotor cortex. I will show here for the first time that the discharge of a major class of M1 output neuron, the pyramidal tract neuron (PTN), is modulated during observation of precision grip by a human experimenter. Many PTNs increased their discharge during observation (facilitation-type mirror neuron), but a substantial number exhibited reduced discharge or stopped firing (suppression-type). Simultaneous recordings from arm, hand, and digit muscles confirmed the complete absence of detectable muscle activity during observation. The discharge of the same population of neurons was compared with activity during active grasp by the monkeys. We found that facilitation neurons were only half as active for action observation as for action execution, and that suppression neurons reversed their activity pattern and were actually facilitated during execution. Thus, although many M1 output neurons are active during action observation, M1 direct input to the spinal circuitry is either reduced or abolished and may not be sufficient to produce overt muscle activity.

Little is known about the gaze pattern during an action observation task and whether gaze direction per se affects the activity of mirror neurons. We analysed the coordination between gaze behaviour and dexterous manipulation in two monkeys that first observed and then grasped three differently shaped objects (execution trials). On randomly interleaved trials, the monkey watched as the same objects were presented to, and then grasped, by a human experimenter (observation trials). During both execution and observation trials, monkeys spent a significant amount of time looking both at the presented object and at the subsequent grasping action. Gaze patterns during execution and observation were well correlated. We recorded from mirror neurons in M1 and F5. The monkey’s eye fixation and saccadic patterns were then analysed in relation to their neuronal firing rates. The correlations with gaze behaviour appeared to arise mostly because of the close co-ordination of eye and hand movements during the task. Detailed analysis showed that when this factor was removed, only a small proportion of mirror neurons showed a significant correlation with gaze direction. Thus monkeys’ gaze behaviour suggests that they show attention to both executed and observed tasks, but this does not significantly affect the activity of the sampled mirror neurons.