| Abstract |
Recent brain imaging studies, using the quantitative 2-deoxy-D-[14C]glucose (2-DG) imaging technique in the Rhesus monkey (Macaca mullata), have demonstrated that the forelimb representation of the primary motor (MI) and somatosensory cortices (Raos et al., Neuroimage 23: 193-201, 2004), as well as several premotor, parietal and occipital areas (Raos et al., J Neurosci 27: 12675-83, 2007 Evangeliou et al., Cereb Cortex 19: 624-639, 2009; Kilintari et al., Cereb Cortex 21 : 949-63), are activated by both action-execution and action-observation, indicating that the spectator mentally simulates the observed action. Moreover, several studies have demonstrated repeatedly that corticospinal excitability is modulated during action observation, providing evidence of an activation of the observer’s motor system. However, evidence for the involvement of the spinal cord in action observation is controversial (Pattuzzo et al., Neuropsychologia 41: 1272-8, 2003; Li et al., J Neurosc 24: 9674-80, 2004).
In the present study we have explored whether and how action-observation affects the spinal cord. For this purpose, we analyzed the forelimb-representation of the spinal cord (cervical enlargement) from eight monkeys. Subjects trained to either execute reaching-to-grasp movements (grasping execution- E) or observe the experimenter performing the same movements (grasping observation- O).
Three E monkeys were trained to reach for and grasp a three-dimensional (3-D) object, with the left forelimb, whereas the right one was restricted. One of the monkeys performed the task under visual guidance and the other two in complete darkness. Three O monkeys were trained to maintain their gaze within the 8°-diameter circular window, with both hands restricted, whereas observing the experimenter
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grasping the ring with the hook grip. The experimenter was standing on the right side of the monkey and performed the reaching-to-grasp movements with the right hand. The rest of the parameters of the O task were similar to the those of the E task. Two motion control (Cm) monkeys were used as control for the O monkeys. Cm monkeys were trained to maintain their gaze within the 8°-diameter circular window, while both hands were restricted, and to observe the moving hand of the experimenter, with the wrist and fingers being extended, without hand preshaping or interaction with the 3-D object. Specifically, the Cm monkeys were exposed to consecutive (and not simultaneous) view of the object and the moving arm. The experimenter was standing at the right side of the monkey.
At the end of each 2-DG experiment the spinal cord was dissected (from the first cervical to the first thoracic segments) and twenty μm thick horizontal sections were obtained, and exposed on autoradiographic films. The autoradiograms were densitometrically analyzed in a computerized image analysis system (MCID, Imaging Research, Ontario, Canada). In each section we sampled the local glucose utilization (LGU) along the rostrocaudal extent of the gray matter (separately for the left and the right side) with a resolution of 50 μm, to produce a data array for the section under analysis. The data arrays of 5 consecutive sections were aligned and averaged to produce a line in the two dimensional map (2-D) of the metabolic activity. To account for inter-individual differences the individual 2-D maps were then transformed to match a common template map.
The results showed that the metabolic activity in the cervical enlargement of the spinal cord is suppressed bilaterally in monkeys observing reaching-to-grasp movements, whereas it is activated ipsilaterally to the grasping hand in monkeys executing the same movements.
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We propose that the elevated activity ipsilateral to the grasping hand, in the ‘executing’ monkeys, may be mediated through MI-corticospinal excitatory signals. The bilateral metabolic depression observed in the ‘observing’ monkeys may represent the sum of excitatory MI-corticospinal and inhibitory premotor cortico-spinal or premotor cortico-brainstem-spinal inputs to spinal local interneurons. All in all, the depression of overall activity in the cervical enlargement of the spinal cord of the O monkeys may explain the suppression of overt movements during action observation, despite the activation of the observer’s motor system.
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