چگونه با هم معادل هستند اجرای اکشن، تصویرسازی و مشاهده حرکات لازم؟ بازنگری مفهوم somatotopy در زمان شبیه سازی عمل

Jeannerod (2001) hypothesized that action execution, imagery, and observation are functionally equivalent. This led to the major prediction that these motor states are based on the same action-specific and even effector-specific motor representations. The present study examined whether hand and foot movements are represented in a somatotopic manner during action execution, imagery, and action observation. The experiment contained ten conditions: three execution conditions, three imagery conditions, three observation conditions, and one baseline condition. In the nine experimental conditions, participants had to execute, observe, or imagine right-hand extension/flexion movements or right-foot extension/flexion movements. The fMRI results showed a somatotopic organization within the contralateral premotor and primary motor cortex during motor imagery and motor execution. However, there was no clear somatotopic organization of action observation in the given regions of interest within the contralateral hemisphere, although observation of these movements activated these areas significantly.

A broad body of literature underpins the notion of a functional equivalence between the execution, imagery, verbalization, and observation of human actions (see, for reviews, Grèzes and Decety, 2001 and Jeannerod, 2001). One comprehensive account of the underlying brain mechanisms assumes that the simulation of body movements is based on own motor representations in the brain (Grush, 2004 and Jeannerod, 2001). Jeannerod has proposed an explanation for this in his mental simulation theory. It states that a movement possesses a covert action stage involving the goal, the means to achieve it, and its consequences (Jeannerod, 2001). Although these covert actions are also actions, they are, nonetheless, actions that have not yet been executed. Situations corresponding to such covert activity are, for example, the conscious, self-intended simulation of one’s own actions (motor imagery) or the perception of actions by others (action observation). The main difference between these two cognitive motor states is that motor imagery is generated internally, whereas action observation is driven by external stimuli. Many studies have delivered evidence that the neural representations for motor imagery and action observation are similar to those for motor execution (e.g., Filimon et al., 2007, Gazzola and Keysers, 2008 and Lotze et al., 1999). They have shown that both motor imagery and action observation are related to activation within motor and motor-related areas, indicating that, to some degree, they use the same brain substrate as the human motor execution system. One central prediction based on these results is that a motor-based action simulation should be composed of action- and even effector-specific motor representations (Fernandino & Iacoboni, 2010). More precisely, simulation of movements with different effectors might engage different effector-specific motor representations, and simulation of different actions with different consequences might be associated with a map for actions leading to a comparable action consequence. Ever since Penfield and Boldery’s (1937) experiments, research has shown that the above – mentioned somatotopic or effector-specific mapping of motor representations within the human sensory and motor systems is a very prominent organization principle. This principle implies that actions relying on specific effectors are represented separately in the motor and somatosensory system. Somatotopic representations within different cortical and subcortical systems can be found in, for example, the primary motor and somatosensory cortex, the premotor cortex, the basal ganglia, the cerebellum, as well as the occipitotemporal cortex (Grafton et al., 1991, Lotze et al., 2000, Orlov et al., 2010, Rintjes et al., 1999, Romanelli et al., 2005 and Schlerf et al., 2010). The most prominent somatotopic representations are found in the precentral and postcentral gyrus of the contralateral hemisphere. Within these areas, the lower extremities are represented near the margo superior cerebri (dorsomedial); upper extremities such as the somatosensory hand area, on the lateral surface of the sensorimotor cortex (dorsolateral) ( Grafton et al., 1991). In line with findings from studies investigating the neural basis of action simulation states (s-states), it has been suggested that motor imagery and action observation also possess a roughly somatotopic arrangement ( Buccino et al., 2001, Ehrsson et al., 2003, Jarstorff et al., 2010, Stippich et al., 2002, Szameitat et al., 2007 and Wheaton et al., 2004). At first glance, these studies provide clear evidence for the notion that motor representations are involved not only in controlling movements of particular effectors, but also during the mere imagination or observation of the action performed by a specific effector. However, these data stem from studies focusing on only one cognitive motor state in one experiment. They compared either action execution with motor imagery or action execution with action observation. Furthermore, the different studies considered different types of movements and used different forms of data analysis and contrast calculation. Hence, until now, no study has systematically investigated somatotopic representations of the same movement for action execution, observation, and imagery. This means that it is still not known whether the execution, observation, and imagery of movements lead to a comparable somatotopic mapping within the primary motor and premotor cortex. To further clarify the issues of functional equivalence and somatotopic organization, this study applied a design with nine experimental conditions in which participants had to execute, observe, or imagine either an intransitive hand movement, an intransitive foot movement, or a concurrent movement of both effectors. In line with the findings in the literature discussed above, the functional MRI measurements covered the brain from the margo superior cerebri to the temporal pole. If a motor-based action simulation is composed of action- and effector-specific motor representations, we expect an effector-specific mapping of motor representations within the primary and premotor area during the motor imagery, action observation, and execution of the single intransitive hand and foot movement in question performed with different effectors, because the movement differs with respect to the effectors used but not with respect to the action goals.

In conclusion, the present data demonstrate that imagination of intransitive movements with different effectors is associated with similar effector-dependent activation sites as well as the specific pattern and, therefore, follows a somatotopic organization principle within the motor and premotor region of the contralateral hemisphere. In contrast, observation of intransitive movements performed with different effectors generates an overlapping activation pattern within the contralateral premotor and primary motor cortex. It is to be hoped that these data might contribute to the ongoing debate over functional equivalence between motor imagery, action observation, and action execution.