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The primary motor cortex is located in Brodmann area 4 at the most posterior part of the frontal lobe. The primary motor cortex corresponds to an output stage of motor signals, sending motor commands to the brain stem and spinal cord. Brodmann area 6 is rostral to Brodmann area 4, where multiple higher-order motor areas are located. The premotor area, which is located in the lateral part, is involved in planning and executing action based on sensory signals. The premotor area contributes to the reaching for and grasping of an object to achieve a behavioral goal. The supplementary motor area, which occupies the mesial aspect, is involved in planning and executing actions based on internalized or memorized signals. The supplementary motor area plays a central role in bimanual movements, organizing multiple movements, and switching from a routine to a controlled behavior. Thus, Brodmann areas 4 and 6 are considered as central motor areas in the cerebral cortex, in which the idea of an action is transformed to an actual movement in a variety of contexts.

Visually guided action is generated via multiple modes of information processing. Here, we discuss three modes of neural processing underlying visually guided action. The first mode involves direct visuo-action association. In this mode, an action is planned to reach and grasp a target based on information about the target position (for reaching) and shape (for grasping). The network connecting the premotor and parietal cortices plays a central role in this mode. The second mode involves conditional visuo-action association. In this mode, a particular action is selected based on a rule associating a visual feature with an action. The third mode involves conditional visuo-goal association. In this mode, a visual signal is associated with a behavioral goal, but not with an action. A particular action is subsequently selected to meet this goal. Areas on the route from the inferotemporal cortex to the dorsal premotor cortex, such as the prefrontal cortex and basal ganglia, play a role in achieving conditional visuo-action and conditional visuo-goal associations. In summary, our analysis suggests the involvement of multiple brain networks converging on the premotor cortex in the three modes of neural processing utilized for generating visually guided action.

The hallmark of higher-order brain functions is the ability to integrate and associate diverse sets of information in a flexible manner. Thus, fundamental knowledge about the mechanisms underlying of information in the brain can be obtained by examining the neural mechanisms involved in the generation of an appropriate motor command based on perceived sensory signals. In this review article, we have focused on the involvement of the neuronal networks centered at the lateral aspect of the frontal cortex in the process of motor selection and motor planning based on visual signals. We have initially discussed the role of the lateral prefrontal cortex in integrating multiple sets of visual signals to select a reach target and the participation of the premotor cortex in retrieving and integrating diverse sets of motor information, such as where should one reach out or which arm is to be used. Next, based on the results of the studies on ideomotor apraxia, we have hypothesized that there are at least 2 distinct levels of neural representation (virtual level and physical level). We have reviewed the evidence supporting the operation of 2 distinct classes of neuronal activities corresponding to these 2 levels. In conclusion, we propose that the frontal cortex initially processes information across sensory and motor domains at the virtual level to generate information about a forthcoming motor action (virtual action plan) and that this information is subsequently transformed into a motor command, such as muscle activity or movement direction, for an actual body movement at the physical level (physical motor plan). This proposed framework may be useful for explaining the diverse clinical conditions caused by brain lesions as well as for clarifying the neural mechanisms underlying the integration of perception and action.

Early studies on cortical motor areas have been centered on their somatotopical organization: a reasonable direction of research from the standpoint of skeletomotor control of limb and body movements. On the primary motor cortex, anatomical and physiological studies revealed aspects of somatotopical organization in progressively finer scales. Earlier studies were directed at elucidating the fine-grain modular organization of the primary motor cortex. Later studies, however, emphasized the diversity of output organization in individual part of the cortex, even at a single-cell level. At present, there is no convincing evidence for the existence of microstructures representing any form of unitary function. As for nonprimary motor areas, the existence of somatotopical organization has been inferred based on anatomical studies and on studies utilizing microstimulation. In the supplementary motor area, the body-part representation is broadly organized rostrocaudally in the order of face, forelimb and hindlimb areas, although with an extensive overlap of each area. In contrast, somatotopy is not apparent in the presupplemenetary motor area; effector-independent control of motor behavior seems to be dominant in this area. In the premotor cortex, motor acts involving the hindlimb appears to be much less represented than actions involving hand-arm and face. Overall, in considering the workings of nonprimary areas, aspects of motor behavior involving sensorial guidance, action-selection, or visuomotor association appear to be of primary importance rather than the determination of body parts to be used.