In anatomical terms, sensation and movement are each represented across distributed regions in the human brain. Somatosensory information— that is, information pertaining to sensation of the body—is gathered from the environment and transmitted to the brain via the spinal cord and via the thalamus, from where it is distributed to three regions of the parietal cortex: the primary somatosensory cortex (located in the postcentral gyrus), the secondary somatosensory area, and the posterior parietal area. These areas are primarily involved in the representation of touch and of limb position, particularly as it occurs in the absence of visual feedback. The motor regions of the brain include the primary motor cortex, located in the precentral gyrus, the premotor cortex, and the supplementary motor cortex. These areas act in concert to contribute to the organization and production of movement and are collectively referred to as the sensorimotor cortex.
First illustrated by Wilder Penfield and Edwin Boldrey in 1937, the sensorimotor cortex, in particular the primary somatosensory and motor cortices, is topographically organized, meaning that particular areas of the body map onto particular areas of the cortex. Furthermore, the size of the representation is proportional to the sensitivity (primary somatosensory cortex) and dexterity (primary motor cortex) of the represented area, rather than its physical size. For example, the somatosensory representation of the thumb and the lips are each approximately as large as that of the entire leg. The reason is that the skin of the thumb and the lips are densely populated by cutaneous receptors, giving these areas a high degree of sensitivity and therefore large cortical representations, while the skin on the leg is far less densely populated by these receptors and therefore less sensitive. A similar topographical map exists in the primary motor cortex, in which parts of the body that perform highly dexterous movements, for example the fingers and tongue, occupy a larger volume of the primary motor cortex with projections to the muscles of these areas, compared with other parts of the body such as the trunk and pelvis.
On a cellular basis, neurons in the primary motor cortex are tuned to the direction in which a movement is performed. These cells are activated strongly when movements are performed in one direction in space and less so for movements in another. It is unclear, however, whether the preferred direction for particular cell populations is represented in an intrinsic coordinate system—for example, flexion and extension of a joint—or an extrinsic coordinate system—for example, movement to the left or the right.
Plasticity of Sensorimotor Representations
For many years, it was thought that there was little capacity for sensorimotor representations to be altered in the adult brain. More recently, it has been demonstrated extensively that sensorimotor representations in the adult brain can undergo functional reorganization, a process known as plasticity, in response to damage to peripheral or central structures, or as a consequence of experience. For example, imaging studies have shown that arm amputees activate a larger than normal area of sensorimotor cortex when performing shoulder movements of the corresponding amputated arm, suggesting that the remaining arm muscles have an expanded representation. Similarly, transcranial magnetic stimulation (TMS), which can be used to map the area of sensorimotor cortex projecting to specific muscles, has demonstrated that motor practice results in an increase in the area of sensorimotor representation of the muscles involved in the practice. These observations confirm that sensorimotor representations are dynamic and malleable, depending on circumstances.
Motor Learning as a Tool to Understanding Sensorimotor Representations
Motor learning can be considered as a form of perturbation in which behavior and neuronal activity is examined before, during, and after a period of adaptation. The pattern of behavior during these three important periods of adaptation can reveal different properties of the movement representation in the brain. In particular, the brain’s ability to generalize motor learning and apply what was learned to different contexts gives important insights into sensorimotor representation in the brain. For example, learning to hit a tennis forehand does not require exposure to all possible ball trajectories, only a subset. Once learned, the challenge is to optimize the generalization of the skill in the appropriate context. Too little generalization results in improvements that are limited to very specific conditions, whereas overgeneralization results in inappropriate actions for a given condition. For instance, it is no use trying to hit a forehand when a backhand stroke is required. The key point here is that patterns and constraints of generalization can reveal how movements are encoded in the brain.
Levels of Sensorimotor Representation
Complex sensorimotor skills can be represented in a number of different frames of reference. In the example of hitting a tennis ball, the brain must first represent the purpose of the movement (hitting the ball), the required end-point result (racquet meeting the ball), the required joint torques to position and orientation of the arm, and finally the required muscle activation pattern to achieve the movement outcome. At each stage, there is a different sensorimotor representation that guides the building of the movement. During motor learning, the exact pattern of errors in the movements can give clues about the neural representation in the brain and the frame of reference that was used during the adaptation phase.
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