The Somatosensory Cortex and Complex Movement
This chapter began by describing the remarkable painting skills of Kamala the elephant. Kamala first needs a plan—some idea of what she wants to paint. She must then execute the movements required to apply paint to her canvas, and she must use so-matosensory information to confirm that she is producing the movements that she intends. So to paint or to perform virtually any complex movement, the motor system jand the somatosensory system must work together. In this final section of the chapter,, we explore that interaction.
The secondary somatosensory cortex plays an important role in confirming which movements have already taken place and in deciding which movements should follow. Damage to the secondary somatosensory cortex does not disrupt the plans for making movements, but it does disrupt how the movements are performed, leaving their exe cution fragmented and confused. The inability to complete a plan of action accurately
Figure 10-26
Somatosensory Plasticity Adapted from "Massive Cortical Reorganization after Sensory Deafferentation in Adult Macaques," by T. P. Pons, P. E. Garraghty, A. K. Ommaya, J. H. Kaas, and M. Mishkin, 1991, Science, 252, p. 1858.
Apraxia. Inability to make voluntary movements in the absence of paralysis or other motor or sensory impairment, especially an inability to make proper use of an object
Figure 10-27
Visual Aid The secondary somatosensory cortex contributes to information flow in dorsal (how) and ventral (what) visual streams
Information from the secondary somatosensory cortex contributes to the dorsal stream by specifing the movement used for grasping a target.
Information from the secondary somatosensory cortex contributes to the dorsal stream by specifing the movement used for grasping a target.
Secondary somatosensory cortex
Visual cortex
Information from the secondary somatosensory cortex contributes to the ventral stream by providing information about object size and shape.
is called apraxia (from the Greek words for "no" and "action"). The following case highlights the symptoms of apraxia:
A woman with a biparietal lesion [damage on both sides of the secondary somatosensory cortex] had worked for years as a fish-filleter. With the development of her symptoms, she began to experience difficulty in carrying on with her job. She did not seem to know what to do with her knife. She would stick the point in the head of a fish, start the first stroke, and then come to a stop. In her own mind she knew how to fillet fish, but yet she could not execute the maneuver. The foreman accused her of being drunk and sent her home for mutilating fish.
The same patient also showed another unusual phenomenon that might possibly be apraxic in nature. She could never finish an undertaking. She would begin a job, drop it, start another, abandon that one, and within a short while would have four or five uncompleted tasks on her hands. This would cause her to do such inappropriate actions as putting the sugar bowl in the refrigerator, and the coffeepot inside the oven. (Critchley, 1953, pp. 158-159)
How does an intact secondary somatosensory cortex contribute to the organ ization of movement? Recall from Chapter 8 that visual information influences movement through the dorsal and ventral streams. The dorsal stream, working without conscious awareness, provides vision for action, as when we use the visual form bf a cup to automatically shape a hand to grasp that cup. The ventral stream, in contrast, works with conscious awareness and provides the vision needed to identify objects.
As Figure 10-27 illustrates, the secondary somatosensory cortex participates in both visual streams. The dorsal visual stream projects to the secondary somatosensory cortex and then to the prefrontal cortex. In this way, visual information is integrated with somatosensory information to produce movements that are appropriately shaped and directed for their targets.
Much less is known about how the secondary somatosensory cortex contributes to the ventral stream, but it is likely that somatosensory information about the identity of objects and completed movements is relayed by the ventral stream to the prefrontal cortex. The prefrontal cortex can then select the appropriate actions that should follow from those that are already complete. Consider the difference in the way wej would reach for an empty glass versus a glass filled to the brim with hot liquid.
Close interaction between the somatosensory system and the motor system exists at all levels of the nervous system. It can be seen in the spinal cord, where sensory information contributes to spinal reflexes. It can also be seen in the brainstem, where various species-specific behaviors, such as attack, withdrawal, and grooming, require both appropriate patterns of movement and appropriate sensory information.
The close interrelation is found as well at the level of the neocortex,, where skilled movements elicited by the motor regions of the frontal lobes require information about actions that have just taken place and about objects that have been or could be manipulated. In short, an interaction between the motor cortex, which decides what should be done, and the sensory cortex, which knows what has been done, is central to how the brain produces movement in the here and now.
Secondary somatosensory cortex
Visual cortex
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