Anatomy Of Praxis

Much of what we know about the anatomical basis of praxis (the ability to perform voluntary skilled movements) comes from the observations of patients with discrete cerebral lesions who have lost these abilities. To perform learned skilled movements, several types of knowledge are required. One must know how to move the limb through space (spatial trajectory) and how fast to move it. To successfully interact with the environment, a conceptual knowledge of praxis is also needed, such as what action is associated with a tool.

Liepmann and Maas'2 were the first to attempt to localize where this movement spatial temporal knowledge is stored. They described a patient with a right hemiparesis who was unable to correctly pantomime to command with his left arm. This patient had lesions of both the pons and the corpus callosum. Because this patient had a right hemiparesis, his right hand could not be tested. Since the work of Broca, it has been known that the left hemisphere in right-handed individuals is dominant for language. Liepmann and Maas could have attributed their patient's inability to pantomime with his left arm to a disconnection between the language and motor areas (i.e., the left hemisphere that mediates comprehension of the verbal command could not influence the right hemisphere's motor areas, which are responsible for controlling the left arm). However, this patient also could not imitate gestures or correctly use actual tools or objects. Therefore, a language-motor disconnection could not account for these findings. Liepmann and Maas posited that the left hemisphere of right-handed individuals contains movement formulas and that the callosal lesion in this patient disconnected these movement formulas from the right hemisphere's motor areas.

Geschwind and Kaplan's! and Gazzaniga and co-workers^! also found that their patients with callosal disconnection could not correctly pantomime to command with their left arm, but unlike Liepmann and Maas' patient, their patients could imitate and correctly use actual tools and objects with the left hand. The preserved ability to imitate and use actual tools and objects suggests that the inability to gesture to command, in these patients with callosal lesions, was induced by a language-motor disconnection rather than by a movement formula motor disconnection. In addition, a disconnection between movement formula and motor areas should produce spatial and temporal errors, but many of the errors made by Liepmann and Maas' patient appeared to be content errors.

Watson and Heilman, '5 however, described a patient with an infarction limited to the body of the corpus callosum. This patient had no weakness in her right hand and performed all tasks flawlessly with her right hand. Yet, as Leipmann might have predicted, this patient could not correctly pantomime to command, imitate, or use actual tools with her left hand. Immediately after this patient's cerebral infarction, she made content errors, but subsequently she made primarily spatial and temporal errors. Her performance indicated that not only language but also movement representations were stored in her left hemisphere, and her callosal lesion disconnected these representations from the right hemisphere. In Watson and Heilman's patient there appeared to be two types of representations that were stored in the left hemisphere: spatial-temporal movement representations (also called visual kinesthetic engrams) and conceptual knowledge about the relationship between tools and the actions required to use these tools.

Heilman and colleagues'^ and Rothi and associates proposed that the movement representations (visual kinesthetic movement engrams), first posited by Liepmann, were stored in the left parietal lobe of right-handed individuals, and that destruction of the left parietal lobe should induce not only a production deficit (apraxia) but also a gesture comprehension-discrimination disorder. Apraxia induced by lesions of the premotor cortex, the pathways that connect premotor areas to motor areas, and the pathways that connect the parietal lobe with the premotor areas, may also cause production deficits. In these situations, patients make spatial and temporal errors, but, unlike parietal lesions, these lesions do not induce gesture comprehension and discrimination disorders. When patients with anterior and posterior lesions were tested, it was found that while both groups of patients were apraxic, the patients with a damaged parietal lobe had comprehension and discrimination disturbances, but those without parietal lesions did not.

As discussed fully in Ch§pie.Ll§., the final common pathway that allows the muscles to move joints involves motor nerves that originate from the spinal cord. These spinal motor nerves are activated by corticospinal neurons, and the corticospinal tract is influenced by the premotor areas. In addition to direct connections with the spinal cord, the premotor area projects to the primary motor cortex. For each specific skilled movement, there is a set of spatial loci that must be traversed in a specific temporal pattern. It is proposed that movement formulas represented

in the inferior parietal lobe are stored in a three-dimensional supramodal code. For the corticospinal neurons to properly activate the motor nerves, the stored spatial temporal knowledge has to be transformed into a motor program.

The medial premotor cortex including the supplementary motor area (SMA) appears to play an important role in mediating skilled movements. Whereas electrical stimulation of the primary motor cortex induces simple movements, SMA stimulation induces complex movements that may include the entire forelimb. SMA receives projections from parietal neurons and projects to motor neurons. SMA neurons discharge before neurons in the primary motor cortex. Studies of cerebral blood flow, an indicator of cerebral metabolism and synaptic activity, have revealed that single repetitive movements increase activation of the contralateral primary motor cortex, but complex movements increase flow in the contralateral motor cortex and bilaterally in the SMA. When subjects remain still and think about making complex movements, blood flow is increased to the SMA but not to the primary motor cortex. Watson and colleagues '8 reported several patients with left-sided medial frontal lesions that included the SMA who made spatial and temporal errors when they attempted to perform learned skilled movements.

The convexity premotor cortex also receives projections from the parietal lobes as well as from the medial premotor cortex, and, like the medial premotor cortex, the convexity premotor cortex also projects to the primary motor area. Many skilled movements require that multiple joints be moved simultaneously. For example, when using a knife to cut a slice of bread, on the forward thrust, the shoulder must be flexed and adducted while the elbow is extended. On the backward thrust, the arm must be extended at the shoulder while the shoulder is abducted and the forearm is extended at the elbow. Lesions in the convexity premotor cortex impair multiple joint coordination. Unlike patients with parietal lesions, patients with convexity and medial premotor lesions could both comprehend and discriminate between well-performed and incorrectly performed pantomimes, demonstrating that their spatial-temporal movement representations were intact but could not be implemented.

It has been suggested that apraxia might be due to deep subcortical lesions including lesions of the basal ganglia or thalamus. A recent analysis by Pramstaller and Marsden'gi reviewed 82 cases of "subcortical" apraxias that were studied by either neuroimaging modalities or neuropathological means. They concluded that isolated damage to the basal ganglia (putamen, caudate, and globus pallidus) does not induce apraxia. However, they noted that small lesions of the thalamus can sometimes cause apraxia, yet the exact role of the thalamus in higher order motor control and apraxia remains to be determined.

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