Peripheral Agraphias

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Clinically, the peripheral agraphias are characterized by defective selection or production of letters in handwriting. In pure cases, the central or linguistic components of writing are intact, and the preservation of orthographic knowledge can be demonstrated via spared output modalities that include oral spelling, typing, and spelling with anagram letters. The major subtypes of peripheral agraphia include allographic disorders, apraxic agraphia, nonapraxic disorders of motor execution, and afferent dysgraphia.

1. Allographic Disorders: Impairments of Letter Shape Assignment

In allographic writing disorders the main difficulty involves the activation or selection of letter shapes appropriate for the abstract graphemic representations held in the graphemic buffer. As noted earlier, it is not clear whether the underlying deficit reflects a failure to access visuospatial letter shape information from an independent allographic memory store or whether it is caused by a functional impairment of the procedures necessary for activating the correct letter-specific graphic motor programs (Fig. 2).

The breakdown of the allographic conversion process can take different clinical forms. In some cases it may result in partial or complete inability to write the intended letters, apparently because of a failure to remember how to create the appropriate physical letter shapes. The letter production deficit may be specific to certain allographic forms. For instance, patients have been described who could write uppercase letters significantly better than lowercase letters, whereas others showed the opposite dissociation. These findings imply independent neural representations for upper- versus lowercase letters that can be selectively disrupted by brain damage. Allographs for different lowercase writing styles may also be organized separately, as suggested by the reported dissociation between printing lowercase letters versus writing the same letters in a cursive style.

Other patients with allographic disorders have no difficulty in producing individual letters but they seem to have trouble specifying the contextually appropriate allographic repertoire, resulting in an uncontrollable mixing of upper- and lowercase letters in handwriting. In still other patients, case and style specification procedures are intact, but written spelling is characterized by numerous well-formed letter substitution errors. Substitution errors typically involve the production of letter shapes physically similar to the intended target, and the occurrence of these errors may be influenced by letter frequency (i.e., more errors on letters used less frequently in the English language). Whether the documented physical similarity effects in letter substitution errors are based on shared visuos-patial features or whether they reflect the fact that graphic motor programs containing similar stroke sequences are more likely to be confused has not been established conclusively.

In the majority of reported cases, allographic writing disorders were associated with left temporo-parieto-occipital damage. Therefore, this posterior cortical region may play an important role in activating and selecting the appropriate letter shapes for written output.

2. Apraxic Agraphia: Defective Motor Programming of Writing Movements

Apraxic agraphia is characterized by poor motor execution of the stroke patterns necessary to produce letters. In order to classify the writing disorder as

"apraxic," it is important to demonstrate that the letter production deficit is not caused by more elementary sensorimotor (i.e., weakness or deafferen-tation), basal ganglia (i.e., tremor or rigidity), or cerebellar (i.e., dysmetria or ataxia) dysfunction affecting the writing limb.

Although patients with apraxic agraphia may be able to grasp the pen correctly, the spatiotemporal attributes of handwriting are severely disturbed and the facile strokes normally used to produce the required spatial trajectory are replaced by slow, effortful, and imprecise movements. In severe cases, all attempts at writing may result in an illegible scrawl. When the writing disorder is less severe, it may be possible to distinguish between various writing styles and between upper- and lowercase forms, although individual letters may be difficult to recognize. Typical errors of letter morphology include spatial distortions, stroke omissions, and the insertion of anomalous strokes resulting in nonletters (Fig. 3A). When letters are more legible, it may be possible to demonstrate that written spelling is actually preserved. The writing difficulty may be specific to letter formation since in some cases numbers could be produced correctly. Patients with apraxic agraphia can sometimes copy letters better than they can write them spontaneously or to dictation. However, copying is slow and fragmented, and it is usually accomplished in a "stroke-by-stroke" fashion relying heavily on visual feedback. These production features are characteristic of unskilled graphomotor performance and are reminiscent of the way children first learn to write.

Apractic Agraphia

Figure 3 (A) Errors of letter morphology in the writing produced by a patient with apraxic agraphia following left parietal lobe damage. (B) Micrographia in Parkinson's disease. Note overall reduction of letter size. Progressive reduction of writing amplitude is seen with repeated attempts to write the same word or letter. (C) Afferent dysgraphia in a patient with right parietal lobe damage. Duplications occur mostly in words with double letters and when writing letters that contain repeated stroke cycles.

Figure 3 (A) Errors of letter morphology in the writing produced by a patient with apraxic agraphia following left parietal lobe damage. (B) Micrographia in Parkinson's disease. Note overall reduction of letter size. Progressive reduction of writing amplitude is seen with repeated attempts to write the same word or letter. (C) Afferent dysgraphia in a patient with right parietal lobe damage. Duplications occur mostly in words with double letters and when writing letters that contain repeated stroke cycles.

Within the framework of our model, apraxic agraphia can be explained by postulating damage to processing components involved in the programming of handwriting movements. Possible neuropsycholo-gical mechanisms include the destruction or disconnection of graphic motor programs or damage to systems involved in translating the information contained in graphic motor programs into graphic in-nervatory patterns (Fig. 2). Apraxic agraphia is dissociable from limb apraxia, suggesting that motor programs for writing are distinct from programs for other types of purposeful skilled movements.

Apraxic agraphia has specific clinicoanatomical correlations. In right-handers, the responsible lesions typically involve the left posterior-superior parietal region. In particular, damage to cortical areas surrounding the intraparietal sulcus (i.e., superior parietal lobule and the superior portions of the angular and supramarginal gyri) is a common finding. In other cases, the lesions involve dorsolateral frontal cortex, including the premotor area at the foot of the second frontal convolution known as Exner's writing center. Finally, apraxic agraphia has been described following damage to the supplementary motor area (SMA). Taken together, these neuroanatomical observations suggest that the motor programming of handwriting is controlled by a distributed neural system that includes parietal and frontal cortical components with distinct functional roles. Specifically, posterior-superior parietal cortex may contain abstract spatiotemporal codes for writing movements (i.e., graphic motor programs), whereas the frontal components of the network (dorsolateral premotor cortex and SMA) may be responsible for generating the appropriate motor commands to specific muscle effector systems. Parietal lesions may cause apraxic agraphia by damaging or destroying graphic motor programs, whereas frontal premotor lesions may interfere with the process of translating these programs into graphic innervatory patterns specifying the proper sequence of muscle activations necessary for producing the appropriate stroke patterns. White matter lesions located deep to these cortical areas may cause apraxic agraphia by disconnecting the parietal and frontal components of the network.

The existence of a distributed frontoparietal cortical system responsible for controlling handwriting movements receives additional support from functional neuroimaging studies in normal subjects (Fig. 4). These studies have consistently demonstrated activation in posterior-superior parietal cortex, dorsolateral premotor cortex, and the SMA during the perfor-

Where Angular Gyrus Axial

Figure 4 Functional MRI scan in a normal right-handed subject demonstrating the cortical network involved in the production of handwriting movements. Regions of activation correlate with writing to dictation minus a control task of drawing circles. SMA, supplementary motor area; PMC, premotor cortex (including Exner's area); IPL, inferior parietal lobule (angular and supramarginal gyri); IPS, intra parietal sulcus; SPL, superior parietal lobule; CS, central sulcus.

Figure 4 Functional MRI scan in a normal right-handed subject demonstrating the cortical network involved in the production of handwriting movements. Regions of activation correlate with writing to dictation minus a control task of drawing circles. SMA, supplementary motor area; PMC, premotor cortex (including Exner's area); IPL, inferior parietal lobule (angular and supramarginal gyri); IPS, intra parietal sulcus; SPL, superior parietal lobule; CS, central sulcus.

mance of various writing tasks. Similar brain regions are activated during imagined writing movements, suggesting that mentally executed and real graphomo-tor gestures are subserved by partially overlapping neural systems. Frontoparietal cortical networks are also implicated in other types of skilled hand movements typically performed under visual guidance, including reaching, grasping, and object manipulation. These observations suggest that distinct frontoparietal cortical systems may form the neural substrate of specific object-oriented motor behaviors.

In most right-handed persons the left hemisphere is dominant for writing. Consequently, writing with the left hand in these individuals must involve transfer of linguistic and motor information from the left to the right hemisphere across the corpus callosum. Consistent with this hypothesis, damage to the corpus callosum in right-handers produces unilateral agra-phia of the left hand. Neuroanatomical observations in patients with callosal agraphia suggest that information critical for programming the skilled movements of writing is transferred through the body of the corpus callosum, whereas linguistic information is transferred more posteriorly through the fibers of the splenium. Furthermore, the fact that unilateral apraxic agraphia and ideomotor apraxia are occasionally dissociable following callosal damage implies that motor programs for writing and programs for other types of skilled limb movements are transferred through anatomically distinct callosal pathways.

3. Impaired Selection and Control of Movement Parameters: Nonapraxic Disorders of Writing Force, Speed, and Amplitude

In addition to determining the correct sequence of muscle activations, the neural systems responsible for generating graphic innervatory patterns must also select the appropriate kinematic parameters for the writing task. The breakdown of these operations may result in the insertion of incorrect movement parameters into otherwise intact graphic motor programs, leading to defective control of writing force, speed, and amplitude.

A typical example of this type of motor production deficit is the micrographia of patients with Parkinson's disease. As its name implies, micrographia is characterized by a striking reduction in handwriting size (Fig. 3B). Letters may get progressively smaller during the writing process. Even though letter size is diminished, writing remains legible in milder cases and there are no stroke-level errors of the type seen in apraxic agraphia. These findings indicate that the control of writing movements at the level of graphic motor programs is preserved. Although patients with micro-graphia can activate the correct muscles in the appropriate sequence, they cannot generate the forces necessary to maintain proper letter size. Writing speed is also significantly reduced. These production features are consistent with the general reduction of movement amplitude and speed in Parkinson's disease.

Micrographia in Parkinson's disease reflects basal ganglia dysfunction caused by the loss of striatal dopamine. Dopaminergic projections to the striatum originate in the substantia nigra of the midbrain. It has been demonstrated that focal lesions of the substantia nigra or the striatum can produce micrographia of the contralateral hand. Basal ganglia structures exert their influence on motor behavior through reciprocal connections to cortical motor areas, including dorsolat-eral premotor cortex and SMA. Operating as a functional unit, the cortical and subcortical components of the basal ganglia-thalamocortical motor loop play an important role in the control of movement force, speed, and amplitude. Consistent with this hypothesis, micrographia has been observed not only in patients with basal ganglia lesions but also following damage to the SMA.

Poor penmanship is also typical of the writing produced by patients with cerebellar dysfunction. Cerebellar lesions interfere with the smooth and automatic execution of the rapid alternating movement sequences that characterize normal handwriting. Writing movements are slow, effortful, and disjointed. Precise control over movement direction, force, speed, and amplitude is no longer possible. As a result, the writing trajectory becomes irregular and may be subject to unpredictable perturbations that the patient is unable to correct. Letters with curved shapes tend to be decomposed into straight lines, reflecting abrupt transitions in movement direction.

Similar to the basal ganglia, the cerebellum is connected to frontal premotor areas via re-entrant neuronal circuitry. Clinical observations in patients with cerebellar lesions suggest that the corticocerebel-lar motor loops are involved in the selection and control of kinematic parameters for skilled movements. The cerebellum also plays an important role in monitoring motor performance by comparing premo-tor commands for the intended movement with sensory feedback about the actual movement taking place. This comparator function is critical for error detection and for adjusting the evolving movement to changing contextual requirements.

In summary, basal ganglia-thalamocortical and cerebellocortical motor networks are possible neural substrates of the system responsible for generating the graphic innervatory patterns that guide the execution of handwriting movements. This conclusion is supported by functional neuroimaging studies that demonstrated activation of premotor cortex, basal ganglia, and cerebellum during the performance of various writing tasks.

4. Afferent Dysgraphia: The Role of Sensory Feedback in Handwriting

Patients with afferent dysgraphia have difficulty using sensory feedback to monitor and control the execution of handwriting movements. The writing errors produced by these patients are similar to those observed in normal subjects under experimental conditions that interfere with the efficient use of visual or kinesthetic feedback. Typical findings include duplications and omissions that are especially common when writing sequences of similar letters or strokes (e.g., "shampoo" written as "shampo" or "shampooo," or the addition of extra loops in writing letters containing repeated stroke cycles) (Fig. 3C). Writing errors are usually not detected by the patients and therefore remain uncor-rected. Cursive handwriting tends to be more severely affected than writing in uppercase print. In addition to letter/stroke duplications and omissions, patients may have difficulty keeping the correct spacing between letters and words and may not be able to write in a straight, horizontal line.

Afferent dysgraphia is typically seen following right parietal lobe damage, although features of the syndrome can occasionally be observed in patients with left parietal lesions. These findings suggest that the parietal lobes in general, and the right parietal lobe in particular, play an important role in monitoring visual and kinesthetic feedback for the proper control of writing movements. Writing produced by patients with right parietal damage also frequently shows evidence of left-sided spatial neglect. However, neglect-related errors (e.g., the tendency to write on the right side of the page or the failure to cross t's and dot i's at the beginning of the word) and feedback-related errors (e.g., letter/stroke repetitions and omissions) are dissociable. Furthermore, the type of neglect that affects the peripheral aspects of writing is distinct from the type of neglect that affects the internal graphemic representations of words and leads to unilateral spelling errors during the readout process from the graphemic buffer. These observations suggest that attention interacts with the writing process at several different levels. Thus, we must distinguish between attentional resources directed toward spatially coded internal graphemic representations computed by central spelling routes and attentional resources directed toward stimuli located in external visual space, including the letters and words produced on the page during writing.

In addition to being mediated by neural systems distinct from those involved in spatial attention, afferent control of stroke and letter production may also be separate from control over the more general (i.e., nonlateralized) visuospatial aspects of writing, such as maintaining the proper horizontal orientation and keeping the correct spacing between letters and words. According to this hypothesis, the core deficit in afferent dysgraphia is the inability to use sensory feedback to keep track of the number of letters and strokes produced in handwriting, whereas the other features of the syndrome may reflect simultaneous damage to functionally independent visuoperceptual and spatial attention modules located in close anatomical proximity within parietal cortex.

Letter/stroke duplications have also been observed in the writing produced by patients with right frontal lobe lesions, and it has been proposed that these errors may represent a form of motor perseveration. In addition, features of afferent dysgraphia have been documented following cerebellar damage. This finding is consistent with the view that the cerebellum is normally involved in the monitoring of sensory feedback during the execution of handwriting movements.

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