Peripheral Components

The peripheral processing components of writing are responsible for converting abstract orthographic information into movements of the pen. This complex sequence, which begins with letter shape selection and ends with neuromuscular execution, is carried out by a set of hierarchically organized processing modules, the operational characteristics of which are discussed in this section. Peripheral conversion mechanisms that subserve other output modalities (i.e., oral spelling, typing, and spelling with anagram letters) will not be discussed here, except to note that these systems most likely diverge from writing at the level of the graphemic buffer (Fig. 2).

1. Allographic Conversion

The first step in producing writing is the allographic conversion process, which involves the selection of the appropriate letter shapes for the string of graphemes held in the graphemic buffer. In handwriting, letters can be realized in different case (upper vs lower) or style (print vs script). The various physical forms each letter of the alphabet can take are referred to as allographs (e.g., B, b, ]¡y-, b). The choice of allographs is influenced by convention (e.g., capitalizing letters in sentence-initial position), contextual factors (e.g., filling out a form vs writing a note or a letter), and individual style.

The exact nature of the representations involved in allographic conversion remains poorly understood. Some investigators have proposed that allographs are stored in long-term memory as abstract visuospatial descriptions of letter shape (the allographic memory store in Fig. 2). Others have suggested that allographs correspond to letter-specific graphic motor programs in which shape information is specified in terms of the sequence of strokes necessary to produce the desired letter. Although it is currently not possible to adjudicate between these competing proposals, the latter interpretation certainly has the appeal of parsimony. In particular, it is not entirely clear why one would need to retrieve abstract visuospatial information about letter shapes once the characteristic stroke patterns of different letters are firmly established in procedural memory and writing becomes an automatic motor task.

2. Graphic Motor Programs

Handwriting is a highly specialized motor activity that takes years to master. Similar to other complex motor skills, the neural control of writing movements is organized in a hierarchical fashion, with the general outline of the movement represented at the highest level and lower levels regulating increasingly specific details of neuromuscular execution. At the highest level, writing movements are controlled by graphic motor programs. It is assumed that these programs are stored in long-term memory rather than being assembled de novo every time a particular letter is written. Graphic motor programs contain information about abstract spatiotemporal movement attributes, including the sequence, position, direction, and relative size of the strokes necessary to produce different letters. However, graphic motor programs do not specify concrete kinematic parameters such as absolute stroke size, duration, and force. Although graphic motor programs are letter specific, they are effector independent in the sense that they do not determine which particular muscle groups are to be recruited for movement execution. An interesting aspect of writing is that it can be performed by using different muscle-joint combinations of the same limb (e.g., writing with a pen is accomplished with the distal muscles of the hand and wrist, whereas writing on a blackboard mostly involves the proximal muscles of the shoulder and elbow) or by using different limbs altogether (e.g., dominant vs nondominant hand or foot). The finding that writing produced by different effector systems displays striking similarities with respect to overall letter shape suggests a high degree of motor equivalence, consistent with the notion of effector-independent graphic motor programs.

3. Graphic Innervatory Patterns

The final stage in handwriting involves the translation ofthe information encoded in graphic motor programs into graphic innervatory patterns containing sequences of motor commands to specific effector systems. It is at this lower stage in the motor hierarchy that the appropriate combinations of agonist and antagonist muscles are selected and concrete movement parameters specifying absolute stroke size, duration, and force are inserted into the program. Since the biophysical context of the actual writing task may vary from one occasion to another (e.g., different writing instruments or surfaces), the motor system must have the flexibility to compute the appropriate kinematic parameters "on-line." Therefore, parameter estimation is viewed as a more variable and dynamic process than the retrieval of stored graphic motor programs specifying relatively invariant movement attributes. Once the kinematic parameters for the given writing context have been selected, the motor system executes the strokes required to produce the desired letters as a rapid sequence of ballistic movements (between 100 and 200 msec/stroke).

4. Afferent Control Systems

Central motor programs for skilled movements can be executed in an "open-loop" fashion (i.e., relatively independent of sensory feedback). Consistent with this general principle of motor physiology, it is possible to write letters in the absence of vision or when the writing hand is deafferented as a result of severe peripheral or central sensory loss. It is also clear, however, that normal handwriting requires afferent input for maximum speed and accuracy. This fact can be readily demonstrated by depriving normal individuals of visual feedback (either by having them write with their eyes closed or by using delayed visual feedback). Characteristic production errors under these conditions include the tendency to duplicate or omit letters or strokes, especially when writing sequences of similar items (e.g., words with double letters or letters containing repeated stroke cycles such as "m" or "w"). These errors can also be observed when subjects are asked to write while performing another task simultaneously (e.g., counting aloud or tapping with the other hand). In the dual-task situation, visual and kinesthetic feedback are available, but sensory information is not being used efficiently because attention is diverted from it by the secondary task. It has been proposed that the accurate monitoring of afferent feedback plays an important role in updating graphic motor programs as to which letters or strokes have already been executed. This "place keeping'' function becomes especially critical when similar or identical stroke patterns have to be produced or when the complex sequence of muscle activations required for handwriting needs to be sustained over longer periods of time. Sensory feedback is also required to maintain the correct spacing between letters and words and to keep the line of writing properly oriented on the page.



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