What's in a Name? Conundrums Common to the Task‐Specific Disorders

Task specificity is a fascinating feature of a subset of movement disorders. In this issue of Movement Disorders in Clinical Practice, Prasad and colleagues describe a highly selective writing deficit for a single symbol in Bengali: “Letter specific dysgraphia: A silent stutter” bringing to the forefront a number of absorbing questions. For example, how do we define whether a disorder is task specific? Frequently, we use clinical markers as our gold standard; subjective impairment described during history taking or an abnormality of movement observed during clinical examination. However, such markers are likely to have a different sensitivity to experimentally derived markers. For example, modern motion-capture technology can record movement with a spatial accuracy and frequency beyond that consciously perceived by the human brain or witnessed by the human eye. Experimentally, task-specific dystonia is associated with abnormal neurophysiological responses (plasticity, inhibition), and these markers are sampled using experimental paradigms removed from the affected task’s context. Therefore, task specificity appears to be a relative task specificity. That is, a particular task is preferentially affected and causes disability to the patient. A gradient of subtle abnormalities for other tasks is likely to be present and skills most similar to the affected task may be the most affected, given that their required neuronal networks are more closely related. Experimentally, we are also able to tap into more-generalized changes within the nervous system, which may be causally related to the pathophysiology or epiphenomena. Task specificity is a feature of a range of neurological disorders. Many clinical examples can be conceptualized as an interaction between the network encoding the task and the network affected by the disorder. Occasionally, focal lesions selective for an essential feature of task performance can lead to a task-specific deficit. A task-specific presentation of a diffuse disease process is more dependent on a threshold effect; once a critical amount of dysfunction to the skill network has been induced by the disease process, a deficit will become apparent. Difficulty with morecomplex motor skills with high performance requirements or unique task features subserved by an otherwise “silent” area of the brain may therefore unmask an insidious disease process because only a small burden of disease may be symptomatic. The sensitivity of the individual to the deficit may also vary depending on influences such as attentional monitoring of the task and the extent the action is required for daily living. Correspondingly, writing dystonia and writing tremor are recognized presentations of the genetic and classically generalized dystonia DYT1 dystonia, and difficulty styling hair has been described as a presenting feature of a retired hairdresser that then developed apraxia and neuroimaging findings suggestive of a neurodegenerative process. Considering the different functions of the neuronal network required for task performance can also help guide our assessment and management. Prasad and colleagues described a deficit writing a spiral symbol common to a Bengali letter and number. It would be interesting to know whether this deficit was also present across other muscle effectors (e.g., if the symbol was still difficult to transcribe with the foot). This would suggest a higherlevel problem in motor planning. If solely in the hand, the deficit of motor control is likely to be encoded at “lower” levels of motor control, which define the spatio-temporal dynamics of the arm muscles activated when writing this symbol. For the isolated task-specific disorders, we then need to select the most appropriate diagnostic label. Prasad and colleagues selected the descriptor dysgraphia: