Anosognosia for left-sided motor and sensory deficits, motor neglect, and sensory hemiinattention: is there a relationship?

In recent years, research on unilateral spatial neglect has focussed on dissociations between different aspects of the clinical syndrome, which is now considered by many students as a multi-componential disorder. Notwithstanding this leading view, there is at least one empirical argument which supports a unitary interpretation of the disorder. This is based on the observation, now replicated many times, that a variety of sensory stimulations (vestibular, optokinetic, transcutaneous mechanical vibration and nervous electrical, visual prism adaptation) involving a lateral change (left-right asymmetry) in the input pattern, affect in a very similar fashion virtually all manifestations of the syndrome, including: visuo-spatial neglect; hemianaesthesia (somatosensory hemi-inattention); extinction, hemiparesis, hemiplegia, and anosognosia for these motor disorders; somatoparaphrenia. These effects may be accounted for with reference to a spatial medium, articulated in a number of specific components, which is modulated by sensory input in a fundamentally similar fashion. Recent investigations concerning the neural bases of some of these stimulations support this view. In this chapter the case of the co-variation of the effects of vestibular stimulation on motor deficits and on anosognosia for hemiplegia is considered. The suggestion is made that one mechanism underlying anosognosia for hemiplegia is unawareness of a deficit of intention, or movement planning component, rather than, or in addition to, unawareness of a primary motor deficit. Temporary remission of anosognosia after vestibular stimulation may represent recovery from this neglect-related component, of which, as of other manifestations of the syndrome, patients are typically unaware. The recovered intention to move may allow the detection by the patient of the presence of a residual primary motor deficit, through a feedback mechanism.