Illusions and delusions in anosognosia for hemiplegia: from motor predictions to prior beliefs

Anosognosia for hemiplegia, the apparent unawareness of one’s contralesional motor deficits, is one of the most counter-intuitive symptoms seen in clinical neurology. Patients with anosognosia for hemiplegia typically have right perisylvian lesions and insist that they can move their paralysed limbs; they may produce elaborate excuses and confabulations when confronted with evidence of their paralysis, or show a marked indifference to their disabilities. Patients’ insight into their illness and corresponding behaviour can vary between sessions, but their anosognosic beliefs seem typically entrenched and can be characterized as delusional. For example, patients may claim that they can move even after clear motor failures, including falls and injuries. Moreover, some patients experience the illusion of moving their limbs when attempting to perform an action, and seem unable to realize that their experience is illusory (Fotopoulou et al ., 2008).There has been a long-held fascination among clinicians on the variability of anosognosia for hemiplegia. Accumulating studies in the second half of the 20th century led to the identification of several concomitant sensorimotor and cognitive impairments and the development of corresponding deficit theories. The theoretical focus on anosognosia for hemiplegia changed, however, in the last two decades as bodily self-consciousness became increasingly recognized as a central scientific topic in cognitive neuroscience. In this context, the understanding of anosognosia for hemiplegia has been enriched by at least four developments: (i) theoretical hypotheses, stemming from philosophical or computational approaches on motor and embodied cognition, that view anosognosia for hemiplegia as a specific disorder of motor awareness rather than a secondary consequence of deficits in other domains (e.g. Frith et al ., 2000); (ii) improvements in neuroimaging research that allowed the identification of brain lesions selectively associated with anosognosia for hemiplegia (Berti et al ., 2005); (iii) well-controlled, psychophysical experiments that supplemented neuropsychological assessments of …