Introduction
The seemingly effortless way that awareness of moving (or not moving) normally occurs might understandably lead to the conclusion that our self-awareness of action is a simple, unambiguous process. However, patients with anosognosia for hemiplegia (AHP) challenge this conclusion, because AHP patients are not aware of being unable to move. More precisely, AHP usually refers to a lack of awareness regarding motor impairment in patients suffering a right hemisphere stroke (Ellis and Small, 1993 and Ellis and Small, 1997). In practice, a variety of clinical presentations are considered characteristic of AHP. Some AHP patients fail to recognise, appreciate the severity, or acknowledge the consequences of paralysis (Orfei et al., 2007). Other patients deny outright any motor impairment, whilst some acknowledge the presence of a motor deficit, but explain it away (Bisiach & Geminiani, 1991). In some cases AHP co-occurs with unilateral neglect (i.e., a failure to respond to stimuli on the contralesional side), whilst in others neglect is absent (Berti et al., 2005 and Jehkonen et al., 2006). Furthermore, AHP can occur independently at verbal and non-verbal (i.e., behavioural) levels (Jehkonen et al., 2006). That is, some AHP patients refuse to acknowledge their paralysis, but are usually content to remain in bed, whereas other AHP patients may verbally acknowledge their paralysis, but attempt to get out of bed, stand, walk or perform other physical tasks that are clearly impossible (Bisiach & Geminiani, 1991). When asked to make self-evaluations, these patients are often unaware of their inability to execute bilateral tasks requiring use of the hemiplegic limb(s) (e.g., clap hands) (Berti et al., 1996, Berti et al., 1998, Marcel et al., 2004 and Nimmo-Smith et al., 2005).
Despite several decades of AHP research, we are still without an adequate explanation, capable of accounting for the diverse clinical, emotional, cognitive and neuroanatomical presentation of AHP (see Adair et al., 1997, Berti et al., 1996, Gold et al., 1994, Hildebrandt and Zieger, 1995 and Small and Ellis, 1996). The heterogeneous presentation of AHP is one factor that has impeded understanding of the disorder. A lack of consensus on how best to characterise and assess AHP means it is difficult to identify patterns in results across studies. Given the dissociation between verbal and behavioural awareness, AHP should be considered present if a patient displays either type of unawareness (Marcel et al., 2004 and Nimmo-Smith et al., 2005). Berti et al. (1996) have developed a rigorous instrument for assessing AHP, which encompasses measures of both verbal awareness and awareness of the behavioural consequences of illness. Employing this type of robust diagnostic method allows more consistent and thorough characterisation of AHP, which can facilitate comparisons across studies and the development of a better understanding of the disorder.
Another major weakness of most AHP studies is a failure to frame their explanations within a robust theoretical model, instead using the disorder itself as a starting point for investigation and constructing an explanation. In contrast, a recent cognitive neuropsychological account of AHP provides a theory-driven, experimentally testable explanation of the disorder (Berti & Pia, 2006). Berti and Pia utilise a ‘forward’ model (Fig. 1) of normal motor control and awareness (Wolpert, 1997 and Wolpert et al., 1995), the utility of which has been established by numerous studies in normal, healthy individuals (Blakemore, 2003, Blakemore et al., 1999, Blakemore et al., 2001, Blakemore, Goodbody, et al., 1998, Blakemore, Rees, et al., 1998 and Blakemore, Wolpert, et al., 1998) patients with schizophrenia (Blakemore et al., 2000, Frith, 2005 and Frith et al., 2000) and recent studies of AHP (Fotopoulou et al., 2008 and Jenkinson et al., in press), and anosognosia for dyskinesias (i.e., involuntary movements) in Parkinson’s disease (Jenkinson, Edelstyn, Stephens, & Ellis, submitted).
