The riddle of anosognosia: does unawareness of hemiplegia involve a failure to update beliefs?

Anosognosia for hemiplegia (AHP) is defined as a lack of awareness for motor incapacity after a brain lesion. The causes of AHP still remain poorly understood. Many associations and dissociations with other deficits have been highlighted but no specific cognitive or neurological impairment has been identified as a unique causative factor. We hypothesized that a failure to update beliefs about current state might be a crucial component of AHP. Here, we report results from a new test that are compatible with this view. We examined anosognosic and nosognosic brain-damaged patients, as well as healthy controls, on a task where they had to guess a target word based on successive clues, with increasing informative content. After each clue, participants had to propose a word solution and rated their confidence. Compared to other participants, anosognosic patients were abnormally overconfident in their responses, even when information from the clues was insufficient. Furthermore, when presented with new clues incongruent with their previous response, they often stuck to their former "false" beliefs instead of modifying them. This impairment was unrelated to global deficits in reasoning or memory, and all patients eventually identified the correct solution of riddles after the last, fully informative, clue. These results suggest that a deficit in the generation and adjustment of beliefs may be a key factor contributing to the occurrence and persistence of anosognosia, when associated with concomitant losses in motor, proprioceptive, and/or attentional functions. Patients may remain unaware of their deficit partly because they cannot "update" their beliefs about current state.