Search

An alien limb is a debilitating disorder of volitional control. The core feature of alien limb is the performance of simple or complex semi-purposeful movements which the patient reports to be unintentional or unwanted, or occasionally in opposition to their intentions. Theories of the mechanism of alien limb phenomena have emphasised the role of disinhibition in the brain, and exaggerated action 'affordances'. However, despite advances in cognitive neuroscience research and a large public and media interest, there has been no unifying computational and anatomical account of the cause of alien limb movements. Here, we extend Bayesian brain principles to propose that alien limb is a disorder of 'predictive processing' in hierarchical sensorimotor brain networks. Specifically, we suggest that alien limb results from predictions about action outcomes that are afforded unduly high precision. The principal mechanism for this abnormally high precision is an impairment in the relay of input from medial regions, predominantly the supplementary motor area (SMA), which modulate the precision of lateral brain regions encoding the predicted action outcomes. We discuss potential implications of this model for future research and treatment of alien limb.

Ischemic lesions within the territory of the anterior cerebral artery present with a variety of clinical signs and symptoms. Among these, frontal alien hand syndrome is rare and easily overlooked in the acute clinical setting, but significantly impacts on functional activities of daily life. Given its rareness, very little is known about its long-term outcome. To shade some more light onto this issue, clinical presentation, course of rehabilitation and outcome of two illustrative cases of frontal alien hand syndrome following anterior cerebral artery stroke are presented. Within seven and nine months from symptom onset, respectively, the clinical symptoms of frontal alien hand had resolved completely in both cases. We conclude that frontal alien hand syndrome has a favourable long-term outcome.

In patients with the callosal type of anarchic-hand syndrome, the left hand often does not act as intended and counteracts the right hand. Reports are scarce about the underlying neurophysiological mechanisms. We report the case G.H. who developed the syndrome after infarction of the left arteria pericallosa. It has been suggested that the syndrome arises out of lacking inhibition from the dominant left hemisphere on the right hemisphere. Yet, in tests of spatial intelligence G.H. performed much better with his "anarchic" left hand than with his dominant right hand, similar to observations commonly reported in split-brain patients. Left-right manual choice responses and event-related EEG potentials to laterally presented stimuli were measured. Asymmetries were evident in G.H.'s behavior and EEG potentials, different from age-matched healthy participants (n=11). His right-hand responses were fast and unaffected by incompatibility with stimulus location, whereas his left-hand responses were variable and accompanied by a large negative central-midline EEG potential, probably reflecting efforts in initiating the response. G.H.'s visual N1 component peaked earlier and was larger at the right than the left side of the scalp, and the P3 component was distinctly reduced at the right side. Both features occurred independent of side of stimulus presentation and side of responding hand. The effort indicated by the midline negativity and the asymmetrically reduced P3 might directly reflect G.H.'s lack of control on his right hemisphere's processing. The faster visual processing of the right hemisphere suggested by the N1 asymmetry might contribute to "anarchic" processing, making the right hemisphere process stimuli before control impulses exert their effect. These neurophysiological results tend to support the split-brain account which assumes that the syndrome arises by the lack of communication between hemispheres that act according to their respective competences.

Diagonistic dyspraxia (DD) is a behavioural disorder encountered in split-brain subjects in which the left arm acts against the subject’s will, deliberately counteracting what the right arm does. We report here an oculomotor and computational study of a patient with a long lasting form of DD. A first series of oculomotor paradigms revealed marked and unprecedented saccade impairments. We used a computational model in order to provide information about the impaired decision-making process: the analysis of saccade latencies revealed that variations of decision times were explained by adjustments of response criterion. This result and paradoxical impairments observed in additional oculomotor paradigms allowed to propose that this adjustment of the criterion level resulted from the co-existence of counteracting oculomotor programs, consistent with the existence of antagonist programs in homotopic cortical areas. In the intact brain, trans-hemispheric inhibition would allow suppression of these counter programs. Depending on the topography of the disconnected areas, various motor and/or behavioural impairments would arise in split-brain subjects. In motor systems, such conflict would result in increased criteria for desired movement execution (oculomotor system) or in simultaneous execution of counteracting movements (skeletal motor system). At higher cognitive levels, it may result in conflict of intentions.

Epilepsia partialis continua (EPC) is clinically defined as a syndrome of continuous focal jerking of a body part, usually a distal limb, occurring over hours, days, or even years. It is considered the status epilepticus equivalent of simple partial motor seizures. A 48-year-old right-handed man with a history of traumatic intracranial hemorrhage was admitted for right-sided hemiplegia and drowsiness after complex partial status epilepticus. An EEG showed periodic lateralized epileptiform discharges over the left hemisphere. Brain MRI revealed extensive multifocal encephalomalaciac changes in the left temporo-parieto-occpital lobe and both frontal lobes with some hemorrhagic residual change. After administration of a loading dose of intravenous phenytoin, his mental status returned to normal. However, his weakness only partially improved. [18F]Fluorodeoxyglucose PET (FDG-PET) demonstrated severe hypometabolism in the left cerebral hemisphere, including the basal ganglia and thalamus, with cerebellar diaschisis. At the 3-month follow-up, he complained of symptoms of alien hand phenomenon. Follow-up MRI revealed more extensive encephalomalaciac changes in previously noted regions with thinning of the posterior end of the body of the corpus callosum. Moreover, FDG-PET demonstrated persistent severe hypometabolism over the left cerebral hemisphere. We suggest that the alien hand phenomenon was a result of thinning of the corpus callosum related to EPC.

A pediatric patient with diagnosis of Parry-Romberg syndrome, or progressive hemifacial atrophy, presented with new-onset unilateral alien hand syndrome, which was attributed to focal progressive atrophy and gliosis in the contralateral thalamus observed on serial neuroimaging. This case illustrates not only the clinical-radiographic correlation between alien hand syndrome and contralateral thalamic lesion, but also involvement of deep gray structure in Parry-Romberg syndrome causing a rare movement disorder.

We present a 41-year-old man who experienced alien limb syndrome as a complication of treatment for recurrent Grade III oligodendroglioma of the right parietal lobe. Alien limb syndrome is a rare phenomenon in which a limb performs involuntary actions and the affected individual feels a sense of estrangement towards the limb. It occurs most commonly as a result of corticobasal syndrome, though a variety of other etiologies have been reported. It is rarely associated with focal lesions, such as stroke or tumors.

Callosal alien hand syndrome is characterized primarily by intermanual conflict and is associated with an anterior callosal lesion. We report a patient who presented with topographical disorientation and the callosal type alien hand sign. An MRI of the brain showed a right parietal lobe infarction. This is a rare example of callosal alien hand sign associated with a right parietal lesion. The right parietal lobe appeared to be responsible for the callosal hand sign in this patient, possibly due to interference with peristriate outflow pathways toward the parietal zones, where visual somatosensory interactions are likely to occur.