what is being produced through efferent outflow to the musculature of the body, and what is being sensed as the presumed product in the periphery of this efferent command signal. in alien hand syndrome, the neural mechanisms involved in establishing that this congruence has occurred may be impaired. this may involve an abnormality in the brain mechanism that differentiates between "re-afference" (the return of kinesthetic sensation from the self-generated "active" limb movement) and "ex-afference" (kinesthetic sensation generated from an externally produced 'passive' limb movement in which an active self does not participate). this brain mechanism is proposed to involve the production of a parallel "efference copy" signal that is sent directly to the somatic sensory regions and is transformed into a "corollary discharge", an expected afferent signal from the periphery that would result from the performance driven by the issued efferent signal. the correlation of the corollary discharge signal with the actual afferent signal returned from the periphery can then be used to determine if, in fact, the intended action occurred as expected. when the sensed result of the action is congruent with the predicted result, then the action can be labelled as self-generated and associated with an emergent sense of agency. if, however, the neural mechanisms involved in establishing this sensorimotor linkage associated with self-generated action are faulty, it would be expected that the sense of agency with action would not develop as discussed in the previous section. loss of inhibitions one theory posed to explain these phenomena proposes that the brain has separable neural "premotor" or "agency" systems for managing the process of transforming intentions into overt action. an anteromedial frontal premotor system is engaged in the process of directing exploratory actions based on "internal" drive by releasing or reducing inhibitory control over such actions. a 2011 paper reporting on neuronal unit recording in the medial frontal cortex in human subjects showed a clear pre-activation of neurons identified in this area up to several hundred milliseconds prior to the onset of an overt self-generated finger movement and the authors were able to develop a computational model whereby volition emerges once a change in internally generated firing rate of neuronal assemblies in this part of the brain crossed a threshold. damage to this anteromedial premotor system produces disinhibition and release of such exploratory and object acquisition actions which then occur autonomously. a posterolateral temporo-parieto-occipital premotor system has a similar inhibitory control over actions that withdraw from environmental stimuli as well as the ability to excite actions that are contingent upon and driven by external stimulation, as distinct from internal drive. these two intrahemispheric systems, each of which activates an opposing cortical "tropism", interact through mutual inhibition that maintains a dynamic balance between approaching toward (in other words, with "intent-to-capture" in which contact with and grasping onto the attended object is sought) versus withdrawing from (that is, with "intent-to-escape" in which distancing from the attended object is sought) environmental stimuli in the behavior of the contralateral limbs. together, these two intrahemispheric agency systems form an integrated trans-hemispheric agency system. when the anteromedial frontal "escape" system is damaged, involuntary but purposive movements of an exploratory reach-and-grasp nature – what denny-brown referred to as a positive cortical tropism – are released in the contralateral limb. this is referred to as a positive cortical tropism because eliciting sensory stimuli, such as would result from tactile contact on the volar aspect of the fingers and palm of the hand, are linked to the activation of movement that increases or enhances the eliciting stimulation through a positive feedback connection (see discussion above in section entitled "parietal and occipital lobes"). when the posterolateral parieto-occipital "approach" system is damaged, involuntary purposive movements of a release-and-retract nature, such as levitation and instinctive avoidance – what denny-brown referred to as a negative cortical tropism – are released in the contralateral limb. this is referred to as a negative cortical tropism because eliciting sensory stimuli, such as would result from tactile contact on the volar aspect of the fingers and palm of the hand, are linked to the activation of movement that reduces or eliminates the eliciting stimulation through a negative feedback connection (see discussion above in section entitled "parietal and occipital lobes"). each intrahemispheric agency system has the potential capability of acting autonomously in its control over the contralateral limb although unitary integrative control of the two hands is maintained through interhemispheric communication between these systems via the projections traversing the corpus callosum at the cortical level and other interhemispheric commissures linking the two hemispheres at the subcortical level. disconnection of hemispheres due to injury one major difference between the two hemispheres is the direct connection between the agency system of the dominant hemisphere and the encoding system based primarily in the dominant hemisphere that links action to its production and through to its interpretation with language and language-encoded thought. the overarching unitary conscious agent that emerges in the intact brain is based primarily in the dominant hemisphere and is closely connected to the organization of language capacity. it is proposed that while relational action in the form of embodied inter-subjective behavior precedes linguistic capacity during infant development, a process ensues through the course of development through which linguistic constructs are linked to action elements in order to produce a language-based encoding of action-oriented knowledge. when there is a major disconnection between the two hemispheres resulting from callosal injury, the language-linked dominant hemisphere agent which maintains its primary control over the dominant limb loses, to some degree, its direct and linked control over the separate "agent" based in the nondominant hemisphere, and the nondominant limb, which had been previously responsive and "obedient" to the dominant conscious agent. the possibility of purposeful action occurring outside of the realm of influence of the conscious dominant agent can occur and the basic assumption that both hands are controlled through and subject to the dominant agent is proven incorrect. the sense of agency that would normally arise from movement of the nondominant limb now no longer develops, or, at least, is no longer accessible to consciousness. a new explanatory narrative for understanding the situation in which the now inaccessible nondominant hemisphere based agent is capable of activating the nondominant limb is necessitated. under such circumstances, the two separated agents can control simultaneous actions in the two limbs that are directed at opposing purposes although the dominant hand remains linked to the dominant consciously accessible language-linked agent and is viewed as continuing to be under "conscious control" and obedient to conscious will and intent as accessible through thought, while the nondominant hand, directed by an essentially non-verbal agent whose intent can only be inferred by the dominant agent after the fact, is no longer "tied in" and subject to the dominant agent and is thus identified by the conscious language-based dominant agent as having a separate and inaccessible alien agency and associated existence. this theory would explain the emergence of alien behavior in the nondominant limb and intermanual conflict between the two limbs in the presence of damage to the corpus callosum. the distinct anteromedial, frontal, and posterolateral temporo-parieto-occipital variants of the alien hand syndrome would be explained by selective injury to either the frontal or the posterior components of the agency systems within a particular hemisphere, with the relevant and specific form of alien behavior developing in the limb contralateral to the damaged hemisphere. diagnosis corpus callosum damage to the corpus callosum can give rise to "purposeful" actions in the person's non-dominant hand (an individual who is left-hemisphere-dominant will experience the left hand becoming alien, and the right hand will turn alien in the person with right-hemisphere dominance). in "the callosal variant", the patient's hand counteracts voluntary actions performed by the other, "good" hand. two phenomena that are often found in patients with callosal alien hand are agonistic dyspraxia and diagonistic dyspraxia. agonistic dyspraxia involves compulsive automatic execution of motor commands by one hand when the patient is asked to perform movements with the other hand. for example, when a patient with callosal damage was instructed to pull a chair forward, the affected hand would decisively and impulsively push the chair backwards. agonistic dyspraxia can thus be viewed as an involuntary competitive interaction between the two hands directed toward completion of a desired act in which the affected hand competes with the unaffected hand to complete a purposive act originally intended to be performed by the unaffected hand. diagonistic dyspraxia, on the other hand, involves a conflict between the desired act in which the unaffected hand has been engaged and the interfering action of the affected hand which works to oppose the purpose of the desired act intended to be performed by the unaffected hand. for instance, when akelaitis's patients underwent surgery to the corpus callosum to reduce epileptic seizures, one patient's left alien hand would frequently interfere with the right hand. for instance, while trying to turn over to the next page with the right hand, his left hand would try to close the book. in another case of callosal alien hand, the patient did not have intermanual conflict between the hands but rather from a symptom characterized by involuntary mirror movements of the affected hand. when the patient was asked to perform movements with one hand, the other hand would involuntarily perform a mirror image movement which continued even when the involuntary movement was brought to the attention of the patient, and the patient was asked to restrain the mirrored movement. the patient had a ruptured aneurysm near the anterior cerebral artery, which resulted in the right hand being mirrored by the left hand. the patient described the left hand as frequently interfering and taking over anything the patient tried to do with the right hand. for instance, when trying to grasp a glass of water with the right hand with a right side approach, the left hand would involuntary reach out and grasp hold of the glass through a left side approach. more recently, geschwind et al. described the case of a woman with severe coronary heart disease. one week after undergoing coronary artery bypass grafting, she noticed that her left hand started to "live a life of its own". it would unbutton her gown, try to choke her while asleep and would automatically fight with the right hand to answer the phone. she had to physically restrain the affected hand with the right hand to prevent injury, a behavior which has been termed "self-restriction". the left hand also showed signs of severe ideomotor apraxia. it was able to mimic actions but only with the help of mirror movements executed by the right hand (enabling synkinesis). using magnetic resonance imaging (mri), geschwind et al. found damage to the posterior half of the callosal body, sparing the anterior half and the splenium extending slightly into the white matter underlying the right cingulate cortex. park et al. also described two cases of infarction as the origin of alien hand symptoms. both individuals had had infarction of the anterior cerebral artery (aca). one individual, a 72-year-old male, had difficulty controlling his hands, as they often moved involuntarily, despite his trying to stabilize them. furthermore, he often could not let go of objects after grasping them with his palms. the other individual, a 47-year-old female with an aca in a different location of the artery, complained that her left hand would move on its own and she could not control its movements. her left hand could also sense when her right hand was holding an object and would involuntarily, forcibly take the object out of her right hand. frontal lobe unilateral injury to the medial aspect of the brain's frontal lobe can trigger reaching, grasping and other purposeful movements in the contralateral hand. with anteromedial frontal lobe injuries, these movements are often exploratory reaching movements in which external objects are frequently grasped and utilized functionally, without the simultaneous perception on the part of the patient that they are "in control" of these movements. once an object has been acquired and is maintained in the grasp of this "frontal variant" form of alien hand, the patient often has difficulty with voluntarily releasing the object from grasp and can sometimes be seen to be peeling the fingers of the hand back off the grasped