Shashwat Goyal
Blog Post-1
Alien hand syndrome (aka
Dr. Strangelove syndrome) is a disorder which causes movements in a person’s
limbs independent of their will. The condition was first described by Goldstein
in 1908 as “a type of apraxia with the feeling of estrangement between the
patient and hand.” While this name can be used for a variety of conditions,
most commonly occurring in the left hand. The movements can include complex
motor functions such as reaching for, holding and manipulating objects or counteracting
the actions of the controlled hand. Patients report having no control over the independent
limb, often having no realization that one of their limbs is moving on its own.
In the following paragraphs, I will be describing the most common variants of
this syndrome, as well as explaining the neurological causes behind it (as
understood currently).
The use of the term
“alien hand” implies a somewhat different condition, as it requires the patient
to feel the hand is a separate entity, one that they may assign personalities.
Patients descriptions of such limbs range from “cheeky” to “monster from the
moon”. One patient even named hers “baby Joseph” as it would often pinch her
nipples, causing a sensation like the biting of a breast-feeding baby.
It’s most commonly seen
in people who’ve had damage or surgery resulting in severing of the corpus
callosum (structure that connects the two hemispheres of the brain). It is also
reported in cases of stroke, infection, tumour, aneurysm, migraine, and degenerative
brain diseases like Alzheimer’s and Creutzfeldt-Jakob. Other areas associated
with this condition are frontal, parietal and occipital. The most common
variants are the callosal and frontal lesions.
The callosal variant
involves damage to the corpus callosum and affects the non-dominant hand.
Patients of this variant commonly show “inter-manual conflict”, where the alien
hand acts against the controlled hand, such as taking a cigarette (that the
patient has put in their mouth using their controlled hand) out of the mouth
and throwing it away. It is thought that there is a conscious agent in the
dominant hemisphere of the brain. This agent links action to language encoded
thought due to direct connection between the agency system and the encoding
system in the dominant hemisphere. Since the actions committed by the
non-dominant limb are often complex and purposeful, the idea that there is only
one agent in the brain, present in the dominant hemisphere is rendered false. Hence,
another subservient agent in the non-dominant hemisphere that is controlled by
the conscious agent is theorized. When the corpus callosum is severed, thus
subservient agent breaks free from the control of the dominant agent. While
this theory does explain why the non-dominant hand is considered alien (as the
sense of agency corresponding to its movement doesn’t develop or is
inaccessible to the conscious agent) and why the alien hand shows complex
actions, it fails to explain why the separate agent causes inter-manual
conflict between the hands.
The frontal variant
involves damage to the frontal lobe and usually affects the dominant hand, but
can affect any hand based on the localization of the damage to the medial
frontal cortex. This variant causes grasp reflexes, reaching for objects and
the inability to release grasp (tonic grasping). Brain scans such as fMRIs have
been used to try and determine the neurological cause of this disorder. These tests revealed that any voluntary movement and the sense of agency associated with it is caused by a decided order of “anterior to posterior” activation of brain
areas, from supplementary and premotor areas like (anteromedial frontal cortex)
to the primary motor cortex in the pre-central gyrus and then to the posterior
parietal cortex. The primary motor cortex is believed to be directly involved in
executing the action while the signal sent to the parietal lobe is an efferent
copy that interacts with and cancels out the somatosensory feedback that the
periphery sends to the brain as a result of movement. Thus the feedback from
the periphery is perceived as re-afference (due to self-generated movement) and
not as ex-afference (due to external stimuli). Damage to the brain can result in
direct activation of the primary motor cortex, leading to the execution of an
action without the sending of an efferent copy to the parietal lobe. Hence, a directly
activating primary motor complex without the premotor and supplementary motor
areas can cause a feeling of a limb moving of its own accord rather than by an
internally generated signal, as is actually the case. The sense of ownership of
the arm seems unaffected by this loss of sense of agency, and the cognitive
dissonance created by the occurrence of movement without a physical external
stimuli or a sense of agency in the patient is resolved by assigning the agency
to the arm itself and treating it as a separate, external entity or an alien.
References
1)
Wikipedia page for
alien hand syndrome
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