Dual consciousness (also known as dual mind or divided consciousness) is a hypothesis in neuroscience. It is proposed that it is possible that a person may develop two separate conscious entities within their one brain after undergoing a corpus callosotomy. The idea first began circulating in the neuroscience community after some split-brain patients exhibited alien hand syndrome (AHS), which led some scientists to believe that there must be two separate consciousnesses within the brain's left and right hemispheres in competition with one another once the corpus callosum is severed.
The idea of dual consciousness has caused controversy in the neuroscience community. No conclusive evidence of the proposed phenomenon has been discovered.
Background During the first half of the 20th century, some neurosurgeons concluded that the best option of treating severe epilepsy was by severing the patient's corpus callosum. The corpus callosum is the primary communication mechanism between the brain's two cerebral hemispheres. For example, communication across the corpus callosum allows information from both the left and right visual fields to be interpreted by the brain in a way that makes sense to comprehend the person's actual experience (e.g., visual inputs from both eyes are interpreted by the brain to make sense of the experience that you are looking at a computer that is directly in front of you). The procedure of surgically removing the corpus callosum is called a corpus callosotomy. Patients who have undergone a corpus callosotomy are colloquially referred to as "split-brain patients". This is because their brain's left and right hemispheres are no longer connected by the corpus callosum.
Split-brain patients have been subjects for numerous psychological experiments that sought to discover what occurs in the brain after the primary interhemispheric pathways have been disrupted. Notable researchers in the field include Roger Sperry, one of the first to publish ideas involving a dual consciousness; and his famous graduate student, Michael Gazzaniga. Their results found a pattern among patients: severing the entire corpus callosum stops the interhemispheric transfer of perceptual, sensory, motor, and other forms of information. For most cases, corpus callosotomies did not in any way affect patients' real-world functioning; however, those psychology experiments have demonstrated some differences between split-brain patients and other subjects.
Split-brain patients and the corpus callosotomy
The first successful corpus callosotomies on humans were performed in the 1930s. The purpose of the procedure was to alleviate the effects of epilepsy when other forms of treatment (medications) had failed to stop the violent convulsions associated with the disorder. Epileptic seizures occur because of abnormal electrical discharges that spread across areas of the brain. William Van Wagenen proposed the idea of severing the corpus callosum to eliminate transcortical electrical signals across the brain's hemispheres. If this could be achieved, then the seizures should be reduced or even eliminated.
The general procedure of a corpus callosotomy is as follows. The patient is put under anesthesia. Once the patient is in deep sleep, a craniotomy is performed. This procedure removes a section of the skull, leaving the brain exposed and accessible to the surgeon. The dura mater is pulled back so the deeper areas of the brain, including the corpus callosum, can be seen. Specialized instruments are placed into the brain that allows safe severing of the corpus. Initially, a partial callosotomy is performed, which only severs the front two-thirds of the callosum. Though the corpus callosum loses a majority of its functioning during a partial callosotomy, it does not completely lose its capabilities; because the back section of the callosum is preserved, visual information is still sent across both hemispheres. If the operation does not succeed in reducing the seizures, a complete callosotomy is needed to reduce the severity of the seizures.
A similar type of procedure, known as a commissurotomy, involves severing a number of interhemispheric tracts (such as the anterior commissure, the hippocampal commissure and the massa intermedia of the thalamus) in addition to the corpus callosum.
After surgery, the split-brain patients are often given extensive neuropsychological assessments. One finding among split-brain patients is that many of them feel normal after the surgery and do not feel that their brains are "split". The corpus callosotomy and commissurotomy have been successful in reducing, and in some cases, eliminating epileptic seizures. This aligns with Van Wagenen's theory.
Alien hand syndrome
Alien hand syndrome, sometimes used synonymously with anarchic hand is a neurological disorder in which the afflicted person's hand appears to act on its own. Alien hand syndrome has been documented in some split-brain patients.
History The first instance of alien hand syndrome was reported in 1908 by Kurt Goldstein. This incident occurred to a woman in her 50s whose left hand grabbed her throat without her making the effort to do so. She was able to remove her hand, but it took a great deal of effort to do so. Upon her death, an autopsy was performed which concluded that the event may have been caused by several strokes in her right hemisphere and corpus callosum. In the 1940s, reports surfaced of patients who had undergone corpus callosotomies that were experiencing uncontrollable hand movements following surgery. In these instances, the actions of one’s left hand conflicted with the actions of one’s right hand. The initial diagnosis for these experiences was "diagnostic dyspraxia".
There are three main types of alien hand syndrome. The first is the frontal variant, which is characterized by the nondominant hand grabbing items and manipulating objects. The second is the callosal variant, which is the most common and is characterized by the uncontrollable and conflicting movement of a right-handed patient’s left hand. Callosal alien hand syndrome often occurs alongside other symptoms of callosal damage such as alexia, the diminished or absent ability to comprehend written language, and visual anomia, the inability to name objects seen in the right vision field. The third type is the posterior variant, which is characterized by the affected hand rising in the air and making non-purposeful movements.
Symptoms The classic sign of alien hand syndrome is that the affected person cannot control one of their hands. For example, if a split-brain patient with alien hand syndrome is asked to pick up a glass with their right hand, as the right hand moves over to the glass, the left hand will interfere with the action, thwarting the right hand's task. The interference from the left hand is completely out of the control of the patient and is not being done "on purpose". Affected patients at times cannot control the movements of their hands. Another example included patients unbuttoning a shirt with one hand, and the other hand simultaneously re-buttoning the shirt (although some reported feeling normal after their surgery). Switching one's attention from one task to another can also lessen the amount of control that they can allocate to their affected hand.
Relationship to dual consciousness When scientists first started observing the alien hand syndrome in split-brain patients, they began to question the nature of consciousness and theorized that perhaps when the corpus callosum is cut, consciousness is also split into two separate entities. This development added to the general appeal of split-brain research.
Gazzaniga and LeDoux's experiment Procedure and results In 1978, Michael Gazzaniga and Joseph LeDoux discovered a unique phenomenon among split-brain patients who were asked to perform a simultaneous concept task. The patient was shown two pictures: of a house in the winter time and of a chicken's claw. The pictures were positioned so they would exclusively be seen in only one visual field of the brain: the winter house was positioned so it would only be seen in the patient's left visual field (LVF), which corresponds to the brain's right hemisphere, and the chicken's claw was placed so it would only be seen in the patient's right visual field (RVF), which corresponds to the brain's left hemisphere.
A series of pictures was placed in front of the patients. Gazzaniga and LeDoux then asked the patient to choose a picture with his right hand and a picture with his left hand. The paradigm was set up so the choices would be obvious for the patients. A snow shovel is used for shoveling the snowy driveway of the winter house and a chicken's head correlates to the chicken's claw. The other pictures do not in any way correlate with the two original pictures. In the study, a patient chose the snow shovel with his left hand (corresponding to his brain's right hemisphere) and his right hand chose the chicken's head (corresponding to the brain's left hemisphere). When the patient was asked why he had chosen the pictures he had chosen, the answer he gave was "The chicken claw goes with the chicken head, and you need a snow shovel to clean out the chicken shed."
Both the winter house and the shovel were being projected to the patient from his LVF, so his right hemisphere received and processed the information; this input is completely independent from what is going on in the RVF, which involves the chicken's claw and head (the information being processed in the left hemisphere). The human brain's left hemisphere is primarily responsible for interpreting the meaning of the sensory input it receives from both fields; however, the patient's left hemisphere had no knowledge of the winter house. Because of this, the left hemisphere had to invent a logical reason for why the shovel was chosen. Since the only objects it had to work with are the chicken's claw and head, the left hemisphere interprets the meaning of choosing the shovel as "it is an object necessary to help the chicken, which lives in a shed, therefore, the shovel is used to clean the chicken’s shed". Gazzaniga famously coined the term left-brain interpreter to explain this phenomenon.
Other experiments
Sperry–Gazzaniga
The Gazzaniga–LeDoux studies were based on previous studies done by Sperry and Gazzaniga. Sperry's experiment included a subject being seated at a table, with a shield blocking the visions from the subject's hands, including the objects on the table and the examiner seated across. The shield was also used as a viewing screen. On the shield, the examiner can select to present the visual material to both hemispheres or to selective hemispheres by means of having the viewing screen. The patient is briefly exposed to the stimuli on the viewing screen. The stimuli shown to the left eye goes to the right hemisphere, and the visual material shown to the right eye will be projected to the left hemisphere. During the experiment, when the stimulus was shown to the left side of the screen, the patient indicated he did not see anything. Patients have shown the inability to describe in writing or in speech the stimuli that was shown briefly to the left side. The speaking hemisphere, which in most people is the left hemisphere, would not have awareness of stimulus being shown to the right hemisphere (left visual field), except the left hand was able to point to the correct object. Based on his observations and data, Sperry concluded each hemisphere possessed its own consciousness.
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