Synchiria is a neurological condition in which a stimulus applied to one side of the body is referred to both sides. If, for example, one’s left hand is touched, he experiences tactile sensations on both hands. People with intact brains do not experience this, probably because of inhibitory mechanisms which prevent activity in one hemisphere of the brain from crossing over to the other.
This phenomenon is therefore very rare, and has only been reported in a small number of brain-damaged patients. It has been described in the auditory sense, whereby a person addresses a patient on the left but is responded to as if they were on the right, and in the visual sense, in which a visual stimulus presented to one eye is described as having been presented to the other. Such patients, in whom the inhibitory mechanisms which normally prevent stimuli from eliciting sensations on both sides of the body are apparently disrupted, provide a unique opportunity for investigating the underlying brain processes.
A new case study, published in Current Biology, shows that the phantom tactile sensations experienced by a stroke patient can be modulated by simple movements. This suggests that the inhibitory processes between the left and right hemispheres of the brain are controlled by limb position, and contributes to our understanding of the complex processes by which different types of information are integrated to produce our subjective sensory experiences.
Jared Medina and Brenda Rapp, of the University of Pennsylvania and Johns Hopkins University, respectively, report the case of a 71-year-old male patient known as DLE, who suffered a stroke 3 years ago. The insult to DLE’s brain was massive – structural magnetic resonance imaging (MRI, below) revealed that the stroke had caused extensive damage to the frontal, temporal and parietal lobes in the left hemisphere. As a result, his right arm and leg were completely paralysed, and his ability to both speak and write were severely impaired.
Medina and Rapp conducted a series of tests on DLE, in which a flat rubber cylinder was used to apply tactile stimuli to his hands. These tests revealed three significant abnormalities in how his brain processes tactile information. First, although he could accurately detect that his right hand was being touched, DLE was unable to localize exactly where the stimulation had been applied, and almost invariably perceived all the stimuli to originate from the third and fourth fingers.
By contrast, he could accurately localize stimuli applied to his left hand, but these also elicited a sensation in the right hand. Stimulation of his left forearm, biceps, chest, cheek, thigh and ankle also elicited tactile sensations in the corresponding part on the right side of the body. Finally, when both hands were stimulated at the same time, the patient failed to report a sensation from the right hand in almost one third of the trials. This may have been an underestimate, because some of the patient’s reports of being touched on the right hand may actually have been based on phantom sensations elicited by stimulation of the left hand.
The researchers then changed what they call the “frame of reference” of the tactile information, simply by asking DLE to move his arms, so that his hands occupied different positions relative to the midline of his body. During the first series of tests, in which phantom sensations in the right hand were elicited by stimulation of the left hand, DLE sat with his hands placed on the table in front of him. However, when his hands were placed to the left of the midline, stimulation of the left hand elicited the same phantom sensations in only 11% of the trials, compared to 55% when the hands were placed to the right.
Under these conditions, the accuracy with which DLE could detect sensations on his hands was also affected. In the initial tests, when both hands were touched at the same time, he failed to report stimulation of the right hand in a significant number of trials. When his hands were placed to the left of his body’s midline, not only did his phantom sensations decrease, but so too did his accuracy in reporting stimulation of the right hand. This supports the researchers’ hypothesis that some of his “correct” responses about being touched on the right hand were based on phantom sensations.
The patient was then asked to turn his head to one side or the other, thus changing the position of the hands with respect to the head’s midline. This too was found to have a significant effect on the phantom sensations elicited in the right hand by stimulation of the left hand- he experienced fewer phantom feelings when his head was turned to the right, so that both hands were to the left of its midline, and more when his head was turned to the left. And again, the patient was less accurate in reporting sensations on his right hand when his head was turned to the right.
DLE’s inability to accurately localize stimuli applied to his right hand, and his failure to report sensations in that hand when both hands were touched simultaneously, is likely due to damage to the left somatosensory cortex, the part of the brain which processes tactile information from the right side of the body. This study suggests that the somatosensory cortical areas representing each part of the body are precisely connected with their counterparts in the opposite side of the brain.
The authors explain their findings in terms of a shift in the brain hemisphere controlling perceptions from the hands and arms, but do not rule out other possible mechanisms. When the right hand is in the right “space” (that is, to the right of the midline of the head and body), sensory information from it passes to the somatosensory cortex in the left hemisphere. Because the left hemisphere is damaged, it does not inhibit the cross-over of information to the right hemisphere, and as a result, phantom sensations are generated and perceived. Conversely, when the hand is in the left space, the sensory information is represented in the undamaged hemisphere, which prevents it from crossing to the left hemisphere, so that no phantom sensations are experienced.
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Medina, J. & Rapp, B. (2008). Phantom Tactile Sensations Modulated by Body Position. Curr. Biol. 18: 1937-1942. DOI: 10.1016/j.cub.2008.10.068